Anal. Chem. 1984, 56,20 R-48 R (15L) Kauffmann, J. M.; Laudet, A.; Vire, J. C.; Patrlarche, G. J.; Christian, G. D. Mlcrochem. J. 1983, 2 8 , 357-62. (16L) Last, T. A. Anal. Chem. 1982, 5 4 , 2327-32. (17L) Li, C.-Y.; Barrett, T. H., Jr.; Lunney. D.; Salt, A. Anal. Chlm. Acta 1982, 134, 167-78. (18L) Lundstrom, K. Anal. Chim. Acta 1983, 746, 97-108, 109-15. (19L) Nagaoka, T.; Okazakl, S. Anal. Chem. 1983, 55, 1836-7. (20L) Oidham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1983. 157, 27-51. (21L) Paul, D. W.; Rldgway. T. H.; Helneman, W. R. Anal. Chlm. Ada 1983, 146, 125-34. (22L) Parus, S. J.; Perone, S. P. Anal. Chem. 1983, 55, 405-7. (23L) Price, J. F.; Cooke, S. L., Jr.; Baldwin, R. P. Anal. Chem. 1982, 54, 1011-15.
(24L) M. E.; Galus, Z.; Adams, R. N. J. Elecfroanal. Chem. 1983, 143, 89-102. (25L) Rusllng, J. F. Anal. Chem. 1983, 55, 1713-18, 1719-23. (26L) Siegerman, H. D. Chem. Blomed. Environ. Instrum. 1983, 12, 373-95. (27L) Tkalcec, M.; Grabaric, B. S.; Fillpovlc, I. Anal. Chlm. Acta 1982. 143, 255-60. (28L) Wang, J. Electrochim. Acfa 1981, 2 6 , 1721-6. (29L) Wang, J.; Freiha, B. A. Anal. Chem. 1982, 5 4 , 334-6. (30L) Wasa, T.; Yamamoto, H. Bunseki Kagaku 1982, 31, T55-T60. (31L) Wasa, T.; Yamamoto, H.; Akimoto, K. Bunsekl Kogaku 1982, 3 1 , T95-T100. (32L) Wasa, T.; Yamamoto, K. Bunsekl Kagaku 1983, 3 2 , T21-T25.
Ion-Selective Electrodes Mark A. Arnold Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
Mark E. Meyerhoff* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
The development and application of ion-selective electrodes (ISEs) continue to be excitin and expanding areas of analytical research. Clearly, the aklity to make direct or indirect measurements in complex samples without concern about sample color or turbidity and the fact that such measurements require relatively inexpensive equipment make ISE based techniques attractive to scientists in many disciplines. This review covers ISE literature published between the Fall of 1981 and the Fall of 1983. We have tried to avoid overlap with the 1982 review wherever possible. However, several articles which appeared in 1981but were inadvertently left out of the 1982 review will be cited here. References to articles published in foreign journals are provided (usually along with the appropriate Chemical Abstracts number) only when such articles appear to offer significantly new information. In an effort to make this review more valuable to the analytical community as a whole, the more important papers concerning new electrode membrane materials, theory of ISEs, new ISE instrumentation, novel measurement techniques, exciting applications of ISEs, etc., will be critically reviewed in the text partion of the article. More routine applications, and new electrode systems which do not appear to offer advantages over existing devices or lack adequate selectivity to be utilized in complex samples, will usually be summarized in appro riate tables. References used within this review were obtaineCY from four main sources: a manual search of major analytical research journals, a manual search of Chemical Abstracts Selects-Electrochemistry, a computer search of Chemical Titles (ACS), and reprints and/or publication lists provided by noted ISE researchers around the world. We apologize, in advance, to those authors whose articles we missed in our literature searches or failed to include due to space limitations and our desire to present the more significant developments in the field.
BOOKS, CONFERENCES, AND REVIEWS There have been relatively few books introduced within the past 2 years that deal exclusively with ISEs. Among those published, a two volume set written by Ma and Hassan (33a, 34a),entitled “Organic Analysis with Ion-Selective Electrodes”, appears to offer the most comprehensive general coverage of the field. Volume 1is divided into three chapters dealing with the theory and practical as ects of ISEs, types and response characteristics of ISEs, a n I a n excellent review of the many measurement techniques used with ISEs (e.g., standard addition, titration, etc.). While volume 1 does not detail the complicated theories of membrane potentials, it does provide 20 R
a good overview of the current state of ISE systems so that scientists who wish to begin using ISEs could easily get started. Indeed, an exhaustive table in Chapter 1 offers selectivity constant data for almost all of the commercially available ISEs. Volume 2 is strictly devoted to ISE applications with, as the title suggests, emphasis on organic determinations. Chapters on ISE measurements of elemental components of organic molecules, organic functional group determinations with ISEs, bioelectrode systems for biochemical analysis, and ISEs in natural product and pharmaceutical assays are presented. A second portion of the volume provides readers with detailed experimental protocols for using various ISEs for specific organic analysis applications. A monograph authored by Cosofret (7a) entitled “Membrane Electrodes in Drug-Substance Analysis” is also primarily concerned with the application of ISEs to organic systems. While a short overview concerning theory and methodology of ISEs is presented, the major portion of the book details techniques for using liquid membrane type ISEs for sensing certain drugs as organic ions, determining inor anic species in pharmaceutical preparations with ISEs, and u t h i n g ISEs for indirect detection of organic drug species which are not ionic. Obviously, the information provided in this book would be most helpful to researchers in the pharmaceutical sciences and to analytical chemists in drug companies who wish to employ simplified electrode techniques for quality control measurements. For those workers just beginning to use ISEs and who have had relatively little background in electrochemistry, Koryta’s new book (28a),“Ions, Molecules, and Membranes”, is must reading. While not exclusively about ISEs, the book offers fine rudimentary coverage of the basic principles involved in a variety of electrochemicaltechniques; from the fundamentals of conduction, ion solvation, and ion motion (e.g., diffusion potentials) to cell organization,double layer the0 ,biological membrane potentials, etc. ISEs are discussed in %tail within the chapter dealing with membranes. The entire book is concisely written and the many illustrations make it easy to follow. The proceedings of several international conferences were also made available in published form. Most notable among these was the symposium held in Matrufud, Hungary, in October of 1980 (45a),published as part of Elsevier’s “Analytical Chemistry Symposium Series”. Plenary lectures by R. G. Bates on the thermodynamics of ISEs, R. P. Buck on the use of impedance methods to study ISE systems, J. Koryta on the analytical capabilities of electrochemistry at
0 1984 American Chemical Society QQQ3-27QQl84IQ356-2QR~Q6.5QlQ
IOKSNCTIVE ELEmooEs
the interface of two immiscible liquids. and W.Simon on the transportof im through neutral csrrier type membranea were the highlights of the meeting. Several keynote lectures and more than 20 eral discussion papers on a wide range ofISE systems are o included. Some of these will be cited later on in this review. of an International Meeting Remarkabl ,the proceed' on Chemical iensors held i n y a u o k a , Japan, in September 1983, have already been published (Ha). Eleven plenary lectures and 121 contributed papers are presented. Many of the papers deal with solid-state semiconductor trpe gas sensing devices which are not ISE-based systems. However, two sections, one on IS& sensors and one on biosensor systems are relevant to this review. Papers regard the lowering of detection limits for preci itate based ISE?Y. Umezawa, E. G. Haranyi, K. Toth, if Fujiwara, and E. Pungor), the shortening of ISE respom times by charge pulsed techniques (K.Kanno, T. Gatayama, and M. Koyama), the use of fluoride lasses as ISE membranes (D. Ravaine, G. Perera, and Z. banne), and the use of a FET type sensor for intravascular H measurements (G. Koning and S. J. Schepel) appear to the most interesting to ISE enthusiasts. Another conference, this one on Physiological Sensors in Medicine, was s ~ by IVAC d Corp. and was held in sunny southern alifornia tn November 1982. The proceedings of the meeting have been published by the company and copies may be obtained by writing directly to them (IVAC Corp.. 10300 Camille Pt. Dr., San Diego, CA 92129). While a wide range of chemical sensing devices were discussed, several key papers on ISE systems were given by prominent ISE researchers, includ R. P. Buck on the history of ISEs as physiological sensors?. A. Rechnitz on bioselective membrane electrodes, J. Janata on the various ISE-based measwing systems (e+, solution/membrane/solution type vs. solution/membrane/conductor type), J. Ladenson on the direct measurement of electrolytes in physiological samples, and W. S. Jordan on the measurement of calcium by ISEs. An additional conference held a t the National Bureau of Standards in May 1983 also had a clinical theme and was titled, Direct Potentiometric Measurements in Blood. The proceedings of tbia meet' have not as yet appeared although publication ia expected.%e majority of the meeting foeused on the measurement of d u m in blood by direct ISE methods and the discrepancies of the values determined by various commercial clinical analyzers. Two key talks concerning
aE".
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general fundamanhb, one by B. E.Conway on the ion specific aspectsof ion hydration and electrolyte behavior and one by R G. Batea regardingscales of ionic activitiesfor standadzing ISEs, were the highlights. In January of 1982,a sym ium on Electrochemical Sensors in Medicine and M e d i & & e a r c hwas held a t Park City, UT. The meeting was sponsored by Critikon Corp. and, while no formal proceedings have been published, abstracts of the speakerstalks were distributed to the attendees. Again, many of the leading ISE researchers made presentations, including R. P. Buck on the biomedical applications of solid-state IS&, D. M. Band on in vivo monitoring with ISEs, R. K. K o h on enzyme based electrochemical sensors, R. A. Durst on reference electrodes and calibranta for ISEs used in biomedical applications, J. Janata on chemically sensitive field effect transistor devices, and W. Simon on the biomedical applications of liquid membrane electrodes. Another, as yet, unpublished conference on was held in Cardiff, England, in April 1983. This meeting, titled International Symposium on Electroanalysis in Biomedical, Environmental and Industrial Sciences, appears to be a biennial affair sponsored by the Electroanalytical Group and Western Region of the Royal Society of Chemistry. Many interesting ISE papers were preaented among the more than 60 total papers. If past bistory holds, the proceedings of this meeting will shortly appear in book form. Numerous review articles about ISEs in general or specitic types and/or applications haveappeared within the past 2 years. Some of these are summanzed in Table 1. Special note should be made to two of the articles cited. Koryta's review (27a) is the fourth in a continuing series appearing in the journal A d y t i c a Chimica Acta and is quite similar in content to this review except for the time period covered. Buck's paper (4a) titled 'Electrochemistry of Ion-SelectiveElectrodes" was published in the inaugural h u e of Sensors and Actuators and presents and exceptionally clear picture of the physical proeesses which lead to the development of membrane potentials, The article is highly recommended to those workers who want to understand how ISEs really work.
GENERAL D I S C U S S I O N S O N ION-SELECTIVE
ELECTRODES In thin section we will review work which in related to po-
tentiometric membrane electrodesof all types Papers dealing with areas of general interest, such as reference electrodes, methods for electrode characterization, standardization materials, and new instrumentation will be presented here. Artides dealing with subjects related to specific electrodetypes will be covered elsewhere under appropriate headings. Impedance measurements have been valuable in understanding the membrane properties responsible for the potentiometric m p o m of IS& and Buck has recently preaented a comprehensive review on impedance measurements asapplied to ISE investigations (96). In this review, Buck dlscws€a the theory, methodology, and interpretation of impedance measurements. Moreover, numerous examples are given for glasa, solid-state, and liquid ion-exchanger-based ISEs as well as for chemically sensitive semiconductor devices. The impedance behavior of poly(viny1 chloride) (PVCbbased IS& has also been studied and multa indicate that membrane hulk and Warburg diffusionprocmes contribute greatly to the ISE membrane behavior ( l b ) . Interestingly, results are presented which suggest that the membrane response mechanism changes with membrane age. In related work, the theory and development of asymmetry membrane potentials have been discussed (286)along with experimental methods for altering such potentials. Novel electrode designs and arrangements have been deseribed which can be employed for ISEs of all typea. Perhaps one of the most promising new electrode arrangements is that proposed by Stepak et al. (41b. 54b) in which IS& are connected in series. Such an electrochemical cell results in enhanced sensitivity with potentiometric slopes n times the Nernstian value, where n is the number of cella in series. An example of this technique was given for a chloride titration with eight silver/silver chloride indicator electrodes in series with eight double junction reference electrodes (54b). Potential changes a t the equivalence point were approximately eight times those of a typical single cell arrangement. Moreover, a second study showed that four cupric ISEs in ANALYTICAL CKMISTRY. VOL. 56. NO. 5. APRIL 1984
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ION-SELECTIVE ELECTRODES
Table I. Additional ISE Review Articles (General and Specialized) no
I
1 2 3
4 5 6 7
8 9
10 11 12
13 14 15 16
17 18 19 20 21
22 23
subject general ISEs physical electrochemistry of IS& ISE bibliographies and recent titles bioelectrodes (enzyme, bacterial, immuno-, etc.) limitations of IS& ISEs in clinical analysis, medicine, and biology neutral carrier-based ISEs ISEs in nonaqueous solvents ion binding studies with ISEs reference electrodes and liquid junction effects in ISE measurements calibration and standards for ISEs micro-ISEs for intracellular measurements dynamic time response of IS& I$Es in automated systems ISEs as detectors in HPLC precision in ISE measurements quqternary ammonium salts for ISE titrations of various species ISEs in the control of fermentation processes methods of thiocyanate determ with ISEs nitrate selective ISEs fluoride electrodes and applications use of ISEs to monitor biological membrane potentials, e.g., bacteria, mitochondria, etc. ISEs based on carbon substrates; coated carbon, impregnated carbon, etc.
series with four reference electrodes resulted in electrode slopes equal to four times the single cell Nernstian value (41b). Also, the relative standard deviation for the method was shown to improve with the square root of the number of cells. Although the system greatly enhances electrode slopes, an increase in the number of cells corresponds to a more complex system which is inherently more prone to operator errors. Future work should concentrate on simplifying the cell design while maintainihg the sensitivity enhancement. Other improvements in ISE technology were proposed. For example, electronic circuits were incorporated into the body of ISEs to convert the high impedance output to a low impedahce si nal in order to improve the signal-to-noise ratio (30b). Mocfified outputs of this type can be transmitted either in an analog form by conventional electrical methods or in digital form by optical fiber methods; the latter being more suitable for long distance applications. Experiments with a fluoride ISE indicate that such modifications are possible and attractive. As will be discussed below, Fjeldly and Nagy (20b) report a solid-state fluoride ISE with integral amplifier electronics. Similarly, Mellor et al. have presented integral electronic-based ISEs for copper, cadmium, and lead using a Teflon-graphite electrode design (38b). Others have also reported on the construction of printed circuit-board ISEs for solid-state ISEs (31b). These electrodes are easy to prepare and are relatively inexpensive. A comparison of this new electrode design to conventional electrodes was presented. In regard to another interesting approach to ISE measurements, Powle and Nieman presented a three part series of papers which &scribe the development and operation of bipolar pulse conductance ISEs (44b-46b). In this method bipolar pulse conductance techniques are used to measure the conductance across the ion-selective membrane as opposed to the comnionly measured membrane potential. The use of calcium and fluoride electrodes are detailed both in static and flowing systems. The electrode syst8ms were shown to respond as a function of ion concentrations as opposed to activities with considerably faster dynamic response then classical potentiometric measurements. Several papers described novel components of ISEs, such as electrode designs, which allow for easy removal and replacement of polymer-based ion-selective membranes (23b, 55b). Moreover, the use of propylene glycol based internal electrolvtes for elevated temr>erature ISE measurements has been proposed (56b). Much attention has focused on cornouter interfacing of ISEs for rapid and simple data acquisition b d manipulati&. Wall et al. described a system for various ISEs in combination with 22R *
ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
ref 9a, 13a, 14a, 23a, 27a, 29a, 35a, 58a 4a 39a, 40a, 41a l a , 3a, 15a, 16a, 17a, 18a, 19a, 36a, 43a, 49a, 50a 5a, 46a loa, 32a, 44a, 52a, 55a, 56a, 57a 2a, 54a 24a 12a 8a 38a, 42a 1l a 30a 47a 2 2a 2 Oa 5 3a 6a 48a 26a, 31a 21a 2 5a 37a an Apple I1 microcomputer (61b). Another system was designed not only to obtain, store, and manipulate ISE output data but also to control other laboratory instruments, such as a motor-driven buret (4b). In a similar fashion microprocessor-controlled pH adjusters have been detailed (25b, 32b, 37b). Of particular interest is the pH-stat design by Lemke and Hieftje (32b) which allows pH control to within 0.0008 pH units. Finally, Efstathiou (17b) proposed an automated method for detection limit and selectivity coefficient determinations for ISEs, using a microcomputer-controlled potentiometric system. Several new pH/mV meters have been reported for use with ISEs. Warner et al. (62b) detail the construction of an inexpensive digital pH meter which can be used in either research or teachin laboratories. In addition, Orion (33b) and Radiometer (IO!) Corporations have both discussed their recent pH meters. Possibly the most important component of an ion-selective electrochemical cell, aside from the ion-selective membrane, is the reference electrode. Unfortunately, the reference component is frequently overlooked as a source of error in ISE systems. We include reference electrodes and liquid junction potentials in this section because of their extreme importance to ISE methodologies. Indeed, Brezinski has published a comprehensive study concerning the response characteristics of commercial reference electrodes (6b). After subjecting these electrodes to large changes in ionic strength and transference, poor transient and steady-state performance was observed, such as slow, inaccurate, and stir rate dependent potential changes. For the most part, ionic interactions within and at the surface of the physical junction can account for these poor response characteristics. Obviously, improved reference junction designs are needed for more accurate reference elements. To this end, microporous Vycer glass and Nafion perfluorosulfonic acid membranes have been proposed and tested as novel junction materials for silver/silver chloride reference electrodes (7b). These materials were shown to eliminate junction clogging from precipitated silver chloride which results in reference electrodes with stable potentials at elevated temperatures and no evidence of thermal hysteresis. Sekerka and Lechner (51b) report a similar reference electrode design using Nafion membranes as an internal junction which yielded satisfactory results when used with a chloride selective ISE. The development and characterization of reference electrodes for use with ISEs in nonaqueous solutions received considerable attention. Coetzee and Gardner (13b) have described an innovative Teflon-based double-junction refer-
ION-SELECTIVE ELECTRODES
ence electrode for use in organic solvents. The novel component of this electrode system is a Teflon plug junction which results in excellent flow rates, moderate impedances, and suitable inertness and durability. Furthermore, a comparison between triiodide iodide and silver/silver(I) ion couples as reference electro e components in organic solvents was reported by Coetzee and Gardner (14b) where their Teflon-plug junction was employed. Their results indicate that the triiodide/iodide couple on platinum possesses excellent exchange current densities and should include the same solvent as that in the analyte solution. Also, a report comparing the analytical usefulness of silver/silver chloride and silver/silver nitrate reference electrodes in ethanolic solutions has appeared (63b). Results suggest that the silver/silver nitrate electrode is suitable for ethanolic solutions; whereas, the silver/silver chloride electrode is not useful owing to hysteresis problems and sluggish response. Finally, an indepth study was presented where the standard potentials of silver/silver chloride electrodes in water/2-ethoxyethanol mixtures were reported (19b). Reliable methods of calculating liquid junction potentials in reference electrodes are needed to account for all components of an electrochemical potential change. Hefter has recently questioned the use of the Henderson equation for calculating li uid junction potentials ( a b ) . He has determined that only unjer certain well-defined conditions, e.g., constant ionic strength, is the Henderson equation adequate for estimating liquid junction potentials. Moreover, Izutsu et al. (29b) present results which suggest that the major component of the liquid junction potential between electrolyte solutions of different solvents is the solvent itself. In their work, measured liquid junction potentials were almost independent of the type and concentration of electrolyte studied for a wide variety of solutes and solvents. Finally, Brezinski (5b) has studied the influence of colloidal charge on the response of pH and reference electrodes (known as the “suspension effect”). He su gests that most pH differences observed between colloidal sefiments and supernatants are real and not junction artifacts. These results are in conflict with the Donnan-like potential explanation and suggest that further studies concerning this phenomenon are needed. Perhaps the most important response characteristic of an ISE is selectivity. Indeed, a nonselective ISE stands little chance of being employed for real sample analyses. Therefore, it is important for the analytical chemist to realize the selectivity limitations of an ISE system of interest. Hulanicki appropriately points out the precautions one must take before applying ISEs to certain analytical problems based on these selectivity limitations (27b). New methods for quantifyin ISE selectivity properties continue to be developed and tested! Three new experimental methods for selectivity coefficient determinations have been proposed by Okunev et al. (39b). These novel procedures involve graphical determination of selectivity coefficients under various experimental conditions. The authors suggest that these methods can accurately assess electrode selectivity irrespective of solution ion concentrations and of ionic charges. These methods compared favorably with the IUPAC recommended separate solution method for a bismuth phosphate based phosphate ISE. However, more experimental results, particular1 with other electrode types, are needed before these method can be fully evaluated. In another study, an extension of the Nikolsky equation was used to examine the selectivity behavior of a chloride ISE in the presence of iodide and bromide (42b). In related selectivity work, a selectivity coefficient determination method has been reported which allows for the calculation of the coefficients for charge carrier-based liquid membrane electrodes irregardless of measured and interfering ion concentrations (53b). Also, the method of linearized multiple standard addition has been applied to selectivity coefficient determinations (35b) where six possible methods are derived. Finally, a general procedure for determining the effect of interfering ions on the performance of liquid-membrane electrodes was described (22b). In response to the call for more selectivity coefficient data by Buck in the 1978 ISE fundamental review (8b),Tuhtar presents a comprehensive study of selectivity coefficient data for the solid-state cyanide ISE (58b). In this paper, selectivity coefficients are given for 29 anions by use of the mixed solution method. Values are compared to those reported earlier for
d
the cyanide electrode and are found to vary considerably. A variety of experimental conditions and electrode membrane compositions are given as possible explanations for the discrepancies. Finally, Tuhtar points out that more uniform work in determining and calculating selectivity coefficient data is urgently needed for many electrode systems. Since the k t review many researchers have reported studies which concern the use of standard addition techniques for ISE analyses. Considerable attention has been devoted to determining the accuracy and utility of the double known addition method (DKAM) in potentiometry. Longhi and coworkers (34b) have reported a theoretical study in which various operational parameters such as standard concentrations and addition volume ratio are considered with respect to method accuracy. These researchers conclude that the DKAM requires a critical inspection of the operational parameters over the expected determinand concentration range in order to minimize errors and that a first to second addition vdume ratio of one is not the optimum for most situations. In addition, Efstathiou and Hadjiioannou (18b) have applied a Monte Carlo simulation to study the error propagation for the DKAM. These latter researchers conclude that by using microprocessors to obtain more accurate potential values and to handle the tedious calculations, improved accuracy of the DKAM should be realized which will make the method more practical. Horvai and Pungor (26b),on the other hand, point out the inherent imprecision of this method which limits its application to relatively unique situations. Furthermore, studies detailing the use of gravimetric standard addition (50b) and dry sample addition ( I l b ) techniques for ISE measurements have been presented. Similarly, a graphical standard addition method was proposed in which a priori knowledge of electrode slope or standard potential is not required (36b). Such a method would be desirable when ISEs are to be used for quick determinations, perhaps using disposable electrodes. Quantification using ISEs in the non-Nernstian response region is an area which requires and deserves considerable attention. Efforts by Frazer and co-workers (21b) show that ISE detection limits can be accurately extended below the IUPAC recommended concentration levels. These researchers provide experimental and calculational techniques for the use of ISEs at low concentration levels. Their study shows that analytical results can be obtained from this region of an ISE response curve down to the potential noise level. At this point it seems appropriate to discuss papers which deal with ion activity coefficients and activity standards because ion activities must be considered during calibration of ISEs. Professor Bates at the University of Florida has recently reviewed the general concept of ion activity in analytical chemistr (2b). Moreover, Bates et al. (3b) have presented a detailedrstudy concerning the determination of mean activity coefficients with ISEs. In this work, sodium nitrate, potassium nitrate, and calcium nitrate activity coefficients were measured in aqueous solutions by usin sodium, potassium, calcium, and nitrate ISEs. An extendej Debye-Huckel equation which includes linear and quadratic terms was employed. Results indicate that accurate data can only be attained when drifts in the standard potential are accounted for experimentally and when electrode calibrations are extended to cover the entire concentration range of interest. Another approach for determining activity coefficients was reported (60b)in which a nonlinear least-squares technique is used to fit data to an extended Debye/Huckel equation. Liquid junction potential changes are taken into account using the Henderson equation and EMF data from cupric and calcium ISEs in cupric and calcium standard solutions, respectively, are used to evaluate the proposed method. A study which attempts to extend the ion-activity scale for sodium chloride and potassium chloride solutions to elevated temperatures has also been published (15b) where the temperature range from 10 to 60 OC was studied and ion activities are calculated by use of the hydration theory. Finally, activity standards for sodium, potassium, and chloride ions were presented (16b) and mean activity coefficients for these ions were measured with eithei. a sodium or potassium glass electrode in conjunction with a silver/silver chloride reference electrode. ISE dynamic and transient behaviors are not well understood at this time and studies have been undertaken to help distinguish what factors influence these electrode response characterisitics. Pungor and Umezawa (48b) remind us that ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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ION-SELECTIVE ELECTRODES
commonly used terminology such as “the electrode response time” is often inaccurate and what is meant in most cases is “the electrochemical cell response time”. The separation of ISE dynamic behavior from the behavior of other cell components is often difficult to accomplish and rarely realized. Hence, most reports on electrode dynamic behavior actually are reporting the entire electrochemical cell dynamic response and results need to be interpreted as such. Toth et al. (57b) have suggested a model to ex lain the components of the transient behavior for the iodiie solid-state electrode in the presence of interfering concentrations of bromide. The initial, rapid “overshoot” response is attributed to the diffusion of the interfering ion through the stagnant solution layer to the electrode surface and to fast interfacial processes such as bromide ion adsorption, iodide desorption, and ion-exchange processes. The relaxation toward an eventual steady-state value was attributed to iodide ion diffusion from or toward the electrode membrane. A f i dynamic response component which is observed as a drift-like potential change is related to slow surface processes which result in changing surface composition and morphology. These authors suggest that this model can be extended to other ISE types; however, experimental data and detailed studies have not yet been reported. Finally, the problem of hysteresis for various ISE types have been measured and large memory effects have been observed with respect to electrode dynamic behavior (59b). This memory effect was shown to be significant for a variety of commonly used ISEs including the fluoride and ammonia electrodes. A number of new ap lications for ISEs have been proposed. Tanaka and Bates (478report the ap lication of various glass and solid-state electrodes for the &termination of acidic dissociation constants of weak bases in deuterium oxide. Also, a novel method for determining macromolecule specific volumes using ISEs was proposed (40b)and was demonstrated for hemoglobin. Moreover, a formula for the equivalence volume in a potentiometric titration has been introduced and illustrated via the microdetermination of various organohalogens (12b). In addition, a novel method of evaluating equilibrium constants was demonstrated (43b)as well as a potentiometric titration method for the determination of nitro heterocycles (52b). Professor Rechnitz at the University of Delaware has proposed an outline for teaching the principles of ISEs and other potentiometric membrane electrodes at the undergraduate level (49b). He points out that most textbooks are considerably outdated in this area and revisions are sorely needed. A teaching approach based on the concept of electrode selectivity is presented in which the various membrane materials are described in terms of bulk and interfacial membrane processes which govern electrode selectivity.
GLASS AND SOLID-STATE MEMBRANE ELECTRODES In this section we review work done with glass membrane electrodes, conventional solid-state ISEs such as LaF3-fluoride electrodes and Ag2S/AgX or Ag2SlYS type pressed pellet electrodes and “nonconventional solid-state membrane electrodes. This latter catagory includes electrodes based on heterogeneous membranes involving cr stalline materials suspended in polymer matrices (so-calle Pungor type electrodes). Solid-state conductors (e.g., glass, carbon, silver, etc.) which have been chemically modified with crystalline-based membranes or coatings will be included. This latter group falls into the broad catagory of chemically sensitive semiconductor devices (CSSDs). Also several papers related to the development of solid-state pH-sensitive electrodes are included. While in the strictest sense, such pH electrodes are not ISEs, their incorporation in this review seems justified. Glass Electrodes. Much of the work involving glass electrodes has been concerned with the continual development and characterization of glass pH electrodes. Although a comprehensive review of work involving the application of glass H electrodes is beyond the scope of this review, we inclu& here a few key papers dealing with fundamental studies concerning pH electrodes. For example, the development of blood capillary glass electrodes for use in medical electronics has been reported by Halder and Guha (59~). Various glass compositions were compared with respect to pH responsiveness, impedance, and mechanical strength for the
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
construction of capillary pH electrodes. Nernstian responses were found with low impedance and small sodium error up to pH 12.0 for certain novel membrane com ositions. Also of importance to those using electrodes in me8cal and clinical applications, Covington and co-workers ( 1 5 1 ~have ) reported a method for testing glass pH electrodes in the physiological pH range and at the physiological sodium ion concentration based on indirect comparison of potentials to the hydrogen gas electrode. The method was shown to be suitable for determining the hydrogen ion sensitivity and the sodium error of glass electrodes with an accuracy of f0.2mV or 0.003 pH units at 37 OC. All commercial glass electrodes tested showed theoretical H response under physiological conditions. In another s t u g , the effect of divalent cations on the steady-state potential of glass pH electrodes was studied and transient potential shifts have been observed (86c). Such transient potential shifts are more pronounced for flow-through electrode arrangements and might be caused by an adsorption of these cations onto the glass membrane. The observed quasi-pH shifts can easily be taken as an actual pH change. Thus, interpretation of pH vs. time data must be done with care when divalent cation concentration changes are possible. Midgley and Torrance (103~) have developed an electrode system which self-compensatesfor temperature changes during pH measurements in boiler feedwater. Their electrode system has an overall cell temperature coefficient between 0.1 and 0.15 mV/OC which corresponds to a pH change of only 0.05 pH units over a temperature range from 15 to 35 OC. Essentially, the temperature coefficients of various cell components are matched as closely as possible by adjusting solution compositions in such a way that the overall cell temperature coefficient is minimized. These pH cells can help establish if a given pH change is caused by a variation in alkalinity or a change in sample temperature. The development of several valuable industrial applications of this approach is likely. With regards to new glass membrane formulations for ISEs, a host of reports have appeared. For example, the molecular composition and structure of iron-doped chalcogenide glass membranes have been extensively studied by using a wide variety of techniques (168~).Electrodes based on Fe2(Gez8Sb were shown to be suitable for determining ferric an$ cupric ion activities in chloride- or nitrate-containing media. Novel glass electrode designs have also been reported where (i) solid-state glass electrodes are constructed with a AgCl-M,Cl eutectic mixture to join the glass membrane to a lead wire (~OC), (ii) the internal reference solution is replaced by a layer of lithium metal deposited on the internal membrane surface for determining pH at high pressures (91c), and (iii) sensitive glass membranes are obtained by sputtering a particular glass film on a porous electroconductive substrate (101~).In further work, the properties of glass membrane electrodes composed of &&-free magnesium phosphate glass containing silver oxide have been reported (113~).These membrane electrodes showed considerable response to various anionic species and to ammonia. Results indicate that a glass membrane composed of 5Ag,0-50P205-40Mg0-5A1~03 responds preferentially to ammonia over common anions. Although it has yet to be determined with certainty, the response mechanism of such an ISE most probably includes silver complexation by the anions or ammonia which establishes a modified membrane potential owing to a decrease in silver ion activity at the glass membrane surface. Fundamental studies to establish the response mechanism are needed. Some selected applications of sodium glass membrane ISEs are summarized in Table 11. Conventional Solid-state ISEs. A number of new solid-state membrane compositions have been tested as suitable membranes for the construction of ISEs for a wide variety of ionic species, Perhaps membrane compositions receiving the most attention are those which involve metal chalcogenides that are mixed and pressed into ion-selective membranes. East and DaSilva (44c,45c)have reported detailed studies characterizing various mercury(I1) responsive solid-state ISEs based on numerous mercury(I1) chalcogenides and their admixtures with silver(1) chalcogenides or elemental mercury. Optimal response characteristics have been found for mercury(I1) ISEs containing HgS (black)/Hg(O)as the membrane composition (4%). Electrodes, which are prepared by activating hydrophobized spectrographic graphite rods with this
ION-SELECTIVE ELECTRODES
membrane material, displayed Nernstian response in the lo-' to IO-' mol L concentration range. Evidence was given which suggests t a t elementry mercury is the active membrane com onent while the HgS (black) acts as an inert matrix. AnJflical applications of these sensors for complexometric titrations have been described (44c). In addition, novel solid-state membrane compositions containing various chalcogenide mixtures have been re orted for cupric (116c, 1 4 8 ~ ) and cadmium (21c, 108c) 158s. Extensive studies on the cadmium ISE revealed considerable enhancement in long-term reproducibility with the new membranes over conventional &,S/CdS membranes (21c). Finally, a series of chalcogenide solid-state membrane ISEs have been reported based on PbSe and PbTe crystals with selective response to silver(I), copper(II), lead, mercury(II), and sulfide ions (117~). Novel solid-state membrane electrodes have been reported for other ions as well, such as the stannous sulfide based sultide responsive ISE (17~).This electrode was prepared by electrolyzing tin onto a platinum wire followed by soaking the treated wire in a saturated hydrogen sulfide solution. Response is linear for sulfide in both a ueous and aqueous-alcoholic media. Sulfide ISEs compose! of Sb-Sb2Sz, k-kzS3, and Mo-MoSz layers on platinum wires have also been studied (146~).In each case, electrode response time is ra id and Nernstian response from to mol L sulfig is observed. Nitrate, thimulfate, and nitrite were s own to interfere when present in the lo-' to mol L concentration range; however, fluoride, chloride, bromi e, iodide, bicarbonate, carbonate, sulfite, and sulfate do not interfere. Numerous solid-state ISEs for halides have been suggested. For example, two reports of calomel-based chloride-selective electrodes have appeared (133c, 144~).Here, pressed pellets of HgS-H &1, were employed. Chloride was successfully determinefin Mg-Zr alloys (144c), boiler discharge water, and triethylenetetramine-epichlorohydrin condensation products (133~) using this electrode. An iodide-sensitive ISE has been prepared by mixing AgI with ferrocene (31c). In addition, various combinations of bismuth, lead, silver, gold, and aluminum with fluoride have been deposited on LaF, crystal membrane by vacuum eva oration and the resulting crystals have been studied as fluorile selective membranes (179~).The BiF3-bismuth41ver system is well suited for use in solid-state fluoride electrodes;however the advantages of these membrane materials over simple LaF3 crystals is unclear. New solid-state ISEs for ammonium (169~)and lithium ( 1 0 0 ~ions ) have been developed. The ammonium ion electrode utilizes a membrane composed of (NH,) PMol 040nHzO and can be used over a wide pH range. $he litkium electrode membrane was prepared by fusing Li2C03-V206(1:6) and crystallizingthe melt in vacuo. A Nernstian response from lo-, to 1 mol/L Li+ was observed over the 5-11 pH ran e. Possible interferents such as Na+, K+, Rb+, Cs+, Mg2+,Sr +, Ba2+,and A13+ elicit only a slight response; whereas, NH4+and Ca2+interfere more seriously. In regard to work dealing with the lanthanium fluoride crystal fluoride electrode, Nagy and co-workers (51c, 109c) have shown that their combination fluoride electrode with reversible solid-state contacts is well suited for fluoride determinations in serum and spinal fluid samples. This electrode design uses a Ag S/AgI pressed pellet membrane electrode as the reference eiement with a constant iodide activity being maintained by the buffer system. Hence, no liquid junction potential is present which results in faster dynamic res onse and greater reproducibility over conventional electroze arrangements. The resulting high impedance is reduced by using a single chip operational amplifier within the electrode body which provides an impedance transformation. Faster analysis times over conventional combination fluoride electrodes were observed with enhanced accuracy. In addition, the dynamic res onse of fluoride ISEs with LaF3 crystalline membranes a n 8 reversible solid-state inner contact have been studied (110~).Results indicate that the new inner connection has no adverse effect on electrode response times. The problem of fluoride ISE stability and drift at low fluoride concentrations has also been addressed by Kissa (84c) and the lower ran e of linearity for fluoride calibration curves has been stutied with various experimental parameters influencing this value (149~). Final1 , a modified total ionic strength adjustment buffer (TIS&) for fluoride measurements has been proposed (137~).
h
i
k
Q
Characterization of the membrane surface for various solid-state ISEs has been the focus of several investigations. Surface analysis techniques which have been employed for the characterization of cupric ISEs include reflected light microscopy, scanning electron microscopy (SEM), and X-ray diffraction (147c). Data indicate that besides excess silver sulfide, only ternary phases of copper are present. ESCA (electron s ectroscopy for chemical analysis) investigations of cupric I ~ E indicate s that small amounts of cupric sulfate are sufficient to deteriorate the electrode performance to unusable levels (46~).Treatment of sulfate contaminated membranes with ascorbic acid is shown to restore electrode response characteristics while regenerating cupric sulfide a t the membrane surface. Similar results are presented for lead ISEs where the A ,S/PbS membrane is exposed to hydrogen peroxide which ctemically oxidizes the membrane surface (131~).Scanning electron microscopy indicates that lead sulfate is formed which suppresses the electrode response. On the other hand, electrochemical (anodic) oxidation of these membranes results in lead and silver oxide formations with a similar decrease in electrode performance. These latter studies show that the entire membrane surface need not be covered to adversely affect electrode performance. Cheng and co-workers (32c) have applied ESCA techniques to 1:l A S PbS membrane ellets and have concluded that PbS, P!bd4, PbO, and Ag28makeup untreated membranes. Exposure to aqueous solutions results in the presense of bound water and adsorbed COz and Pb(CO& on the membrane surface. Membranes treated with HC104appear to be cleansed of lead sulfate and oxide contaminations while treatment with basic EDTA appears to remove lead sulfate and enhance lead oxide levels. Finally, Pungor and co-workers have studied the adsorption of copper ions on the surface of Ag2S/CuS membranes (62c) and the adsorption-desorptionof silver and iodide ions on the surface of Ag,S/AgI membranes ( 6 1 ~ ) Results . from these studies indicate that super- and sub-Nernstian responses at low analyte activities can be explained by ion adsorption-desorption desorption processes. Response time is one of the most important characteristics of an ISE especially when establishing the practicality of a particular system. Unfortunately, considerable confusion with respect to a universal response time definition has made comparison of experimental data difficult. Uemasu and Umezawa (161c) have suggested that ISE response times be defined as the time necessary to achieve a specified rate of potential change. These researchers correctly point out that conventional response time definitions necessitate one to attain a final equilibrium potential so that the response time can be calculated either from a specified fraction of the overall steady-state response or from 1mV before the final equilibrium potential. Equilibrium potentials are not easily obtainable in the strictest sense which makes these response time defiiitions impractical in many circumstances. If the response time is defined as the time necessary to achieve a specified potential drift rate, this problem can be avoided and response time data consistent from laboratory to laboratory could be achieved. Although Uemasu and Umezawa demonstrate their response time definition with a cupric ISE (161c), this definition should be appropriate for ISEs of all types. It is interesting to note that the new Beckman pH/mV meters include an "autoread" function which displays the potential when a specified potential drift rate is obtained. Unfortunately, this drift rate is not adjustable and users are bound by the manufacturers drift rate specification of 0.04 mV/s for the 3 71 pH/mV meter. This drift value corresponds to a potential rate change of 2.4 mV/min which is too large for most ISE work. In other reports concerning response times for solid-state electrodes, Lindner et al. have recently reported detailed studies concerning the dynamic response of solid-state iodide ISEs (93c-95c). The dynamic response in the detection limit concentration range was determined to be controlled primarily by electrode factors other than simple diffusion of the analyte to the electrode surface (93~).In a second study, these researchers have found that a f i i diffusion model is appropriate to explain the effect of various experimental parameters on the transient signals observed in the presence of interfering ions (94~).According to this model, surface exchange reactions are fast in comparison to species diffusion through a stagnent solution layer at the membrane surface and the existence of ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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ION-SELECTIVE ELECTRODES
Table 11. Analytical Applications of Glass and Solid-state ISEs
electrode sodium (glass)
fluoride (LaF, crystals)
silver or sulfide (Ag,S)
application determ of sodium in food stuffs cheese carnallite and dehydration products seawater the Bayer process cerebrospinal fluid of conscious rabbits microdeterm of cyanide estimation of free and combined soda determ of fluoride in foods bovine milk breast milk toothpaste soils aluminum production environmental samples geothermal-geopressured brines ore leach solutions uranium compounds low-melting fused salt systems precipitation samples industrial waste waters water determ of fluorine in concentrated sulfuric acid concentrated phosphoric acid glass ceramics silicate materials environmental standard reference materials organic compounds detection of fluorinated compounds in air determ of sulfate fluoride stability constants with alkaline-earth metals potentiometric titration of oxalate tellurium determ of hydrogen sulfide in effluents sulfide after cephalosporins degradation sulfide by sulfate-reducing bacteria total sulfide in the 7.5-11.5 pH range ovalbumin cyanide in blood sulfonamides sulfur in raw materials sulfur forms in sulfur-alkali solutions thiols, cationic surfactants, and halides field electrode method for sulfide determinations measurement of photosynthetic sulfide oxidation by chloro bium
halide (Ag,S/AgX)
iodide (Ag,S/AgI)
chloride (Ag,S/AgCl)
bromide (Ag,S/AgBr) copper (Ag,S/CuS) 28R
monitoring halides in electroplating baths monitoring oscillating reactions formation of zinc(11) halide complex in methanol determ of cationic surfactants determ of tungsten(V1) mercury in the presence of iron(II1) micro amounts of arsenic(II1) micro amounts of mercury iodide-131 total residual chlorine in drinking water acetylenic hypnotics evaluation of reaction kinetics determ of chloride in cheese milk plant tissue concentrated sulfuric acid determ of chlorine in geological materials as detector for gas chromatography study of metal(I1) chloride formation constants determ of bromine in biochemical preparations halothane determ of copper in copper plating solutions
ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
ref 8c 87c 145c 176c 14c 80c 92c 19c 27c llc 48c 3 6c 49c 120c 173c 119c 128c 69c 9c 167c 15c 105c 75c S8C 50c 16c 27c, 166c, 174c 42c 20c 96c 123c 124c 57c 52c 3c, 4c, 5c 58c 83c 157c 97 c 1oc 55c 126c 175c 23c 155c 114c, 115c 41c 53c 81c 150c 34c 28c, 138c 60c 139c 136c 67c 127c 13c 7 4c 22c 6c 130c 40c 39c 129c 170c
ION-SELECTIVE ELECTRODES
Table I1 (Continued) electrode
cadmium (Ag,S/CdS)
lead (Ag,S/PbS)
cyanide (Ag,S/AgCN) misc
application
ref
archaeological and corrosion samples organic compounds determ of zinc in fertilizers metallochromic dyes citrate by titration L-alanine, L-serine, tyrosine and histidine study of copper binding to serum albumin copper iminodiacetate cornplexes titration of soil-derived fulvic acid sequential multielement analysis microdeterm of aliphatic and polyamines study of cadmium iniinodiacetate coniplexes chelatometric titration of nickel(I1) indirect determ of sulfide microdeterm of nitro and nitroso groups in organic compounds microdeterm of catechols determ of phosphates in water sulfate sulfite determ of total cyanide in thiocyanate-containing wastewaters semiautomated determ of electrolysis in electroplating baths
an outer membrane layer having a modified selectivity pattern is not necessary. In another paper, Morf offers a more mathematical explanation for the transient responses of silver halide membrane electrodes to interfering ions ( 1 0 7 ~ ) . The selectivity of various solid-state ISEs has also received considerable attention. An equation has been developed which describes the selectivity characteristics of the nonstoichiometric copper sulfide electrode immersed in Cu(1) and Cu(I1) solutions containing interfering chloride ions (68c). The derived equation is more accurate than the commonly used Nicohky equation under the conditions employed. Moreover, studies have been reported which test the variability of solid-state ISE selectivity coefficients with respect to numerous experimental parameters such as temperature and stir rate ( 7 1 ~ ) Selectivity . coefficient variability on these parameters is suspected to be partially responsible for the wide variations in literature values. Solid-state halide selective electrodes have also been studied in detail with respect to selectivity considerations. The interference of bromide, iodide, sulfide, and nitrogen-base pH adjustors on chloride determinations in feedwater with an ISE measurement has been studied in detail (111~).Results indicate that, as others have noted before, interferences are not always in accord with solubility product considerations alone and free amines affect the stability of the observed potentials. Also, the res onse of halide sensing ISEs based on Ag/AgX and Ag/Ag28/AgX membranes have been studied in the presence of numerous redox systems (63c). Results show that 2S-basedelectrodes respond significantly to strong oxidants, w ? ereas l AgCl and AgBr electrodes are resistant to redox interferents provided that the halide film is nonporous. Sample pretreatment can often eliminate interferences with the halide electrodes. Indeed, a method for the selective microdeterminationof certain halides has been reported where interfering halides are selectively oxidized to the corresponding halogen prior to the analysis (29c). Procedures for the determination of chloride and bromide are described. The importance of metal ion buffers for constructing accurate calibration curves for ISEs is stressed by Avdeef et al. (7c). In their study, a cu ric calibration c w e with a Nernstian response down to 10-1g,mol/L cupric ion was obtained by employing cupric ion buffers. The main problem is the lack of accurate data concerning stability constants for various metal ion-ligand complexes which can be used as buffer components. These researchers have used their low detection limits to study copper binding to biomolecules. The usefulness of metal ion buffers for cadmium ISE response curves has also been reported (180~).In this latter study, a variety of possible ligands are tested as buffer systems and cadmium-ligand stability constants are given for many ligands. Two other repo& have appeared which detail the effect of complexing
165c 30c 72c 9oc 118c 104c 106c 164c 18c 64c 66c, 141c, 142c 164c 160c 25c 65c 140c, 143c 26c 89c, 163c 56c 37c 88c
agents on the remonse of cadmium ( 1 3 2 ~and ) lead ( 4 7 ~ ) did-state ISEs. The response of cupric ISEs based on silver sulfide membranes has been evaluated with respect to cuprous ( 1 5 4 ~and ) mercuric ( 1 3 4 ~ions. ) In the former studv, the solid-state electrode is shown to respond in a Nernstian manber to cuprous ions in the presence of acetonitrile, while in the latter paper, the application of the cupric electrode system for the measurement of mercuric activities is documented. In addition, the cupric ISE has been found to be an excellent tool for monitoring amine contaminations in nonaqueous solvents ( 3 5 ~ )Coetzee . and Deshmukh have found that impurities as low as mol L can be detected with the cubric electrode. Fluoride and p electrodes are also shown to be useful in this regard. Evidence is presented which indicates that an ion pair formation is responsible for the cupric electrode response. The performance of the Orion 95-70 total residual chloride electrode system (43c) and of a bromide ISE-redox electrode cell for bromine and free residual chlorine ( 1 0 2 ~have ) been evaluated. The Orion electrode system has been found to be suitable for total residual chlorine measurements down to levels of 1-5 pg/L when modifications in the Orion procedure are employed. The bromide ISE system can be used for free residual chlorine determinations down to 50 pg/L. Finally, the effect of temperature on the measurement of sweat chloride concentrations using ISEs has been studied ( 1 2 ~ ) Results . indicate that electrode calibration should be done at the same temperature as the measurement for optimal accuracy. Hence, calibrations at individual skin temperatures are recommended. Solid-state ISEs continue to be used in a wide array of applications and some of these are listed in Table 11. Nonconventional Solid-state ISEs. A number of novel ISEs have been reported in which ion exchange materials are impregnated into polymeric inert matrices which then serve as the ion-selective membrane. A variety of weak cation exchangers have been studied as membrane components for monovalent ISEs (122~).The selectivities of these ISEs were interpreted on the basis of thermodynamic effective charge densities of the membrane. Epoxy resin impregnated electrodes for thallium(1) have been reported where a mixture of araldite and zinc molybdophosphate was employed in one case (98c) and nitropyridine tungstoarsenate was used in the other (2c). Similarly, a copper hexacyanoferrate(II1) based thallium(1)-sensitive electrode was suggested (78c). Molybdate ISEs have been proposed by several workers (99c, 152c, 162c) and two reports of rare-earth ISEs have appeared (177c, 1 5 6 ~ ) . The incorporation of zirconium tungstoarsenate into a polystyrene matrix has been used for zirconyl ISEs (77c, 1 5 3 ~ ) . Oxide vanadium-bronze has been examined as an exchange material for a copper(I1) ISE with a linear calibration curve
L
ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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ION-SELECTIVE ELECTRODES
from to lo-‘ M (172~).A copper(I1) ISE has also been studied where a CuS/Ag2S mixture was suspended in a poly(viny1 chloride) support matrix (54c). Lead ISEs have been developed by using lead(I1) antimonate in an araldite matrix (158~).In addition, the construction and characterization of sulfate (73c) and nitrate (IC)ion exchanger based solid-state ISEs have been presented. Finally, an ion-exchanger membrane electrode for tris(ethy1enediamine)cobalt(3+) has been studied (79c). Pandey and Tripathi (121~)have extensively studied impregnated ion-exchange membrane electrodes. These resebchers conclude that these types of electrodes can be used to determine the activity of counterions in single salt solutions as well as in mixtures of counterions, if the selectivity coefficients of the electrode for the various counterions differ considerably. These workers also point out that difficulties with respect to electrode reproducibility, aging, and nonuniformity in structure severely limit the analytical usefulness of these types of electrodes. Solid-state pH Electrodes. Studies continue in the development of nonglass pH sensors. stabilized ceramic membrane electrodes show considerable promise as pH sensors at elevated temperatures (112c, 159~).In this case, the membrane is a yttria-stabilized zirconia layer (Zr02 17% Y203) which displays excellent Nernstian response to solution pH between 25 and 275 “C (159~).Moreover, a direct comparison between ceramic and conventional glass pH electrodes shows that the former displays little, if any, sensitivity to alkali ions in basic solutions and, therefore, is free from the “alkaline error” associated with glass pH electrodes. Other nonglass pH electrodes which have been studied include antimony wires (85c), chemically modified platinum and vitreous carbon electrodes (33c), sputtered iridium oxide films (82c), and polymeric sulfur nitride based electrodes (135~).Cheek and co-workers have studied the pH response of platinum and vitreous carbon electrodes with and without polymer films (33c). These researchers have found that, contrary to previous thinking, the electrodes’ pH response is principally due to the platinum and vitreous carbon surfaces and not to the polymeric layer. The polymer layer does appear to enhance electrode selectivity, however, by sterically hindering large redox systems from approaching the electrode surface. The results of an extensive study to characterize the pH response of reactively sputtered iridium oxide film electrodes has been presented (82c). This pH electrode type was shown to respond in a near-Nernstian fashion between 0 and 100 OC. Preliminary tests show that these electrodes are stable in aqueous solution up to 200 OC. Finally, the use of polymeric sulfur nitride (SN), membrane electrodes for pH titrations was presented (135c). Although these electrodes exhibit limited linearity, a super-Nernstian response is observed from pH 10 to 12 which is shown to be quite advantageousfor the titration of weak acids (pK, = 9) with a strong base. Finally, pH sensors based on tungsten-bronze crystals have been discussed (24c).
+
LIQUID AND POLYMER MEMBRANE ION-SELECTIVE ELECTRODES
As evidenced by the over 140 references in this section, the development, study, and application of liquid and polymer membrane ISEs remain the prime focus of many investigators. In this section we separate liquid membrane electrodes into two categories; those based on neutral carrier molecules and those based on organic ion exchangers. Because it is now commonplace to immobilize wet membrane components containing the ion carriers or exchangers within polymer membrane matrices, usually poly(viny1 chloride) (PVC),we will not separate the conventionalpure liquid membrane type electrodes from the polymer membrane type. Indeed, the polymer membranes are merely highly viscous forms of the their wet liquid counterparts and as such they still should be classified as liquid membrane systems. However, as noted in the 1982 review, some differences in the response properties including selectivities may be observed when membrane components are used in pure liquid form vs. polymeric form. In addition, certain liquid membrane electrodes have been developed specifically for clinical chemistry applications and these will be discussed later in this review as will liquid membrane-based devices which do not employ internal reference solutions (e.g., CWEs, ISFETs, etc.). Before we consider membrane electrodes which incorporate 28R
ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
specific carriers or ion-exchangers, there were several general investigations concerning electrochemistry at the interface of two immiscible electrolyte solutions (ITIES) or solution/ membrane interfaces which may be of fundamental importance in understanding the potential develo ing mechanisms at the surfaces of liquid and polymer memirane ISEs, or in the ultimate development of new electroanalytical devices. For example, Buck and co-workersat the University of North Carolina have published a series of related papers dealing with the potential profiles attained at the interface of waternitrobenzene solutions as a result of the single and dual ion transport across such interfaces (due to partitioning) (91d, 92d, 93d, 108d). By use of chronopotentiometry and the measurement of interfacial tensions, rates of ion transfer across the interface and estimates as to the space charge distribution of the resulting double layer region may be made. Since the electrical response to liquid membrane ISEs arises from the selective transfer of ions at the sample/membrane interface, such studies may provide insight into the processes which result in sub- and super-Nernstian behavior as well as those that limit the dynamic measuring ranges for certain liquid membrane ISEs. Several other groups, most notably Koryta’s laboratory in Czechoslovakia, have also been studying electrochemical behavior of ITIES systems. Indeed, the entire concept of ion transfer across water/organic phase boundaries and the analytical applications that may result was reviewed by Koryta (784 for both nonfacilitated and facilitated systems (i.e., with ion carriers or exchangers). In a variety of research reports, cyclic voltammetry procedures were used in conjunction with the ITIES systems to develop analytical methods for the detection of antibiotics (814, ion carriers (794, and even the enzyme acetylcholinesterase (1314 (based on the difference in acetylcholine and choline transfer between aqueous and organic phases). Further, it has been shown that when ion carriers (e.g., valinomycin, crown ethers, etc.) are incorporated into the organic phases of ITIES systems, cyclic voltammetric methods can also be used to accurately determine the formation constants of various ions with the ionophores (514 130d). Obviously, such information could be useful in predicting the resulting selectivity constants for potentiometric liquid membrane electrodes which utilize these same ionophores. Furthermore, insights into fouling of liquid membrane electrodes by adsorbing species may also be gained from this technique since, when such processes occur at the interface, rates of ion transfer decrease (e.g., cyclic voltammetry currents decrease) (80d). Finally, in a related approach, and one that may also lead to a new electroanalyticalmethod, Thompson and Krull in Canada (123d-126d) have studied the possibilities of using changes in the conductance of phospholipid bilayer membranes as the basis for developing selective chemical sensing systems. The theoretical models for such sensing have been worked out and some preliminary data for valinomycin and carbohydrate (using membrane imbedded lectins) detection have been obtained. This work demonstrates that when formed properly, thin lipid bilayer membranes are not as fragile as one might expect and it suggests the possibility that perhaps such bilayers could ultimately be used in potentiometric membrane electrode configurations. Now for literature regarding the more classical liquid and polymer membrane ISE devices: Neutral Carrier Based ISEs. Efforts to synthesize new cyclic and noncyclic organic carriers that possess suitable ion specificity for use in liquid membrane ISEs remains an important area of research. With regard to the cyclic molecules, the crown ethers have traditionally received considerable attention and a recent review on the use of such molecules in chemical analysis has a section devoted to their application in liquid membrane ISEs (142d). In a fundamental study, Lamb et al. (84d) investigated the influence of macrocyclic ligand structure on the selectivities and transport rates of cations through bulk liquid membranes. Such data can probably be utilized to predict the selectivities of liquid membrane ISEs prepared with any of the more than 20 compounds examined. Numerous research reports have appeared suggesting the use of new crown type compounds for the fabrication of potassium selective electrodes. Electrodes based on polymer membranes in which bis(benz0-15-crown-5)derivatives were incorporated have been shown to exhibit high selectivity for potassium over sodium (log k = -3.2) and may be useful for
ION-SELECTIVE ELECTRODES
applied as an indicator electrode in the potentiometrictitration the direct determination of potassium in urine samples (714 of sulfate while the nickel electrode was used to determine 72d, 136d). A PVC type membrane electrode based on riathe formation constant of Ni2+with sulfosalicylicacid. Simphtho-15-crown-5 was also shown to possess high selectivity ilarly, researchers in Japan (58d) prepared various unsymfor potassium over sodium ( k = 4 X lo4) (140d). This crown metrical bisbenzocrown-5-benzo-crown-6ligands and used compound was incorporated in the PVC membrane along with these in PVC polymer membrane electrodes selective for rup-nitrophenyl octyl ether and di icrylaminate. The dibidium over potassium and cesium. In addition, an electrode picrylaminate was robably addec r to prevent negative inwith selective response to lithium was described in which terferences from hy rophobic anions in the sam le, much like dibenzo-14-crown-4 was used as the active component of a tetraphenylborate has been used in the past. WKlle suggested PVC membrane (99d). In other neutral carrier work, new as satisfactory alternatives to valinomycin, these new potascalcium electrodes based on oligo-oxa-alkanes(67d) were resium ISEs do not display the selectivity over sodium that ported and a patent was granted to Kodak on the use of valinomycin-based electrodes do, although their dynamic 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile in PVC measuring ranges and response times are indeed quite similar. as a polymer membrane pH sensor (164. Several larger macrocyclic structures have also been reFor the first time, the utilization of macrocyclic poly(thia ported for the fabrication of potassium electrodes. A Chinese paper suggests the use of 3,3’-n-pentadecyldibenzo-30- ether) compounds in liquid membranes has been demonstrated. Kamata et al. (664 in Japan have incorporated the crown-10 in dioctyl phthalate plasticized PVC membranes macrocyclic 13,14-benzo-1,4,8,11-tetrathiacyclopentadecane (1444. Selectivity over lithium and ammonium ions was as a neutral carrier in PVC to prepare a functional Cu(I1) better than that observed with valinomycin, while selectivity selective probe. Unfortunately no data concerning the seover sodium was somewhat less (k = 3.3 X IO4). Of particular lectivity of their electrode over other transition metals were note was the rather large improvement in linear response range provided in their communication. The electrode did, however with this new carrier when com ared to valinomycin-based electrodes (linear range 1 X 10-F to 1 X lo-’ mol/L). More respond in a Nernstian fashion to Cu2+in the range of lo-‘ common dibenzo-30-crown-10or diethyldibenzo-30-crown-10 to 10-1mol/L. In view of these promising preliminary results, it seems likely that more work with thia ether type compounds compounds were also used to prepare potassium sensors (98d). is on the horizon. Hopefully, some of these macrocylics will Selectivity over sodium was not nearly as good as when using exhibit selective binding to certain transition metals and the the dipentadecyl derivatives. A recent patent (128d) also resulting electrodes may prove useful as alternatives to the points to the use of various 30-crown-10 compounds as neutral solid-state type of membrane electrodes (e.g., Ag2S-CuS, etc.) carriers in potassium selective electrodes. now being used for the determination of such metals. In a relatively new direction, Gajowski and co-workers in As has been the case since 1969, when they initially introGermany ( 3 3 4 investigated the use of cryptand 222B (2,3,5,6,8,9,11,12,14,15-decahydro-4,13-(ethanoxyethanoxy-duced the valinomycin-based membrane electrode for poethano)-4H,13H-1,7,1O,l6,4,13-benzotetraoxadiazacyclo- tassium, Simon and his colleagues in Zurich continue to make valuable contributions to the literature. In a presentation at octadecine) in pure liquid diphenyl ether to prapare potassium a bioinorganic conference (110d), Simon provided an overview ISEs. While the cryptand type ca e compounds have been of his groups most recent efforts including the prospects for around for several years, there have een relatively few reports preparing a-bicarbonate-selectiveliquid membranes through concerning their use in liquid membrane ISEs. In this case, the use of trioctyltin compounds as the membrane mediators. despite the more complex geometrical constraints of the With regard to more specific research reports, the Swiss group cryptand‘s structure, the final electrode exhibited only marstudied the possibility of using noncyclic N,N’-dioctadecylginal selectivity over sodium ( k = 0.1). However, selectivity substituted monoamides of certain dicarboxylic acids as over lithium was quite acceptable (k = 0.0003) and the linear to carriers in PVC membranes to prepare alkaline earth (Ca2+ range for potassium response extended from and Mg2+)selective electrodes (88d). By use of N,N-dioctamol/L. Similar results were obtained when the cryptand was decylmalonamide in p-nonylphenol plasticized membranes, utilized in a PVC type membrane. In view of the growing number of cryptand molecules now being synthesized, it seems electrodes with selectivity constants, kMq ranged from 0.1 to 0.01 for cases where i = Na+, K+, and LI . Essentially equal likely that certain structures will ultimately possess ion selectivities that will enable their use in improved liquid memresponse toward calcium was observed. However, as may be brane ISEs. expected, the membrane has a rather large response to hydrogen ions (k = 1000) owing to the free carboxylic acid A host of new carriers have also been proposed for use in function in the carrier. In a different investigation (134d), sodium selective liquid membrane ISEs. Shono et al. in Japan another class of amides, and in particular, 2,2’-[1,8(117d, 118d) have employed bis( 12-crown-4) compounds in naphthalenebis(oxy)bis(N,N-dicyclohexylacetamide)],was PVC membranes to fabricate sodium electrodes with excellent shown to behave as a reasonably selective carrier for sodium selectivities over many common monovalent cations (e.g., k N a ions over potassium ( k = 0.05). The selectivity over other = 0.009, kNaLi = 0.001). Such selectivities are similar to those cai5ons was quite good with the exception of lithium (k = 4.0). found when using the more traditional sodium liquid memThe Zurich group also proposed the use of noncyclic ligands brane components such as monensin and Simon’s synthetic for lithium and uranyl ion electrodes based on substituted ionophore ETH 227. Surprisingly, these workers found that diphenylmaleimide species (132d). PVC type membrane the dynamic measuring range of the electrodes was profoundly electrodes fabricated with one of the compounds prepared had affected by the concentration of NaCl used as the internal selectivity for lithium over sodium by a factor of 10 (12 = 0.1) reference solution. With high concentrations internally, linear while another ligand of the same class yielded selective reresponse toward sodium was observed to about 3 X 10-4mol/L sponse to uranyl ions over Ag+ ( k = 0.1) and even greater with slightly sub-Nernstian slopes of 52-55 mV/decade. Others have used noncyclic ligands as carriers for new sodium selectivity over Li+ ( k = 0.01). liquid membrane ISEs. For example, Materova et al. (89d) In a major contribution, the Swiss workers also proposed reported the use of a variety of 1,2-bis(benzylcarbamoyla rather unique type of bicarbonate responsive liquid memmethoxy)benzene derivatives as sodium carriers while Xue brane electrode (314. The membrane phase consisted of (1374 1394 used triglycyldibenzylamine and amides as active various ethanolamine compounds in plasticized PVC memcomponents in PVC membrane electrodes. Both of the acyclic branes. However, the amines are hydrogen ion carriers, not type carriers yielded sodium electrodes with poorer selectivities bicarbonate carriers. Thus, the resulting pH electrode rethan existin devices although the dibenzylamide based sponds to bicarbonate in buffered samples (pH 8.0) via a compounds &d exhibit some selectivity over potassium (k = mechanism that resembles a gas sensor. Indeed, the electrode 0.16). is in reality a pseudo-gas-sensing device. Background levels Aside from sodium and otassium electrodes, new ionoof C02 in the bufferd bicarbonate samples can diffuse through phores were also introducecffor the fabrication of electrodes the PVC membrane into an unbuffered internal reference selective for several other cationic species. For example, the solution. A steady-state change in the H+ activity at the barium complex of benzo-15-crown-5and the nickel complex internal interface of the PVC membrane results. Since the of 1,4,8,11-tetraazacyclotetradecane in araldite-based memouter membrane potential does not change (sample solution branes were studied as potential barium and nickel selective is buffered), the electrode exhibits a negative slope to the electrodes (83d). Selectivity coefficients of less than 1 were bicarbonate activity in the sample (as if the ISE was directly observed for most cations tested. The barium electrode was responding to bicarbonate ions). Obviously, selectivity over
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other anions is excellent because none of these species can pass through the PVC membrane and change the pH of the internal solution. As a finale in the development of new neutral carrier systems, one paper reported on rather unique types of sodium and potassium PVC-membrane electrodes based on a cation-proton countertransport mechanism (324. In this work, researchers utilized 18-crowq-6 and 15-crown-5compounds which had been derivatized with lipophilic carboxylic acid residues. Thus, the final membranes prepared with these compounds behaved as dual ion carriers for both protons and alkali metal ions. In order to obtain potentiometric response to sodium or potassium, it was found that the internal reference solution must be acidic while the sample solution must be basic. Only in this situation will the membrane potentials reflect the concentration of sodium or potassium in the sample. It is theorized that these new membrane electrodes function via a pumping mechanism whereby the alkali metal ions are transported against their concentration gradient as a result of the high transmembrane proton gradient. The 18-crown compounds were used for potassium sensing while the 15crown derivatives were used for detecting sodium. While interesting from a mechanistic standpoint it is doubtful that such electrode systems will realize considerable analytical utility because they are sensitive to the pH of the sample whereas conventional sodium and potassium liquid membrane ISEs are not. Several fundamental studies concerning existing neutral carrier based ISEs were also published in the last 2 years. For example, Griffin and Christian ( 4 0 4 performed a study on the long term stability of valinomycin-based potassium electrodes. Freshly prepared PVC type electrodes were compared to ones fabricated 3 and 10 years earlier. Surprisingly, slopes and selectivity data for the older electrodes were quite similar to the new ones. However, response times were much longer for the aged electrodes, e.g., 3-6 min as compared to 30 s for freshly prepared devices. Apparently, storage of the electrodes in water extends the lifetime by minimizing the loss of volatile components from the PVC membrane (membranes do not dry out as they do when stored in air). Since the stored electrodes were not repeatedly used in fresh volumes of water samples, the loss of valinomycin from the membrane was also minimized (controlled by the partition coefficient between the membrane phase and water). Thus, the shelf life of the potassium electrodes can be extended by storing them in a fixed volume of solution. Others also found that even short exposure of valinomycin-PVC type electrodes to air, e.g., 5 min, can be enough to cause alterations in membrane potentials and response times in solution (28d). Ammonium electrodes prepared with nonactin derivatives were also the focus of some fundamental studies including investigations on the conducting mechanisms of such systems (109d) and the effect of sample pH and ionic strength on the response to ammonium ions (32d). Finally, there have been a considerable number of reports lately concerning the use of PVC membrane electrodes which do not make use of specific ion conducting or carrier molecules (see several additional references in CWE section of this review) within the sensing membranes. For example, Vytras et al. (1354 used 2-nitrophenyl P-ethylhexyl ether plasticized PVC membranes which were responsive to 4-bromobenzenzenedizonium ions for the titration of various organic coupling reagents (most were large organic sulfonic acid type species). Others (100d) have claimed that dibutyl phthalateand dioctyl phthalate-PVC membranes have selective responses toward calcium and uranyl ions. In these systems, it is believed that the carbonyl groups of the plasticizers present in the membranes (phthalates, adipates, sebecates, etc.) behave as psuedo-neutral-carrier moieties. Indeed, a detailed study on the mechanism of response for such PVCplasticized membrane electrodes toward hydrophobic cations, e.g., alkyl quaternary ammonium species, has been reported (85d). Electrodiffusion experiments suggest that the cations themselves are capable of carrying charge through the membrane via selected solvation with the carbonyl sites of the plasticizers. Actual potential development is believed to be localized in a small region at the membrane/sample interface. Thus, it seems probable that with the judicious choice of plasticizers, PVC membrane electrodes selective for certain cations can be developed without the need for specific neutral 30R
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carriers provided that the plasticizers themselves display some selective interaction with the ion. Ion-Exchanger Based ISEs. The most interesting development in the use of organic ion exchangers in liquid or polymer membrane ISEs concerns the fabrication of new types of calcium sensors. Traditionally, calcium ion exchanger based ISEs have been prepared by incorporating various alkyl phosphates in dioctyl phenylphosphonate solvent in pure liquid or polymer membrane form. In such systems the ionexchange sites of the alkyl phosphate compounds are mobile within the membrane and the addition of phosphonate cosolvents improve selectivity for calcium. Unfortunately, the lifetime of these sensors is limited by the loss of the exchanger and cosolvent from the membranes. In attempt to overcome this problem, two groups in England have studied the feasibility of grafting the ion exchanger alone, or simultaneously with the phosphonate cosolvent,to the backbone of a polymer matrix. Hobby et al. (494 50d) prepared such electrodes by grafting decyl and 4-(1,1,3,3-tetramethylbutyl)phenylphosphate to vinyl chloride-vinyl alcohol copolymers. In some cases the solvent, octyl phenylphosphonate was also covalently attached to the polymer. The resulting electrodes possessed fixed ion-exchange sites and had considerably longer lifetimes than conventional calcium sensors. Unfortunately, selectivity over sodium and magnesium was not as good as traditional PVC membrane electrodes with mobile sites. Even poorer results, in terms of selectivity, were obtained by Ebdon and co-workers (144 25d) who immobilized dialkyl phosphate groups onto poly(styrene-b-butadiene) matrices. These workers observed significant interferences from various transition-metal ions with their new sensors. Several other new ion-exchange type electrodes responsive to a variety of cationic species are also deserving of comment. Most notable among these are new electrodes selective for transition metals, particularly copper, which utilize various dialkyl phosphorodithioate copper complexes in chloroform as the active membranes (119d, 1334. Mercury and lead ions were major interferences with all the electrodes studied, yet some selectivity over other divalent transition-mea ions was observed. Two new uranyl ion (UO,+)selective electrodes with dramatically improved selectivities were also described. Serebrennikova et al. (1164 in Russia, used di-2-ethylhexyl phosphate and tributyl phosphate in various organic solvents as liquid membranes whereas Luo and co-workers in Taiwan ( 8 6 4 employed a mixture of tributyl phosphate, tri-noctylphosphine oxide, and sodium tetraphenylborate in PVC membranes to obtain responsive electrodes. Selectivity constants for both ion-exchange systems were quite comparable, with selectivity constants generally to for most common divalent and monovalent cations tested (using optimized membrane formulations). The effect of pH on a similar uranyl ion sensor was also examined in a separate paper (IOId)and it was shown to be significant only at low pH values, e.g., below pH 5.0. With re ards to new anion responsive electrodes, relatively few new jevelopments have been made. Greenberg and Meyerhoff ( 3 9 4 did investigate the use of Aliquat 336 and trifluoroacetyl-p-butylbenzenein a plasticized PVC membrane for use in a carbonate-selective electrode. The trifluoro compound was shown previously to promote selective response to carbonate ions in pure liquid membrane electrode systems (along with the Aliquat ion exchanger) although the exact mechanism for this selectivity has never been established. As demonstrated by Greenberg and Meyerhoff, even when incorporated into a polymer membrane form, this cosolvent/ ion-exchange mixture still maintains considerable selectivity for carbonate over many common anions (e.g., Nos-, Cl-, Br-, etc.). However, selectivity over salicylate and perchlorate is essentially nonexistent. Thus, the polymer electrode could not be used to accurately detect total carbon dioxide species in blood samples where salicylate may be present (aspirin users). In another interesting report, Hara and collaborators in Japan ( 4 5 4 evaluated several anion responsive membrane electrodes prepared with "solid solvents". That is, the active membranes were composed of a methyltrioctylammonium exchanger in solid octadecanol. Such electrodes responded toward p-toluenesulfonate in a manner similar, in terms of selectivity characteristics, to analogous liquid membranes prepared with pure decanol as the solvent. While the authors suggest that the solid liquid membranes may have some ad-
ION-SELECTIVE ELECTRODES
vantages over the pure liquid type (ease of preparation, rugged, etc.), in fact, such membranes are quite brittle and can easily crack leading to electrode failure. It would seem that simple incorporation of the exchanger with decanol in a PVC membrane would yield similar results. The development and application of membrane electrodes selective for organic cations and anionic species remains an active area of investigation. Numerous electrodes of this type based on a wide array of ion-exchanger and solvent systems have been proposed, incliidin ones responsive to trinitrobenzenesulfonic acid (TNUS) (5124, dichloroacetate (1044, naproxinate (52d), scopolamine (59d), flufenamic acid ( 7 3 4 , and salicylate ( 1 2 4 , as well as a host of others summarized in Tables I11 and IV. While in many instances selectivity toward the organic ion over inorganic cations or anions is quite good, selectivity over other organic species with similar structure and functional group moieties is generally poor and analytical applications in complex samples are limited. Nonetheless, as may be noted, many of the species sensed are drugs and, therefore, the electrodes developed can be very useful for direct or indirect assays of such species in pharmaceutical preparations. Furthermore, some of these organic ion sensors can be useful as detectors in fundamental studies or in novel assay arrangements. For example, the flufenamic acid electrode was cleverly employed to study the binding of flufenamate to proteins in solution (734, whereas the TNBS sensor was used to determine amino acids based on the reaction of TNBS with primary amino groups (1124. Some additional new ion-exchanger based liquid and polymer membrane electrodes responsive to both organic and inorganic species are listed in Table I11 along with the membrane compositions that yield the desired response. Further applications of new and existing ion-exchanger based electrodes are provided in Table IV. Various liquid and polymer membrane ion-exchanger electrodes have also been the focus of fundamental studies regarding interferences and response mechanisms. For example, Hulanicki and co-workers in Poland found that calcium electrodes based on alkylphosphate exchangers suffered interference from lipophilic anions which could lead to false low values for calcium determinations in certain samples (56d, 5 7 4 . They surmised that the response to anions is due to calcium binding to the solvent mediators in liquid membranes (e.g., phosphonates) or the plasticizers in PVC membranes. This process gives rise to positively charged anion exchange sites at the membrane/sample solution interface along with the desired negative sites of the alkylphosphate compounds. Obviously, the use of alternative solvents and plasticizers which do not behave as psuedoneutral carriers for calcium ions can solve this problem. Indeed, Frend, et al. (30d) showed that PVC type calcium electrodes prepared with 1-decanol or 1-dodecanol rather than dioctyl phenylphosphonate had reduced interference from dodecyl sulfate anions. Changing the polymer matrix from PVC to poly(viny1idene chloride) or a vinyl chloride-vinyl alcohol copolymer without changing the cosolvent mediator did not alter the anion effect. Others have also examined the effect of surfactants on the behavior of the calcium electrode as well as the nitrate electrode (554. In general, liquid and polymer membrane ISEs of any type will exhibit large positive responses to hydrophobic ions which possess charges similar to the principal species the electrode was designed to sense as well as negative response toward large surface active ions with opposite charge when analyte ion concentrations are high. This latter response arises from the mechanism mentioned above; Le., the solvent or plasticizers in the membrane a d s as a neutral carrier for the analyte giving rise to oppositely charged membrane sites at high analyte levels. Response to nonionic surfactants is generally minimal. However, barium selective electrodes, prepared with a PVC membrane phase of tetraphenylborate ion pairs with barium complexes of some nonionic surfactants (e.g., nonylphenoxy(ethyleneoxy)ethanol),respond directly to nonionic surfactant concentrations (614 62d). Indeed, it is possible to detect critical micelle concentrations of these Surfactants by utilizing the break points in the log [surfactant] vs. EMF plots since once in micellular form, the potentiomentric response is diminished. In a series of important papers relating to the barium selective membrane electrode, Moody and Thomas’ group in Wales used 133Ba2+ tracers and showed that the barium ions
do not permeate the PVC membrane to any significant degree (membranes prepared with barium-surfactant complexes of tetraphenylborate) (224 23d). The membrane does uptake the tracer to some extent but does not actually transport the ion through the membrane in accordance with a concentration gradient across the membrane, at least not during the time scale of their experiment (up to 15 days). This behavior is just the opposite to that observed in most PVC membrane type electrodes including the calcium probe based on alkyl phosphate exchangers. Here, the Wales group demonstrated that calcium ions are actually transported through the membrane and when an external potential is applied across the membrane, a steady ion current is established due to the migration of calcium in the membrane (96d). As expected, in neither case does a tracer counterion, e.g., 36Cl-,enter into the membrane. If it did, then of course a negative interference from the anion would occur for each electrode. There have been a few fundamental studies concerning anion responsive membrane electrode systems based on ionexchanger species (usually quaternary ammonium type exchangers). Beg and Nabi ( 4 4 performed experiments to test the theory regarding detection limits for liquid membrane electrodes based on ion exchangers. Using benzyltrialkylphosphonium and ethyltriphenylphosphonium halides in 1-butanol, they concluded that the detection limits for anion response are determined by the solubility of the ion exchanger in the aqueous test solution. Lowering the concentration of the exchanger in the membrane phase improves the detection limits by reducin the amount of exchanger that can move into the sample. 8 n the basis of these studies, it is suggested that the mechanism which determines lower limit of detection for liquid anion exchange electrodes is analogous to that of solid-state membrane systems (e.g., Ag2S/AgI,etc.) where the K of the crystalline material controls the detection capabitties. With regard to the selectivity of liquid membrane anion exchange systems, Gao and co-workers in China have examined the effect of increasing alkyl chain length of membrane alkylammonium compounds on the selectivity of perchlorate electrodes over other anions, both organic and inorganic (354 36d). As expected from Beg and Nabi’s work cited above, increasing the hydophobicity of the exchanger (by increasing the length of the alkyl chain) improves the linear response range and detection limits of the perchlorate sensor; however, for small inorganic ions, the Hofmeister series selectivity pattern is still maintained although absolute selectivity coefficients do change somewhat. Exchangers with very long alkyl chain lengths are not recommended due to difficulties in purifying these compounds and their low solubility in PVC. Another group in China also studied perchlorate response of electrodes prepared with various triphenylmethane dye-perchlorate complexes as the active membrane components. An empirical formula for predicting selectivity coefficients over anions was derived and shown to be dependent on the thermochemical radii of the various anions (46d). In a related paper, Tahara and Oka (1224 claimed that the selectivity of anion responsive membranes can be improved somewhat by doping the membranes with small amounts of cation exchangers. A similar approach has been used previously for cation responsive electrodes where tetraphenylborate has been added to eliminate negative interferences from lipophilic anions. In the case of anion responsive electrodes, the negative sites of the cation exchanger may repel1 certain anions from entering into the membrane phase, and provided that the main anion exchanger can overcome this repulsion for certain ions, alterations in the Hofmeister selectivity pattern may be observed. Finally, in other miscellaneous studies related to ion-exchanger based liquid membrane electrodes, Moody et al. investigated the effect of pH on a beryllium ISE prepared with organophosphate exchangers (954; Selinger performed conductance and extraction studies to help understand the response of tetraphenylborate based membrane systems to various pharmaceutical cationic species (113d); a specific method for calibrating calcium electrodes in the low concentration range via the use of calcium EGTA solutions was suggested ( 5 4 ;and Smolyakov and Kokovkin looked at the effect of size and dimensions of tetraalkylammonium species on the response of general cation exchange electrodes toward such species (1204. In this last paper it was once again shown that selectivity over various organic cations is directly related ANALYTICAL CHEMISTRY, VOL. 50, NO. 5, APRIL 1984
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to the resolvation energy of the ions in the organic membrane phase (i.e., like dissolves like!).
COATED WIRE ELECTRODES, ION-SELECTIVE FIELD EFFECT TRANSISTORS, AND MICROELECTRODES The development of miniaturized and microsized ISE probes continues to be a "hot" area of research. Most commercial ISEs are considered macro in size with tip diameters on the order of 3-15 mm. Such sensors are not suited for measurements in small volumes of sample nor for the desired in vivo applications of ISEs that biomedical researchers have long awaited. One way to reduce the size of conventional ISEs is to remove the internal reference solution; that is, place the ion conducting membrane material in direct contact with the electronic conductor, e.g., metal wire, or the insulating gate of a semiconductor device. This approach has resulted in the development of coated wire electrodes (CWEs) and ion-selective field effect transistors (ISFETs). Such devices are often classified as coated solid-state devices (CSSDs). As mentioned, the majority of interest in these new devices has come from the biomedical community who would like to have simple catheter type sensors for continuous in vivo measurements of not only ions but also drugs, metabolites, hormones, etc. The development of suitable devices that are stable and sensitive enough for this urpose is obviously a formidable task. Indeed, Pinkerton (%e) recently reviewed the prospects and problems involved in preparing such devices particularly with regard to in vivo drug sensing. Others have also addressed the current state of ISE technology with regard to fabricating catheter type devices (48e). In addition, further miniaturization of ISE probes is needed by physiologists who wish to study the processes that go on within intact cells. Here, electrodes with tip diameters between 0.1 and 10 pm are required. In this section of the review, we will cite recent articles relating to the development, study, and application of both CSSDs and conventional microelectrodes devices. Many of the reported sensors utilize liquid or polymer membranes (with either neutral carriers or ion exchangers) as the outer ion-selective phases such as those reported in the preceding section. With regard to simple miniaturization of ISEs, there is no doubt that the coated wire approach is an attractive one. However, as mentioned in the 1982 review, there has been a lingering controversy over the performance of these devices in terms of stability and reproducibility and the exact mechanism by which they operate. In connection with this controversy, several important papers have been published over the past 2 years. Trojanowicz and co-workers (58e) detailed a comparison study on the performance of valinomycin based potassium electrodes prepared with direct solid contacts and those prepared with conventional internal reference solutions. They found similar response properties in terms of detection limits, selectivities, and dynamic response times although the choice of plasticizers for the membrane had a larger affect on the solid contact type electrodes (dioctyl-adipate better than dibutyl-sebecate). Contrary to many previous reports, these researchers also observed that the long term stability of absolute potential values was better for the solid contact electrode than the conventional device. It should be pointed out that the electrodes used in this study were not in the strictest sense CWEs but rather flat silver electrodes pressed tightly against precut polymer membranes. Nonetheless, such a probe should behave exactly like a CWE and the lack of long term drift is rather surprising in view of the findings reported by another research team at the same University. Indeed, Maj-Zurawska and Hulinicki ( B e ) showed that the potentials of nitrate selective CWEs prepared by coating a PVC-nitrate ion-exchanger membranes on platinum contacts were dramatically affected by the surface oxidation state of the platinum. In their experiments, preoxidation of the platinum with sulfuric acid resulted in base line potentials that were 170 mV more positive than electrodes in which the latinum surface had first been reduced with ferrocyanide %eforecoating. These results suggest that reducing or oxidizing agents present in real samples that can permeate the ion-selective membrane of the CWE devices and react with the metal surface to cause errant potential changes. Since oxygen gas can permeate PVC, such electrodes may actually 32R
ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
respond to oxygen over a long term via this mechanism as suggested by other workers several years ago. A possible solution to this problem may lie in stabilizing the redox potential of the polymer membrane itself, and this interesting approach was examined by Stefanova and co-workers in Russia (53e). They showed that the potentials of solid contact type devices could be stablized somewhat by doping the PVC ion-selective membranes with organic redox species, e.g., heme, cation exchanger in mixed form Fe(I1) Fe(III), etc. Clearly more basic research of this type must e undertaken before the CWE devices become widely accepted. While long term stability may be a problem, CWEs have been found to be quite useful for many direct determinations provided that the electrodes are calibrated often. Further, when used as indicator electrodes in titrimetric procedures, the slight drift in absolute potentials does not affect the titration curves, and results are usually as accurate as when using conventional IS&. Thus, a host of new CWEs have been reported along with some interesting applications and some of these are summarized in Table V. Most notable among these are the drug sensors prepared by Cunningham and Freiser (12e) which may be quite useful in pharmaceutical methods or studies, and the new sodium and potassium CWEs based on bis(crown ether) compounds (56e). Most of the CWEs prepared to date have utilized solid metals such a platinum, copper, or silver as the internal contact wires. Recently, Selig (49e) has proposed the use of spectroscopic graphite rods as internal electronic conductors for the fabrication of simplified CWE devices which may be used in an array of potentiometric titrations. In his work, the coated PVC-plasticized membrane utilized does not contain a neutral carrier or ion-exchanger species. Consequently, the plastic-coated carbon responds nonselectively to a variety of quaternary ammonium compounds (used as titrants) because the plasticizers act as psuedo-neutral carriers for these hydrophobic cations in the coated membrane (see discussion in Neutral Carrier-Based Liquid Membrane section). Selig also studied the possibility of using noncoated graphite as an indicator electrode in titrations involving cetyl-pyridinium ion as the titrant (50e). He found that the coated graphite electrodes yielded much better titration data than the noncoated probes for the determination of nitroform and perchlorate, and that bis(ethylhexy1)phthalate was the optimum plasticizer to use in the preparation of the coated graphite sensors. In the ever growing area of ISFET technology, several important developments are noteworthy. Janata and co-workers at the University of Utah continue to lead the way in studies regarding the fundamentals of ISFET design and performance. Most importantly, it now appears that problems concerning the poor adhesion of PVC type ISE membranes to the gates of FETs can be overcome by suspending a polyimide mesh over the gate of the FET device used to prepare the sensor (9e). As the polymer membrane is cast onto the gate and the casting solvent evaporates, the polymer f i i becomes anchored in place by the mesh. Thus, failure of the final ISFET devices due to detachment of the ion sensing membrane is minimized. In several other important papers, the Utah workers have proposed a new chip circuit for the preparation of ISFETs which prevents damage to the devices by electrical charge during their handling (52e), studied the source of noise in ISFET based measurements via noise spectra analysis (41e), and develo ed a novel FET device that is responsive to dipolar molecules b e ) . In another fundamental investigation, KOand coresearchers at Case Western Reserve University have examined the question of whether nonuniform electric fields created by the source-draincurrent in ISFETs could influence the gate potentials measured and consequently the accuracy of these devices (28e). No noticable effect was observed for pH sensitive ISFETs based on SN., or SiOz gates. In other interesting work, researchers have studied the cation response of ISFETs prepared with hydrophobic Teflon-coated gates (41e), developed sodium and potassium sensitive ISFETs with tip diameters of only 30 Km using alumina-silicatesas the outer ion sensing layer (43e), and fabricated new sodium selective ISFETs by ion implantation of lithium and silicon into alumina based gates (47e). This latter study is particularly interesting since the resulting gate material, formed by the ion implantation process, mimics the composition of glass membrane sodium electrodes.
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Table 111. Additional New or Improved Ion-Exchanger Based Liqpid and Polymer Membrane ISEs principal species sensed ref membrane components no. 17d 1 tetrazole-cetyltrimethylammonium ion pair in silicon cetyltrimet hylammonium ions rubber; polymer Cr,0,229d 2 triheptyldodecylammonium iodide in PVC; polymer CuL,' L = 2,9-dimethyl-l,lO-phenanthroline 70d 3 CUI L,-picrate ion pair in dibutyl phthalate; liquid 10d cs 4 cesium-triphenylcyanoborate in PVC; polymer (note: membrane pressed from solid components) alkaline-earth metals 87d 5 calcium trioctyloxybenzenesulfonate in various organic solvents; liquid 97d c10,6 cetyltriheptylammonium perchlorate, dioctyl phthalate, in PVC; polymer 103d 10,7 ethylviolet periodate ion pair in PVC; polymer benzoate 90d 8 tetradecylammonium benzoate, p-trifluoroacetylbenzoic acid hexyl ester, dioctyl phthalate, in PVC; polymer benzoate 7d 9 dimethyldicetylammonium or benzyldimethylcetylammonium benzoate in various organic solvents; liquid 121d 10 tetraphenylborate-calcium (Triton X-100) complex, ethylene oxide type surfactants in PVC; polymer 115d NO,11 various nitrate salts of triarylmethane, xanthene, oxazine, and cyanine dyes in nitrobenzene or nitrotoluene; liquid 53d NO,12 nitrate complexes of gentian violet, tetraphenylphosphonium and tetraphenylarsonium in nitrobenzene; liquid 129d 1 3 myristyltrimethylammonium-chloridein undisclosed c1membrane phase Alizarine-S 145d 1 4 trioctylmethylammonium chloride, dibutyl phthalate, in PVC; polymer ephedrine 146d 15 ephedrine-tetraphenylborate complex in PVC; polymer 6d, 107d Au(CN),1 6 trinonyloctadecylammonium-dicyanoauratein toluene-nitrobenzene mixture; liquid 2d, 60d AuC1,- and Au(CN),17 tetraphenylarsonium and tetradecylphosphonium ion pairs with AuC1,- or Au(CN),', dibutyl phthalate, in PVC, polymer 54d 18 tetraphenylarsonium-AuC1,- or safranine-AuC1,AuC1,ion pairs, dibutyl phthalate, in PVC, polymer 19 hexadecyltrioctylammonium dicyanoargentate, in 143d AgCN,nitrotoluene, with or without PVC; liquid or polymer 20 picric acid-thiourea-aurate or thiourea-argentate 106d Ag(SC(~H,),),+or Au(SC(NH,),); complexes in nitrobenzene, chlorobenzene or 1,2-dichloroethane; liquid 21 zephiraniine-thiocyanate ion pair in dichloroethane; SCN77d liquid 22 trihexylcet ylammonium thiocyanate in PVC ; SCN138d polymer 23 tri-n-octylmethylammonium thiocyanate in Urushi SCN 47d (an oriental lacquer); polymer-solid 24 various thiourea derivatives complexed with CuSCN SCN 63d in nitrobenzene or chloroform; liquid 25 tetradecylphosphonium hexafluorophosphate, PF,7 5d dibutyl phthalate, in PVC; polymer 26 tetradecylphosphonium trichloromercurate(I1) in HgC1,74d, 76d PVC (no plasticizer); polymer 27 complexes of neutral organophosphorus compounds NO,114d (e.g., tributylphosphine oxide) with uranyl nitrate in chloroform; liquid membrane 28 epoxy-based cation exchanger heterogeneous Caz+ 34d membrane (no information on exchanger); polymer 29 strontium salt of methylphosphonic acid monoSIz+ 38d isooctyl ester in organic solvent; liquid 30 complex of dodecyltriheptylammonium with InBr,141d indium tetrabromide in PVC; polymer 31 ethylviolet-picrate complex in PVC; polymer picrate 102d 32 various quaternary or phosphonium salts of 2,42,4-dinitrophenolate 24d dinitrophenolate, 2-nitrotrimethylsilylbenzeneas plasticizer, in PVC; polymer +
As the applications of existing ISFET devices for ion measurements continue to grow, interest has developed in the possibilities of fabricating FET based devices for sensing species other than common electrolyte ions. Indeed, ISFET based probes for the detection of dissolved gases have been
described (36e) and the prospects of developing chemically sensitive FETs for direct sensing of organophosphate molecules have been discussed (26e). Further, using a polymer membrane outer membrane containing a general anion exchanger species, Covington and associates developed a pheANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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ION-SELECTIVE ELECTRODES
Table IV. Some Additional Studies with and/or Applications of Ion-Exchanger Based ISEs no. electrode used study or application 1 nitrate in studies of nitrate uptake by barley plants determ of nitrate 2 in calcareous soils 3 in palladium electroplating baths 4 in drinking and mineral waters 5 in plant tissues 6 as thiocyanate sensor for measuring bacterial membrane potentials 7 calcium in titrimetric determ of aminocarboxylic acids, fluoride, oxalate, etc. deternz of calcium 8 in rocks 9 in human milk 10 determ of ionic association constants for CaZ+with various organic acids 11 determ of formation constants for calcium and magnesium phosphate complexes 12 to study affects of various electrolytes on rates of CaSO4-2H,O precipitation 13 picrate to study the stoichiometry and kinetics of Jaffe rxn (creatinine with picrate) 14 determ of atropine by titration with picrate 15 determ of N-cetylpyridinium ions by titration with picrate 16 tetraphenylborate responsive to measure membrane potentials of P. denitrificans cells 17 as detector in titrations of various organic cations (e.g., surfactants, drugs, etc.) with tetraphenylborate 18 perchlorate as periodate transducer for kinetic end point detection of titrations of polyhydroxy1 compounds with periodate 19 cholic acid responsive determ of cholic acids in pharmaceutical products chloramine-T responsive 20 in titration methods to detect catalytic effect of excess Ititrant on chloramine-T rxns crystal violet responsive 21 in 2-phase titrations of various phenolates with aqueous crystal violet titrant nonselective carboxylate and 22 to quantitate structure-activity relationships of various organic sulfonate anion responsive acids with respect to biological effects due to hydrophobic interactions 23 cationic and anionic surfactant in studying aggregation of homologous series of long chain responsive surfactants nobarbital responsive ISFET ( I l e ) . In addition the use of immobilized enzymes in conjunction with ISFET sensing was also recently demonstrated when workers prepared a microsize urea sensor by immobilizing urease at the surface of a pH res onsive ISFET (40e). &e relatively new application of ISFETs is their use as detectors in flowing streams. In connection with this, Haemmerli et al. (21e) reported on a flow system arrangement which may be used to evaluate the response times of microISFETs (or conventional microelectrodes). By flow injection analysis (FIA) measurements it was found that the response of such sensors is sufficiently fast so that they may be used to measure changes in ion activities in cells that occur on the time scale of a few seconds. Ruzicka and Ramsing (46e) published on the use of pH and calcium selective ISFETs as detectors for FIA determinations of pH and calcium directly in blood samples. Similarly, Sibbald and co-workers used a potassium selective ISFET as an on-line detector for the continuous measurement of potassium in blood W e ) . Of note in these last two papers was the relatively large potential drifts encountered with the ISFET detectors. While frequent calibration of the flowing systems or use of differential dual channel systems can eliminate some of the concerns about such drift, it is nonetheless disturbing that more detailed investigations as to the causes have not been made. Clearly, if, as often proposed, ISFET t e devices are to become useful as truly continuous monitors nonautomated systems where repeated calibration is not possible), than this problem of drift must be overcome. Since this drift is most often observed when the devices are used in real samples, e.g., blood, as a first step, perhaps a more thorough study of ISFET response to other species which may be found in these samples is warranted. Finally, in connection with conventional microelectrode systems, the application of these devices for intracellular studies continues to grow at almost an exponential rate. We have no intention of reviewing or even listing the vast number 34R
ANALYTICAL CHEMISTRY, VOL. 58, NO. 5, APRIL 1984
ref 18d, 19d 44d 37d 105d llld 82d 65d lld Id 21d 15d 94d 21d 4 3d 41d 68d, 69d 13d, 20d 2 6d
8d 127d 42d 48d 64d
of physiological and biological studies which have utilized such electrodes over the past 2 years. However, several such studies were flagged in our computer searches of the general ISE literature and a few representative examples are cited in Table V. More importantly for this review, there were several fundamental reports regarding the general use of microelectrodes as well as some new microelectrode systems and these should be mentioned here. For example, one paper focused on the major question concerning the use of micro-ISEs for intracellular studies (39e);that is, do the potentials observed with these electrodes yield accurate intracellular ion activity data? Obviously, as pointed out in this paper, the selectivity constants of the microelectrodes over other intracellular species are critical to their successful application. Having some idea as to the relative concentrations of potentially interfering ions within the cells and the appropriate selectivity constants for the given electrode to be used can aid in assessing the ultimate accuracy of the results obtained. Sodium and calcium liquid membrane microelectrodes based on neutral carriers (ETH 227, and ETH 1001) do indeed have adequate selectivity coefficients to be used routinely for intracellular studies (13e). The detection of intracellular calcium ion activity is of particular interest at this time and two additional papers focused solely on making this measurement with micro-ISEs (23e,59e). In another important paper, Lanter et al. (34e) critically evaluated the application of various calcium selective liquid membrane microelectrodes and their fabrication. They reported that various microelectrodes with tip diameters of less than 1pm often possess super-Nernstian response and have higher detection limits than larger devices, probably due to slow kinetics at the phase boundary of the liquid membrane and the sample solution. With a membrane phase consisting of the calcium selective carrier ETH 1001 ([R-(R*,R*)13,17,18,22-tetramethyl-14,21-dioxo-16,19-dioxa-l3,22-diazatetratriacontanedioic acid, diethyl ester) sodium tetraphenylborate, and PVC, plasticized with nitrophenyl octyl ether, micropipet electrodes with tip diameters of less than
ION-SELECTIVE ELECTRODES
Table V. Additional References Concerning Development and Applications of CWEs, ISFETS, and Microelectrode Devices ref CWEs no. 1
2 3 4 5 6 7 8
9 10 11
12 13 no. 14 15 16 17 18 19 20 no. 21
22 23 24 25 26 27 28
general report on multisensor probes based on CWE technology new sodium and potassiuni selective CWEs based on novel bis(crown ethers) CWEs based on dinonylnaphthalene sulfonic acid for determ of basic drugs (i.e., methadone, cocaine, etc.) bismuth(II1) responsive CWE based on Aliquat-BiC1,- complex in PVC CWE for determ of actinyl(V1) cationic species based on N,N'-diheptyl-N,N',6,6tetramethyl-4,8-diaxauddecanediamidein PVC with l-chloronaphthalene as plasticizer various miniature potassium CWEs based on valinomycin; CWEs using internal Ag/AgC1 wires are suggested chloride selective CWE based on trioctylammonium chloride in epoxy membrane coated onto Ag/AgCl electrode various anion responsive CWEs based on Aliquat-336 various patents on CWE type devices titr determ of anthraquinone dyes with CWEs responsive to quaternary ammonium titrants titr determ of triarylmethane dyes by titration with tetraphenylborate and a CWE responsive to cationic dyes determ of sodium and potassium in urine with bis(crown ether) based CWE described in item 2 determ of alkaline earth ions in dimethyl sulfoxide using copper electrode coated with gelatin membrane ISFETs use of pH ISFET as detector for microprocessor-based coulometric pH stat F - and Cl- selective ISFETs as detectors in potentiometric titrations of C1- and F - in electroplating baths new dielectric layers for pH sensitive ISFET devices new reference FET for use with ISFETs miniature pH and pC0, ISFET devices patent on combination type ISFETs (e.g., ref and ion-sensing in one probe) misc patents on ISFET devices microelectrodes determ of calcium with micro-liquid membrane ISEs in frog skeletal muscle fibers in squid giant axons in intact lenses (using ligand ETH 1001) improved solid-state micro-Ag/AgCl electrode for intracellular measmt of C1improved liquid membrane ion exchanger based chloride microelectrode using Corning 477913 exchanger comparison of chloride liquid membrane microelectrode vs. other methods for intracellular determ of Clcomparison of glass micro-pH electrodes vs. other methods for intracellular pH measurements construction and results with new double barreled intracellular pH microelectrode measmt of pH in plant vacuoles with micropipet type liquid membrane pH electrode based on tridodecylamine carrier construction and evaluation of various acetylcholine and choline micropipet type liquid membrane electrodes
1 pm can be fabricated which have detection limits on the order of mol/L and selectivity coefficients nearly equivalent to macrosized electrodes prepared with the same membrane phase. The authors suggest that the addition of PVC to the membrane phase makers a substantial difference when fabricating the very small electrodes. In one final paper concerning new microelectrode designs, Haaemmerli, Janata, and Brown at the University of Utah continued to study their unique concept of mounting conventional liquid membrane micropipet type electrodes directly on the gate of FETs. This approach supposedly reduces the noise associated with the overall micro-ISE assembly and in their most recent work (20e),they examined the electrical characteristics of potassium and chloride microelectrodes of this type. GAS SENSORS AND SELECTIVE
BIOELECTRODE SYSTEMS We review here literature related to the introduction, development, and application of potentiometric gas-sensin membrane electrodes, biocatalytic membrane electrodes, and other potentiometric membrane electrode systems coupled with biocatalyzed reactions. Gas Sensors. Several new potentiometric gas sensor designs have been proposed for carbon dioxide measurements. Meyerhoff and co-workers (595)introduced a polymer-membrane electrode-based potentiometric carbon dioxide gas
17e 5 6e 12e 3e 7e 19e 24e 54e 18e, 30e, 62e, 63e 61e 60e 57e 42e ref 6e 2e 44e 55e 16e 4e 31e, 32e ref 37e 15e 2 5e 35e 5e le 10e 14e 33e 27e
sensor in which a carbonate ISE is employed as the internal sensing element. Alternatively, these researchers have introduced and developed gas sensors fw carbon dioxide and ammonia in which a neutral carrier-based polymer membrane pH electrode serves as the internal sensing element. Both electrode designs take advantage of the low cost and durable nature of the polymer membranes as opposed to the conventional glass membrane based gas sensors. Moreover, Meyerhoff et al. (60f) have reported theoretical considerations to explain the steady-state response characteristics of polymer membrane electrode-based gas sensors in general. Sensor slopes, detection limits, linear ranges, and selectivities were shown to be a function of the internal electrolyte pH, ionic strength, and equilibrium constants. Predicted response characteristics for carbon dioxide and ammonia gas sensors based on carbonate and ammonium internal sensing elements, respectively, show excellent agreement with experimentally observed values. An interesting approach to liquid and gaseous sample carbon dioxide measurements has been reported by Scarano and co-workers (73f) in which a carbon dioxide gas-sensing membrane electrode is positioned in the line of a carrier gas stream which contains the sample. In this arrangement the sample is blown onto a gas-permeable membrane and a pH change of the internal electrolyte is monitored with time by a glass pH electrode. Carbon dioxide concentrations as low ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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ION-SELECTIVE ELECTRODES
as 1.5 X M are reported to be easily quantified. Fundamental electrode response characteristics and theoretical considerations were presented. Future studies with this electrode arrangement should be dedicated to improving the rather long electrode recovery times. In addition, a new carbon dioxide gas sensing membrane electrode was described in which a novel pH sensing film was used to monitor internal electrolyte pH changes (68f). This pH sensing film was composed of Si3N4,A1203,and/or Taz06 deposited on an electrical conductor. Because transcutaneous carbon dioxide measurements can provide a simple, noninvasive method to continuously monitor pC0, levels from a bedside station, the development of reliable sensors and methods for such measurements is an active area of clinical research. Currently popular transcutaneous carbon dioxide sensors are similar to common carbon dioxide gas sensors with either a silicon rubber or microporous Teflon membrane separating the skin surface from a bicarbonate internal electrolyte layer and a combination glass pH electrode. For transcutaneous measurements, the sensor surface and affected skin area are typically heated to approximately 44 "C to enhance pCOz permeation. Wimberley and co-workers have recently shown that transcutaneous carbon dioxide measurements correlate quite well with capillary pC0, levels in healthy adults (94f). Moreover, these researchers conclude from their studies that rapid capillary pC02 changes can be reliably monitored. Kost and co-workershave evaluated such sensors for neonate capillary (47f) and arterial (46f) carbon dioxide levels. In these latter studies, a glycerol-water based internal electrolyte was employed which resulted in enhanced electrode response times in comparison to the more common water-based internal solutions. Results indicate that the trauma of common sampling procedures can induce significant transient fluctuations in blood as tensions. Such fluctuations may decrease the clinical reliaklity of conventional capillary pC0, values (47f). Also, Kost et al. (46f) have suggested that continuous transcutaneous pC0, measurements on neonates only be used to follow short-term trends in arterial pC0, levels. Overall, the reliability of transcutaneous pC0, measurements has not been well established. The optimal location for these types of clinical measurements has not yet been determined for adults or infants (42f). Future studies must aid in resolving these important questions. Recently presented clinical trials are a promising sign (6lf). In view of the great interest in this area, numerous patents have appeared concerning the fabrication of various transcutaneous carbon dioxide sensors (40f, 67f, 83f-85f). Fundamental studies to characterize gas sensor dynamic response behavior have appeared for carbon dioxide and ammonia gas sensing electrodes. Buffle and Spoerri (9f) derived a mathematical expression to compute response times for any potentiometric-based gas sensor. Their equation was tested with respect to the dynamic response of a carbon dioxide as sensor and their experimental data justify the use o f t is equation. The data presented show that when diffusion of the gaseous species of interest through the membrane material becomes very fast, the ultimate limit in sensor response rate is governed by species diffusion within the internal electrolyte layer. Hence, internal electrolyte layer thickness and species diffusion coefficient within this layer are the dynamic response limiting parameters under optimal membrane conditions. In related work, Arnold has studied the effect of membrane composition on the dynamic response of ammonia gas sensors (3f). This report points out that the rate-determining step for many ammonia as sensor based procedures is not the sensor response time %utthe recovery time where the recovery time is the time necessary to reestablish the base line potential between samples. Results indicate that of the commercially available Teflon membranes, the 0.02 pm pure, microporous Teflon is the membrane material of choice. Surprisingly, results suggest that support polymer membrane pore size can alter sensor dynamic behavior. Two papers have appeared in which the ammonia gas sensor was evaluated with respect to its ability to accurately follow the rate of ammonia production from various reactions. Efstathiou et al. (20f) present a technique to calibrate the ammonia sensor under dynamic conditions when relatively slow response of the sensor is a problem (i.e., low sample ammonia concentrations). The calibration technique is based on a mathematical comparison of potential-time data obtained
a
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ANALYTICAL CHEMISTRY, VOL. 56,NO. 5, APRIL 1984
from the reaction of interest to potential time data obtained under identical conditions from a simulated reaction in which ammonia standards are delivered at an accurately known rate. This method was successfully applied to study the alkaline hydrolysis rate of nicotinamide. Similarly, Hato and coworkers (34f) have reported fundamental investigations concerning the application of the ammonia gas sensor to directly follow the rate of enzymatic reactions. These researchers have found that the slope of a total ammonia-nitrogen concentration vs. time curve is an accurate measure of initial rates for enzymatic reactions. Total ammonia nitrogen concentrations are obtained at certain time values from conventional ammonia calibration curves based on steady-state potential measurements. The success of this method depends on the presence of a specified amount of ammonia before the reaction begins. A specified background ammonia concentration is necessary so that the response is sufficiently rapid to accurately follow the generation of ammonia. This technique was applied to the in situ determination of serum leucine aminopeptidase by the same researchers (35f, 36f) and provides a simple means for the determination of enzymatic activites from potential vs. time data. Unfortunately, the technique is restricted to relatively high enzyme activity units (Le., greater than 3 I.U.) owing to the required ammonia background levels. The question of gas-sensingmembrane electrode selectivity has been recently addressed for both the carbon dioxide and ammonia gas sensors. A study reported by Kobos et al. (44f) showed how the selectivity behavior for the carbon dioxide gas sensor is dependent on the type of gas permeable membrane employed. When the commonly used silicon rubber membrane was employed, the sensor displayed a large response to nonvolatile organic acids, such as benzoic, cinnamic, and salicylic acids. This response was shown to arise from the solubility of the protonated acid in the membrane material. Microporous Teflon membrane based sensors were shown to discriminate against nonvolatile organic acids; however, poor selectivity over volatile organics and acidic gas was observed. Homogeneous Teflon membranes resulted in sensors with excellent selectivity for carbon dioxide but sensor response times were dramatically slowed. Overall, the membrane material of choice for carbon dioxide measurements depends on the anticipated interferences present in the sample. Studies by Fraticelli and Meyerhoff (Wf)as well as by Lopez and Rechnitz (54f) point out the problem of amine interference with the ammonia gas sensing membrane electrode. In their study, Fraticelli and Meyerhoff showed how the selectivity of the ammonia sensor can be improved significantly over volatile amines by replacing the pH sensing element with a nonactin-based ammonium selective polymer membrane electrode (25f). Lopez and Rechnitz, on the other hand, showed that amine basicity is more important than volatility in determining the selectivity behavior of ammonia gas sensors based on pH internal sensing elements (54f). Moreover, this latter paper introduces an equation which is derived from fundamental considerations that can be used to calculate theoretical selectivity coefficients for ammonia over interfering amines. Parris and Foglia (69f)have taken advantage of the ammonia gas sensor amine interference in their procedure to determine the concentrations of ammonia and volatile amines in meats. In other gas sensor work, a comparison study of four different hydrogen cyanide sensing air gap type gas sensors has been reported by Fligier et al. (23f). In this study, the dynamic and steady-state response characteristics of various hydrogen cyanide gas sensors were compared. Sensors based on silver, silver sulfide, silver iodide, and silver (sulfide-iodide) mixture internal sensing elements were studied in conjunction with an internal electrolyte composed of 0.1 mM dicyanoargentate(I), 0.1 M sodium tetraborate, and 1%(w/w) methylcellulose adjusted to pH 9.3. Results show that the iodide containing electrode systems display near conventional Nernstian slopes, whereas the other two gas sensors display the expected double-Nernstian slope. These researchers demonstrated via common ionic equilibria expressions that the presence of iodide should indeed lower the slope to a normal Nernstian value of 60 mV per concentration decade. Table VI summarizes some of the applications which have been reported for ammonia and carbon dioxide gas sensors. Many applications include gas sensors in conjunction with
ION-SELECTIVE ELECTRODES
Table VI. Gas-Sensing Membrane Electrode Applications ref electrode type analytical application ammonia
carbon dioxide
determ of ammonia in various types of water effluent water seawater meat tissue industrial samples electroplating baths methanogenic sludge culture broth and filtrate serum determ of ammonia released from rubber caps total nitrogen determination in wines products of microbial synthesis determ of nitrite in aqueous samples determ of &-aminohydroxy compounds determ of drugs having a carboxyamide group carbothionamido group analysis of nicotinamide hydrolysis in alkaline solutions determ of leucine aminopeptidase kinetic determination of trypsin determ of carbon dioxide in water blood con tin uous measurement of total organic carbon in water microbiological assay of gentamicin, streptomycin, neomycin microbiological assay of tetracycline determination of nicotinamide adenine dinucleotide and glutathione reductase
llf
65f 82f 69f 81f 58f 52f 6f 15f, 16f, 36f 49f, 50f
26f 22f 13f 64f 86f 87f 20f 35f, 36f, 75f 7 6f
12f If
72f 79f 78f 31f
_ _ I
biocatalyzed reactions to quantify either the enzyme, substrate, cofactor, prosthetic group, inhibitor, or activator. Seegopaul and Rechnitz have detailed methods for the enzymatic determination of thiamine pyrophosphate (74f) and the kinetic determination of the proteolytic enzymes leucine aminopeptidase (75f) and trypsin (76f). Moreover, Hassan and Rechnitz (31f) have described methods for the determination of nicotinamide adenine dinucleotide phosphate (NADP+) and glutathione reductase by usin a carbon dioxide gase sensor with the enzyme malate fehydrogenase (decarboxylating). An enzyme cycling approach was employed which results in extremely low detection limits. Also, the carbon dioxide gas sensor has been employed to determine vitamin BB (pyridoxal phosphate) using the enzyme tyrosine decarboxylase (70f)and to characterize the enzyme carbonic anhydrase (7f). An interesting extention of the ISE-biocatalyzed reaction approach is detailed in two papers by Simpson and Kobos (78f, 790 in which assay procedures for numerous antibiotics are described. In these procedures, antibiotic inhibition of carbon dioxide production from suspended bacterial cells is utilized for antibiotic quantification. A decrease in carbon dioxide is measured after a fixed time by a potentiometric carbon dioxide gas sensor and is related to the antibiotic concentration in a logarithmic fashion. This technique is essentially a
microbiological assay procedure; however, instead of requiring 1to 2 days, as in conventional microbial assays, the proposed procedure requires only 1 to 2 h. This analysis time enhancement is the principal advantage of this method and is possible because the new method monitors microbial metabolic changes while conventional microbial assays monitor cell growth changes. Potentiometric based microbial assays for tetracycline (78f),gentamicin, streptomycin, and neomycin (79f)have been described. Other researchers recently have proposed similar assay procedures using either amperometric-based sensor or glass pH electrodes (450. The use of potentiometric membrane electrodes to monitor enzyme labels for homogeneous and heterogeneous immunoassay procedures is a rapidly growing area. The major advantages of potentiometric systems, which includes the insensitivity to sample turbidity, encourages the development of potentiometric membrane electrode-based enzyme immunoassays. Gebauer and Rechnitz (270 have reported a comparison study which evaluates a number of deaminating enzymes as possible immunoassay labels. The enzymes studied included adenosine deaminase, asparaginase, and urease. Ammonia gas sensors were employed to quantify these enzyme labels. Asparaginase was found to be the most effective label when binding ability, retained activity, and stability properties are considered for dinitrophenyl-enzyme conjugates. Moreover, these researchers described a cortisol immunoassay procedure in which aspara inase is employed as the enzyme label. In addition, Alexanjer and Maltra (2f) have reported the use of a fluoride ISE to detect the enzyme label horseradish peroxidase when p-fluoroaniline is used as the enzyme substrate. No interference in enzyme detection is noticed from serum matrix components. Using a similar approach, Mascini et al. (570 have demonstrated that acetylcholinesterase can be used as an immunoassay label using a pH electrode as the detector. Other enzyme-ISE systems which are not mentioned in Table VI include the enzymatic determination of nicotinamide adenine dinucleotide phosphate (33f) and glutathione (32f) by Hassan and Rechnitz using the enzyme glutathione reductase in conjunction with a silver sulfide membrane electrode. A separate assay procedure for the enzyme glutathione reductase in the concentration range from 0.4 to 4.0 mIU/mL was also detailed (32f). Similarly, Siddiqi details an assay procedure for peroxidase and peroxidase-coupled reactions based on a fluoride ISE (77f). In this study, 26 possible fluorine-containing compounds were evaluated as suitable analytical substrates for peroxidase. Finally, pH changes during enzymatic reactions are the basis for numerous assay procedures including those for urea (88f), acetylcholinesterase (62f), and other biochemicals (55f). Biocatalytic Membrane Electrodes. A number of overview-type articles have appeared reviewing recent advances in the area of biocatalytic membrane electrodes (Sf, 17f, 71f). Rechnitz (71f) points out the major processes common to all potentiometric sensors with an emphasis on bioanalysis. Recent applications and future prospects for bioanalysis with membrane electrodes of various types are also discussed. Bowers, on the other hand, provides an overview of immobilized enzymes in chemical analysis which includes enzyme electrodes (8f). In this latter paper, the response mechanisms of both potentiometric and amperometric-based enzyme electrodes are briefly discussed. Numerous papers dealing with the theory of biocatalytic membrane electrodes have appeared. Hameka and Rechnitz (30f) presented a theoretical interpretation of the mechanism of such sensors with an emphasis on the time dependence approach to the steady-state potential. They concluded that the enzyme layer thickness and the effective solution substrate diffusion constant are important parameters in determining electrode response times. This latter parameter was directly related to the degree of external solution stirring. According to this study, the response time appears to be dependent on the bulk solution substrate diffusion coefficient as opposed to the enzyme layer substrate diffusion coefficient. Jochum and Kowalski (41f) presented a new model for enzyme electrodes in which two compartments represent the enzyme layer. One compartment contains the enzyme while the second is a solution layer which separates this enzyme layer from the electrode surface. Essentially, the enzyme layer compartment is described by enzyme kinetics and diffusion processes and ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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ION-SELECTIVE ELECTRODES
Table VII. Some New Potentiometric Biocatalytic Membrane Electrodes detected membrane electrode measd species species type acetylcholine n-acetyl-L-methionine arginine arginine creatinine guanine histidine L-lysine penicillin penicillin pyruvate tyrosine tyrosine urea urea urea urea urea urea
H' 3" 3 "
3" 3" 3 "
CO, CO,
H+ "
COZ
CO, COZ
H+ H' H'
NH,' 3 " 3"
glass
gas sensing gas sensing gas sensing gas sensing gas sensing gas sensing gas sensing glass glass gas sensing gas sensing gas sensing glass glass iridium oxide polymer gas sensing gas sensing
the second compartment is governed by simple molecular diffusion. This two-compartment model avoids limiting assumptions and represents experimental reality quite well. Finally, with respect to theoretical models, Arnold and Rechnitz (5f) have tested the frequently employed assumption that biocatalytic membrane electrodes are nondestructive devices. These researchers showed that under certain conditions a considerable amount of substrate can be consumed in a relatively short time period. These resulta are important for future theoretical models and for future studies involving in vivo applications of biosensors. Two papers have appeared which discuss new methods of enzyme immobilization at electrode surfaces. Mascini and co-workers (56f) described a new procedure for coupling enzymes with nylon meshes for improved enzyme electrodes. The rocedure is relatively quick and simple and can be appliezunder mild conditions. Another immobilization procedure for enzyme electrodes was detailed in a German patent (93f).
Because the Jaffe method lacks selectivity and the measurement of creatinine is critical in clinical laboratories, considerable effort has been placed on the develo ment of creatinine-selectivebiosensors Guilbault and Coulet &9f) have reported two creatinine sensors where the bacterial enzyme creatinine deiminase is immobilized at the sensing surface of an ammonia gas sensor. These electrodes seem to be quite stable with response times ranging from 2 to 10 min. Most importantly, these electrodes are sufficiently sensitive to detect normal creatinine levels in serum. One problem with such a biosensor system is background ammonia particularly if applied to urine samples. The replacement of isolated enzymes with other, more crude, biocatalytic preparations is one direction of biosensor development which has received considerable attention. Di Paolantonio and Rechnitz demonstrated the enhanced stability which can be attained by using intact bacterial cells as the biocatalytic layer (19f). In this study, a pyruvate biosensor was developed and the response properties were determined. Results indicate that proper electrode storage can si enhance electrode lifetimes. Indeed, the detaile bacterial electrode was shown to last considrably longer than electrodes prepared with an isolated enzyme, a dual enzyme-cofactor mixture, or plant tissue slices as biocatalysts. Moreover, these same researchers demonstrated that biochemical induction of an enzymatic activity within a bacterial strain can be a powerful method for preparing bacterial electrodes ( l 8 f ) .An electrode for tyrosine was described in which the biocatalytic activity of interest was induced to practical levels within the bacterial cells by adjusting the growth medium. Furthermore, it was demonstrated that selective inhibition of interfering activities can be employed to enhance electrode selectivity to a practical level. Often bacterial electrodes suffer from the problems of low slopes and rather long response times owing to insufficient amounts of biocatalytic activity a t the electrode surface.
biocatalyst
ref
acetylcholinesterase acylase/L-amino acid oxidase arginaselurease streptococcus lac tis creatinine deiminase rabbit liver lactobacillus 30a extract lysine dicarboxylase penicillinase penicillin amidase Streptococcus faecium L-tyrosine decarboxylase Aeromonas phenologenes urease urease
10f 65f 66f 28f 29f 4f 48f 90f 51f 51f 19f 37f 18f 91f 5 3f 39f 38f,89f 21f,24f 92f
urease
urease urease bacteria
Kovach and Meyerhoff (48f) have shown that the use of a concentrated enzyme extract from bacterial cells can eliminate these problems. These researchers developed a highly selective histidine biosensor using this strategy and have shown that the enzyme extract is considerably better than the intact bacterial cells with respect to many electrode response characteristics. Once again it was demonstrated that isolated enzymes are not always necessary to prepare highly selective biosensors. In other biosensor work, an optimization strategy for tissue-based biosensors was proposed by Arnold and Rechnitz ( 4 f ) . This strategy was demonstrated through the development of a guanine selective electrode where a slice of rabbit liver is immobilized at the surface of an ammonia gas sensor. The strategy involves optimizing parameters which affect substrate diffusion and biocatalytic activity at the electrode surface. Moreover, a selectivity enhancement strategy was illustrated for tissue electrodes in general and the resulting sensor was shown to compare favorably to an isolated enzyme-based guanine biosensor. Finally, the development of polymeric membrane electrodes that respond to large binding proteins such as antibodies continues. Keating and Rechnitz report an electrode which responds selectively to cortisol antibodies and detects this antibody at ng/mL concentration levels (43f). Collins and Janata (14f) have critically evaluated the potential response mechanism for these types of electrodes, particularly ones based on ISFET technology. Their studies provide strong evidence that the interaction of proteins with the membrane surface changes the interfacial potential by altering the ionexchange process to give a mixed potential. Hence, it seems that the electrode response to small inorganic ions is modified in some way by antibody binding to the electrode surface. Earlier work by Solsky and Rechnitz support this conclusion @Of).
ION-SELECTIVE ELECTRODES IN FLOW-THROUGH ARRANGEMENTS AND CLINICAL ANALYSIS SYSTEMS
YfiCantlYIn this section we review developmentswith regard to ISEs
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
in automated arrangements (e.g., continuous flow, flow injection, chromatographic systems, etc.) as well as in biochemical and clinical analysis methodologies. While most of the clinical applications utilize flow-through electrode designs, several do not, and such static electrode systems will also be covered here. Indeed, a variety of new membrane electrodes specifically formulated for biomedical work will be cited. With respect to ISEs in nonclinical flow systems, one of the more exciting developments involves the use of ISEs as detectors in chromatography. Several interesting papers have recently appeared. Contrary to the usual requirements that ISEs have high selectivity, their use as detectors in ion chromatographic systems mandates just the opposite. For example, Suzuki and co-workers (94g)utilized coated wire type
ION-SELECTIVE ELECTRODES
PVC membrane electrodes made with valinomycin, nonactin, tetranactin, and benzo-15-crown-5 as detectors for ion-exchan e chromatography of monovalent cations, e.g., Li+, Na+, NH., 8,K+, Rb+, and Cs+). On elution with dilute nitric acid, detection limits for the separated ions varied depending on the response of each detector toward the individual ions. The tetranactin based electrode, being the least selective,displayed the greatest sensitivity to the widest range of ions in a given sample mixture. In another example Hershcovitz et al. in Israel (49g)found that plain silver wires coated with sparingly soluble silver anion salts were more sensitive detectors for ion chromatography of halides and thiocyanate than conventional conductometric detectors. As with classical solid state ISEs, detection limits toward the various anions depended on the particular salt coated on the wire and the solubility product of the anions with silver. Similarly, workers in Japan reported the use of commercial chloride and bromide solid-state ISEs as ion chromatography detectors for the simultaneous determination of bromide and chloride in natural waters (Ig). Again, nitric acid served as the eluent. Recognizing that detection in chromatography requires detectors with minimal dead volumes and that this characteristic is an even greater prerequisite when dealing with modern micro open tubular columns, Manz and Simon (68g) proposed the use of a micro-liquid-membrane electrode as a detector for alkaline metals and quaternary ammonium compounds after separation on silanized and unsilanized silica capillary columns. By use of a micropipet electrode with a tip diameter of only 1pm, filled with the general cation exchange solution of tetra-p-chlorophenylborate in 2,3-dimethylnitrobenzene, detection of picomole amounts of some of the ions was possible. While the open tubular column used did not have sufficient efficiency to separate a complex mixture of the cations, these workers showed that the effective dead volume of their microelectrode detector was, amazingly, on the order of 500 pL. Clearly, in view of the large number of nonselective liquid membrane electrodes already prepared for various classes of organic compounds (see Cosofret’s book cited in the first section of this review, ref 7a) it seems likely that such electrodes could fiid wider applicability as simplified microdetectors in an array of chromatographic systems. Novel flow-through arrangements for ISEs continue to be examined. Meyerhoff and Kovach (76g) detailed the procedures necessary to prepare a simple tubular polymer membrane electrode by developing an ISE-flow injection analysis teaching laboratory experiment. Tubular potassium selective electrodes were prepared by replacing the wall of a narrow (e.g., 0.8 mm i.d.) piece of Tygon tubing with PVC-valinomycin membrane. The tubular electrodes may be used as detectors for FIA measurements of potassium in serum. In a related paper, Degawa et al. (25g) studied the response time of such tubular polymer electrodes (ammonium selective membranes in this case) by using a pseudo-stop-flow-type arrangement. Increasing flow rates yielded faster steady-state response times (as fast as 50 ms). Two theoretical models were used to fit the data: one based on the diffusion rate of the ion in the stagnant layer adjacent to the membrane, and one based on the rate of ion mi ation in the polymer membrane. Data fit the former model l%st, at least for the time response of the electrode over the first 50 me. Thus, it is presumed that faster flows improve response times of such electrodes by decreasing the thickness of the stagnant layer. The use of ISEs as detectors for continuous flow gas measurements has also received considerable attention. Bhargava and Gmitro (9g)designed an automated system for the continuous monitoring of H2Sin coke oven gas. Analyte gas in the sample stream was continuously absorbed into a stream of NaOH to form free sulfide ions which subsequently flowed to a flow-through solid-state sulfide electrode. Gas dialysis cells in flow systems have also been used in conjunction with ISEs for gas analysis. One patent (75g)describes a continuous monitor for cyanide based on the detection of HCN from acidified sample with a gas permeable tube, a reci ient stream of NaOH, and a flow-through cyanide electro%e. Similarly, Marhsall and Midgley (70g) reported the use of a HgS/HgCl solid-state electrode to measure sulfur dioxide chemically liberated from the desired analyte, sulfite, in water samples. The solid-state membrane responds to both chloride and sulfite ions an thus could not be used to directly measure sulfite in waters due to the significant chloride in-
terference. In their final flow-injection arrangement, the sulfite in the sample is converted to SO2with acid. The SOz formed in the sample stream then diffuses across a gaspermeable membrane of a dialysis chamber into a recipient flowing stream of dilute acid in which sulfite is re-formed and detected as this stream flows over the surface of the solid-state membrane electrode. Static (nonflow) air-gap type electrodes for SO2were also prepared by using this detection principle. As flow-injection analysis (FIA) techniques become more and more popular as a means of automation, the use of ISEs as detectors in such systems also grows. A review of FIA methods as applied to clinical chemistry cited several references and examples of ISE-based systems (86g). Interestingly, Simpson and Holler (92g) reported a novel FIA instrument for titrimetric determinations using a flow-through glass pH electrode and resulting peak width data as the quantitative index of concentrations. Other interesting systems reported included the use of multiple flow-through ISEs in se uence for the simultaneous determination of several ions (K7, Na+, Ca2+,and Cl-) in serum (101g),the use of a simple Ag/AgCl wire as a detector in a chloride-selective FIA system (98g), the use of solid-state chloride and cyanide electrodes in FIA arrangements particularly for the measurement of chloride in blood (79g), and a fully automated FIA instrument for performing fluoride determinations based on a flow-through fluoride selective electrode (1OOg). An important theoretical paper concerning theuse of ISEs in FIA arrangements also appeared (99g). This paper dealt with the limitations of the linear response for ISEs in FIA systems using the chloride, iodide, fluoride, and copper selective electrodes as models. The authors attempted to explain why dynamic measuring ranges for ISEs in FIA systems are always much poorer than when the same electrodes are used in classical batch modes. Obviously dispersion of the sample within the FIA network will always make detection limits higher for FIA methods vs. batch methods. However, the authors also concluded that the concentration yielding the smallest peak height which is still on the linear portion of the calibration curve is governed by the mechanism determining the detection limits for the electrode in general, e.g., solubility of the membrane material, surface adsorption, or contamination. These conclusions fit well with the bulk of the existing data on the use of ISEs in FIA systems. Aside from FIA systems, ISEs continue to be widely used in classical continuous flow type arrangements (e.g., Technicon autoanalyzer type). For example, chloride produced from the thermal degradation of poly(viny1 chloride) was continuously monitored via an autoanalyzer equipped with a flow-through solid-state chloride electrode (105g). Others have reported ISE-based continuous flow arrangements for the the detection of chloride (77g) and thiocyanate (60g) in wastewaters, the detection of a wide range of ions with a multipurpose automated system based on interchangeable ISE flow-through modules (56g),and the measurement of copper(I1)in corrosive industrial samples by using a specially designed high volume (3000 mL/min) flow cell and a solid-state copper electrode (12g). The concept of applying the Grans addition method for ISE determinations in continuous flow systems was cleverly demonstrated by Landry and co-workers (63g) using fluoride as the analyte. In their system, three different standard solutions of fluoride ion are added to the sample stream at timed intervals and known flow rates. The resulting spiked samples then flow through a fluoride electrode where peak potentials are observed for the three successively spiked portions of the sample along with a prior nonspiked sample. By use of the appropriate Grans addition equations and by consideration of dilution of the original sample as a result of the addition of the standard solutions, an accurate measurement of the fluoride content of the original sample is possible (f5%). However, the additional pumps and timing circuits required for addition of the spiking standards make the resulting apparatus much more complex, and in view of the only average precision obtained, it would appear that a normal continuous flow method with intermittent calibration would offer comparable results, at least in the case of fluoride measurements. The determination of biochemical species with ISE-based continuous flow systems, particularly with regard to clinical analysis applications, has also been the focus of considerable attention. Mascini and co-workers in Italy have reported the ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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ION-SELECTIVE ELECTRODES
use of immobilized creatininase enzyme (immobilizedin a flow reactor tube) in conjunction with a flow-through ammonia electrode for the automated determination of creatinine in blood (73g) and urine (72g) samples. The immobilized enzyme converts creatinine in the sample to ammonia and Nmethylhydantoin. Background endogenous levels of ammonia in the samples must first be measured without the enzyme and these values are then subtracted from the total ammonia detected after the enzymatic process in order to accurately determine creatinine levels. This problem of background ammonia in physiological samples was subsequently addressed in a paper by Meyerhoff and Fraticelli (37g). These workers demonstrated that even extremely elevated amounts of endogenous ammonia in plasma samples could be almost completely removed within an automated flow arrangement by first passing the sample through a 3-m on-line gas dialysis tube composed of microporous Teflon. Following such treatment, the sample may be mixed with an ammonia-liberatingenzyme and the total ammonia then roduced can be detected downstream with a flow-through&E-based ammonia detector. As a model system, the direct determination of asparagine in plasma samples via the use of asparaginase enzyme was performed. In another paper involving enzymes, Mascini and Palleschi described a novel flow-though detector for the simultaneous determination of glucose and urea in serum samples (74g). Dual enzyme electrodes in which urease and glucose oxidase were immobilized on the surfaces of ammonia and oxygen electrodes, respectively, served as the detectors. Several papers reported biochemical assays without the use of enzymes. Diamandis and co-workers in Greece have cleverly adapted a flow-through liquid membrane periodate selective electrode to the determination of glucose in serum (26g) and sucrose in natural or industrial products (29g). Reaction of the reducing sugar with the strong oxidizing reagent, periodate, results in a decreased amount of that ion in the flow-through network. Thus,peak electrode potentials observed correspond to loss of periodate from the reaction stream and are directly proportional to the concentration of the sugar in the sample. Unfortunately, for measurements of glucose in serum, the presence of other reducing sugars or carbohydrates may yield falsely elevated values. In similar work, the Greek group also developed an automated method for creatinine determinations in urine samples based on the selective reaction of picrate ions with creatinine (the so-called Jaffe reaction) and a flowthrough picrate selective liquid membrane electrode (28g). This same analytical reaction scheme was incorporated into a clinical chemistry experiment for undergraduates in which creatinine in urine is determined kinetically with a static type picrate selective electrode (27g). With regard to direct ion measurements in clinical samples, whether automated or manual, a host of fundamental and practical papers have appeared. For starters, Simon's group in Zurich has now demonstrated that a previously described polymer membrane pH electrode (based on the neutral carrier tridodecylamine) can be used directly to measure the pH of whole blood samples (3g). Correlation with the conventional glass electrode method was excellent and it now seems likely that such electrodes will eventually replace the more costly glass electrodes in commercial biomedical instruments. To enable the accurate measurement of blood pH by any electrode, a reference method was detailed by Maas et al. (66g) in which NBS buffers (pH 6.836 and 7.392at 37 "C)were used. The proper protocols for sample handling and measurement were also provided. Back in Zurich, Simon's group further reported on the use of an improved sodium selective liquid membrane electrode for the measurement of sodium in undiluted serum and urine samples (4g). The electrode was based on the neutral ligand, l,l,l-tris(2-oxa-4-oxo-5-aza-5methyldodecany1)propane. Results on undiluted serum and urine samples correlated quite well with flame photometry values after correction for the volume occupied by proteins and lipids in the serum. For workers who eventually would like to use this or other ISEs in vivo, a new reference electrode specifically designed for such purposes has been develo ed (69g). The electrode consists of a conventional Ag/A C1 [alf cell, Rin ere solution as an internal electrolyte, an hemocompata%le poly(hydroxyethy1 methacrylate) ( HEMA) as an outer membrane making contact with the Blood. The resulting reference catheter was stable to +0.92 mV for 6-8 h during animal trials.
d
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In a particularly important paper directed toward the clinical chemistry community, Koch and Ladenson (59g) presented their findings that a low molecular weight anion component of certain urine samples can interfere with valinomycin-based PVC membrane electrodes yielding erroneously low potassium values when such electrodes are used in undiluted samples. Simon and his fellow investigators had previously observed these results and ascribed them to a hydrophobic anion present in urine which can partition into the PVC-valinomycin membrane reducing the amount of positive surface charge. Unfortunately, Koch and Ladensen's paper does not identify this anionic species. The entire problem can be solved by replacing the PVC with silicone rubber. Indeed, in two separate papers (2g, 5g), the Swiss group demonstrated that silicone rubber-valinomycin membranes yield values in close agreement with flame photometry for potassium measurements in all undiluted urine samples tested. Apparently, in switching to silicone rubber as the principal membrane material, the partition coefficient of the unknown anion component into the membrane is greatly diminished. The controversy over the various factors which may influence values obtained for direct ISE measurements of cations, particularly sodium, in undiluted blood still rages. The fact that sodium values are often observed to be less than the 7% higher values (compared to flame photometry methods) predicted due to the volume occupied by protein and lipids in undiluted blood, fuels this controversy. Clearly, differences in liquid junction potentials between standards and the actual blood samples are a major factor and several papers have looked at this problem (19g,23g, 24g). Eurythrocyte effects on the junction potential appear to be most pronounced in instruments that use static types of liquid junctions (log,91g). Other reports suggest that sodium ion binding to bicarbonate may account for the decreased sodium values obtained in both undiluted serum and urine samples (16g,42g, 43g). However, in one paper (57g),sodium binding to all the components of serum was extensively studied by both electrode and NMR techniques and it was shown to be on the order of only 1%. Regardless of the small discrepancies as to what should be the true value reported, it is clear that direct ISE determinations of sodium in blood yield more relevant values than corresponding flame emission methods in instances when patients have certain disease states including multiple myoloma (62g). Indeed, several studies report on the differences in values obtained by direct ISE techniques vs. flame methods for samples containin various levels of lipids and proteins (8g, 40g, 61g, 97g). &e additional factor that can affect sodium levels reported is the presence of lithium ions, often used as an antipsychotic drug. In one report (48g), normal doses of lithium were shown to cause positive errors in blood sodium results obtained on several commercial ISE-based electrolyte analyzers. Similarly, certain organic drugs were also shown to cause slight positive biases for sodium measurements with some instruments (81g). There are already numerous commercial ISE-based electrolyte analyzers on the market and seemingly new ones appear each month. Many reports concerning the evaluation of these instruments as well as the situations which may cause erroneous results with them have been published. References to some of these reports are provided in Table VIII. Also listed are some studies with noncommercial electrolyte systems and some general articles about making ISE-based clinical measurements of electrolytes (particularly calcium). As can be seen from Table VI11 and the above discussions, the detection of cationic electrolytes in physiological samples is now routine, using either solid-state glass electrodes (for H and Na+) and/or polymer membrane electrode systems lased on ion exchangers or neutral carriers (for Na+, K ,Ca2+, and Li+). Unfortunately, there are few ISEs which may be utilized directly in undiluted biological samples for desired anion determinations (e.g., HCO,, C1-, PO4*, etc.). Even when there are ap ropriate electrodes for a gwen assay, interferences can be a pro!lem. For example, it was recently suggested that chloride ions interfere with the measurement of iodide levels in urine samples when utilizing a solid-state type iodide electrode (Ag2S/AgI) (18g). At average concentrations of the two anions in urine, a +5% error results in the iodide value. Similarly, chloride determinations in blood samples are now most often made with a simple Ag/AgCl electrode; however,
ION-SELECTIVE ELECTRODES I _
-
~-~
Table VIII. Summary of Reports Evaluating or Describing Commercial and Noncommercial Electrolyte Analyzers and Some Additional General Papers on Electrolyte Measurements with ISEs instrumentation no. 1 eval of Kodak Ektachem ISE slide system 2 description and principles of Kodak Ektachem ISE systems
3 eval of NOVA-2 and Orion 55-20 Caz+analyzers 4 eval, bias, and interferences with NOVA-2 Caz+analyzer analyzer 5 eval of NOVA-6 Na+/K+/Ca2+ 6 eval of NOVA-7 total and ionized calcium instrument 7 comparison of ionized calcium by three instruments: Orion 55-20, NOVA-2, and Radiometer A/S 8 eval of Radiometer Caz+and pH instrument 9 a noncommercial combination ISE for simultaneous determ of K + , Na+,C1-, and Caz+in serum 10 eval of Du Pont ACA electrolyte analyzer 11 eval of Ionetics manual K+-ISEsystem 1 2 eval of Beckman E4A electrolyte analyzer 1 3 eval of Bectin Dickinson QEA Na+/K+analyzer 14 K+ interference in urine Na+ values with Beckman Electrolyte-2 analyzer 15 description of Beckman Electrolyte-2 analyzer 1 6 drift and inadequate sampling in Beckman ASTRA-8 17 eval of NOVA-1 Na+/K+analyzer 18 eval of Corning 902 Na+/K+analyzer 19 eval of IL 502 Na+/K+analyzer 20 comparison study of four electrolyte analyzers (SMAC, ASTRA-8, AMT-721, IL 343) 2 1 eval of Technicon RA-1000 Na+/K+/CO,system 22 Instrumentation Laboratories patent on disposable K + electrode no. general 23 24 25 26 27
28 29
ref
effect of sample freezing on ISE-based Ca2+determ in serum quality control solutions and serums for ISE-based Ca*+determ inexpensive ISE method for ionized calcium determ in serum determ of ionized calcium in serum and saliva free Ca2+in aerobic and anerobic blood as measured with ISEs ref intervals for ionized calcium values comparison of undiluted Na+/K+measurements by ISEs and flame photometry (diluted)
such electrodes are prone to interferences by serum proteins which can bind silver ions causing large changes in the electrode potential. While this problem can be partly overcome by covering the Ag/AgCl electrode with an appropriate semipermeable membrane (89g),a number of efforts have been made to develop a liquid membrane electrode system selective for chloride ions (using either ion exchanger or neutral carrier molecules). Many chloride electrodes of this type have been proposed over the past 10 years, primarily ones that employ quaternary ammonium compounds as the active membrane molecules. However, most of these suffer severe bicarbonate and salicylate interferences rendering them useless in clinical samples. During the past 2 years, Nova Biomedical, Inc., reported a new liquid membrane chloride electrode which has reduced salicylate interferences and could be utilized to measure chloride directly in undiluted urine samples (104g). Furthermore, several Japanese patents have also reported on the development of ion-exchanger based chloride selective liquid membrane electrodes which supposedly have ample selectivityto be used in all body fluids (5Og-52g). The addition of various aryl alcohols to the membrane phase may be the key factor in generating improved selectivity toward chloride. Aside from chloride measurement systems, a polymer membrane electrode selective for carbonate ions is now incorporated into Technicon's new RA-1000 instrument for the determination of total carbon dioxide species in blood (15g). In this approach, the sample is first adjusted to pH 8.6 to liberate the carbonate ions from the bicarbonate and GO2 in the blood. Attempts to prepare phosphate selective electrode for use in physiological samples have also been made. Montalvo et al. (78g)described a redox type phosphate electrode based on a chemically treated iron wire. In the presence of constant oxygen tension, the electrode performed surprisingly well when used to directly detect phosphate in serum samples. However, redox electrodes of this type will always be sensitive to the redox environment of the sample and it is doubtful that such devices could be routinely used as direct sensors to obtain accurate results in all types of samples having varied concentrations of oxidizing or reducing agents. Thus, additional efforts to prepare a truly ion-selective membrane type phosphate electrode as well as electrodes selective for other
anions are necessary. Perhaps the use of recently synthesized anion binding macrocyclic ligands will prove valuable for this purpose.
ACKNOWLEDGMENT The authors gratefully acknowledge the support of the Chemistry Departments at the Unversity of Iowa and the University of Michigan for providing resources used in the preparation of this review (e.g., library facilities, computer searches, word-processing equipment, etc.). In addition, the secretarial work of Claudia Bishop, Greta Bocciardi (U of M), and Joan Zoekler (U of I) and the proofreading assistance of MAA's and MEM's research groups are also acknowledged. LITERATURE CITED BOOKS, CONFERENCES, AND REVIEWS
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GENERAL DISCUSSION ON ION-SELECTIVE ELECTRODES ( l b ) Ahmad-Bitar, R.; Abdul-Gader, M. M.; Zihlif, A. M.; Jaber, A. M. Y. J. Electroanal. Chem. Interfaclal Electrochem. 1983, 143, 121-133. (2b) Bates, R. G. Stud. fhys. Theor. Chem. 1983, 2 7 , 237-250. (3b) Bates, R. G.; Dlckson, A. G.; Gratzl, M.; Hrabeczy-Pail, A.; Lindner, E.; Pungor. E. Anal. Chem. 1983, 55, 1275-1280. (4b) Ben-Yaakov, S.; Ravlv, R.; Guterman. H.; Dayan, A.; Lazar, B. Talanta ’ 1982, 29, 267-274. (5b) Brezlnski, D. P. Talanta 1983, 30, 347-354. (6b) Brezlnskl, D. P. Analyst (London) 1983, 106, 425-442. (7b) Brezinski. D. P. Anal. Chim. Acta 1982, 134, 247-262. (8bj Buck, R.’P. Anal. Chem. 1978, 50, 17R-29R. (9b) Buck, R. P. Ion-Selectlve Electrode Rev. 1982, 4 , 3-74. (lob) Busch, J.; Graabaek, A. M.; Malamvig. H. Int. Lab. 1982, 14, 92, 94-96; Anal. Absh. 1982, 6 , J96. ( l l b ) Chapman, B. R.; Goldsmlth, I.R. Analyst (London) 1982, 107, 1014-1018. (12b) Cheng. F. W. Microchem. J. 1982, 27, 401-407. (13b) Coetzee, J. F.; Gardner, C. W., Jr. Anal. Chem. 1982, 54, 2625-2626. (14b) Coetzee, J. F.; Gardner, C. W., Jr. Anal. Chem. 1982, 5 4 , 2530-2532. (15b) . . Deak, E. Anal. Chem. SymD. . . Ser. 1981, 6 , 203-213; Chem. Abstr. 1982, 97, 16203g. (16b) Dolbniak-Leonowicz, E.; Bulawa, J. Chem. Anal. (Warsaw) 1981, 2 6 , 357-360 (Pol); Chem. Abstr. 1982, 9 6 , 62184d.
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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(93f) Weise, H.; May, U.; Scheller, F.; Nentwig, J. Ger. (East) (Patent) 153,237 Dec 30, 1981, Appi. 223,924, Sept 10, 1980; Chem. Abstr. 1982, 9 6 , 2 1 3 8 4 9 ~ . (94f) Wimbereiy. P. D.; Pederson, K. G.; Thode, J.; Fogh-Anderson, N.; Sorensen, A. M.; Siggaard-Anderson, 0. Clin. Chem. ( Wlnston-Salem, N . C . ) 1983, 2 9 , 1471-1473. ION-SELECTIVE ELECTRODES I N FLOW-THROUGH ARRANGEMENTS AND CLINICAL ANALYSIS SYSTEMS (lg) Akaiwa, H.; Kawamoto, H.; Osumi, M. Talanta 1982, 2 9 , 689-690. (2g) Ammann, D.; Anker, P.; Jenny, H. B.; Simon, W. Anal. Chem. Symp. Ser. 1981, No. 8, 179-184. (3g) Anker, P.; Ammann, D.; Simon, W. Mikrochim. Acta 1983, 1 , 237-242. (4g) Anker, P.; Hano-Beat, J.; Wuthier, U.; Asper, R.; Ammann, D.; Slmon, W. Clin. Chem. (Winston-Salem, N . C . ) 1983, 2 9 , 1508-1512. (5g) Anker, P.;Hans-Beat, J.; Wuthler, U.; Asper, R.; Ammann, D.; Simon, W. Clin. Chem. (Winston-Salem, N . C . ) 1983, 2 9 , 1447-1448. (6g) Annan, W.; Klrwan, N. A.; Robertson, W. S.;Teasdale. P. R.; Ager, B. P. J . Autom. Chem. 1980, 2 , 212-220. (7g) Ansari, A.; Drew, C. R. Clln. Chem. (Winston-Salem, N.C.)1982,28, 1630. (8g) Apple, F. S.; Koch, D. D.; Graves, S.; Ladenson, J. H. Clin. Chem. (Winston-Salem, N.C.)1982, 2 8 , 1931-1935. (9g) Bhargava, 0. P.; Gmitro, M. Am. Lab. (Fairfleld, Conn.) 1983, 15, 28-32. (log) Bijster, P Vader, H. L.; Vink, C. L. J. Ann. Clin. Biochem. 1983, 2 0 , 116-1 20. ( I l g ) Bijster, P.; Vader, H. L.; Vink, C. L. J. J. Autom. Chem. 1982, 4 , 125-128. (12g) Bond, A. M.; Hudson, H. A.; Van den Bosch, P. A.; Waiter, F. L.; Exelby, H. R. A. Anal. Chim. Acta 1982, 136, 51-59. (13g) Burr, R. G. Clin. Chem. (Winston-Salem, N.C.)1982, 2 8 , 1710-1 7 11. (14g) Butler, S. J.; Payne, R. B. Clln. Chem. (Winston-Salem, N.C.)1983, 2 9 , 585-586. (15g) Chapoteau, E.; James Scott, W. Clin. Chem. (Winston-Salem, N.C.) 1983, 2 9 , 1187. (16g) Coleman, R. L.; Young, C. C. Clin. Chem. (Winston-Salem, N.C.) 1982, 2 8 , 1705-1706. (17g) Coolen, R. B. Proc.-Int. Congr. Clin. Chem. 1982, 1 1 , 1173-1177. (18g) Cooper, G. J. S.;Croxson, M. S. Ciin. Chem. (Wlnston-Salem, N . C . ) 1983, 2 9 , 1320. (I9g) Cormier, A. D.; Fejes, A. M. Clin. Chem. (Winston-Salem, N.C.) 1982, 2 8 , 1632. (20g) Costello, P.; Kubaslk, N. P.; Brody, B. B.; Sine, H. E.; Bertsch, J. A,; D'Souza, J. P. Clin. Chem. (Winston-Salem, N . C . ) 1983, 2 9 , 129-132. (21g) Crawhall, J. C.; Purdy, W. C. Trends Anal. Chem. 1982, 1 , 184-187. (22g) Croweli, J. A.; Moore, R. E.; Bowers, G. N. Clin. Chem. (Winston-Sa/em, N . C . ) 1983, 2 9 , 1187. (23g) Czaban, J. D.; Cormier, A. D.; Legg, K. D. Clln. Chem. ( Wlnston-Sa /em, N.C.)1982, 2 8 , 1703-1705. (24g) Czaban, J. D.; Cormier, A. D.; Legg, K. D. Clin. Chem. (Winston-Saiem, N . C . ) 1982, 2 8 , 1936-1945. (25g) Degawa, H.; Shinozuka, N.; Hayano, S. Chem. Lett. 1983, 7 , 25-28. (26g) Dlamndis, E. P.; Efstathlou, C. E.; Papastathopoulos, D. S.; Hadjiioannou, T. P. Mlcrochem. J. 1983, 2 8 , 227-234. (27g) Diamandis, E. P.; Koupparls, M. A,; Hadjiioannou, T. P. J. Chem. Educ. 1983, 60, 74-76. (289) Diamandls, E. P.; Hadjiioannou, T. P. Microchem. J. 1982, 2 7 , 5 12-51 8. (29g) Diamandls, E. P.; Hadjiioannou, T. P. Analyst (London) 1982, 107, 1471-1478. (30g) Diebler, H.; Adler, H.; Svenjak, D.; Johnson, R.; Lanza, H.; Herron, R. Clin. Chem. (Winston-Salem, N.C.)1983, 2 9 , 1193. (31g) Dllena, B. A.; Walmsley, R. N.; Fraser, C. G. Clin. Chem. (WinstonSalem, N.C.)1983, 2 9 , 1856-1857. (32g) Drop, L. J.; Tochka, L. N.; Misiano, D.R. Clin. Chem. (Winston-Sa/em, N.C.)1982, 2 8 , 129-133. (33g) Drop, L. J.; Mlslano, D. R.; Tochka, L. N. Clin. Chem. (Winston-Saiem, N.C.) 1982, 2 8 , 2448. (34g) Everitt, M. T.; Kenny, M. A.; Delaney, C. J. Clin. Chem. (Winston-Sa/em, N . C . ) 1982, 2 8 , 1630. (35g) Felstel, C. C.; Matsuyama, 0. Clin. Chem. (Winston-Salem, N.C.) 1982, 2 8 , 1631. (36g) Flores, 0.; Buzza, E. Clin. Chem. (Winston-Salem, N.C.)1982, 2 8 , 1233. (37g) Fraticelli, Y. M.; Meyerhoff, M. E. Anal. Chem. 1983, 55, 359-364. (38g) Fyffe, J. A.; Jenkins, A. S.; Cohen, H. N.; Dryburgh, F. J.; Gardner, M. D. J. Autom. Chem. 1980, 2 , 85-89. (39g) Fyffe, J. A. J. Autom. Chem. 1982, 4 , 79. (409) Gourmelin, Y.; Truchaud, A.; Hersant, J.; Gllkmanas, G.; Bigorle, B. Clin. Chem. (Whston-Salem, N . C . ) 1983, 2 9 , 1193. (41g) Graham, G.; Burritt, M. Clin. Chem. (Winston-Salem,N.C.)1983, 2 9 , 1187. (42g) Graves, S . W.; Landt, M. L.; Smith, C. H.; Compton, R.; Ladenson, J. H. Clin. Chem. (Winston-Salem, N . C . ) 1983, 2 9 , 1272. (43g) Graves, S. W.; Koch, D. D.; Ladenson, J. H. Clin. Chem. (WinstonSalem, N.C.)1982, 2 8 , 1631-1632. (44g) Gross, S. E.; Khayam-Bashl, H. Clin. Chem. (Winston-Salem, N.C.) 1982, 2 8 , 1629-1630. (45g) Hall, G. S.;O'Leary, T. D.; Prior, A. P. Clin. Chem. (Winston-Salem, N.C.)1982, 2 8 , 1248. (46g) Happe, T. M. Clin. Chem. (Winston-Salem, N . C . ) 1983, 2 9 , 1310. (47g) Harff, G. A. Clin. Chem. (Wlnston-Salem, N.C.) 1982, 2 8 , 1232-1233.
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Anal. Chem. lQa4. 56. 40 R-63 R (48Q) Harff, G. A.; Van Leeuwen. C. Clin. Chem. (Wlnston-Salem, N.C.) i082, 28, 2003. (4%) Hershcovitz, H.; Yarnitzky, Ch.; Schmuckler, G. J. Cbromtogr. 1082, 252. .- , 113-119 . .- . . - . (50g) Hitachi, Ltd. Jpn. Tokkyo Koho JP (Patent) 57 57,655, Dec 6, 1982; Appl. 771156,374, Dec 27, 1977; Cbem. Abstr. 1083, 99, 35661n. (51g) Hitachi, Ltd. Jpn. Kokai Tokkyo Koho JP (Patent) 82 50,649, Mar 25, 1982; Appl. 801125,923, Sept 12, 1980; Chem. Abstr. 1082, 97, 35681~. (52g) Hltachi, Ltd. Jpn. Tokkyo Koho JP (Patent) 57 57,656, Dec 6, 1982; Appi. 771156,375, Dec 27, 1977; Chem. Abstr. 1083, 99, 3 2 4 7 8 ~ . (53g) Ishll, S.; Flores, 0.;Belisle, T. J. Clln. Lab. Autom. 1982, 2, 336-344. (54g) Jalandra, P.; Ho, T. T. Clln. Chem. (Wlnston-Salem, N . C . ) 1082, 28, 1631. (55g) Kearney, S. D.; Dreier, G. H.; Cormler, A. D. US. US (Patent) 4,366,038, Dec 28, 1982; Appl. 175,052 Aug 4, 1980; Chem. Abstr. 1082, 9 8 , 8 5 7 8 2 ~ . (56g) Kiselev, G. G.; Mezhburd, T. A.; Nikonov, V. N. Zavod. Lab. 1082. 48, 3-6 (Russ); Cbem. Abstr. 1983, 88, 2 6 9 0 2 ~ . (57g) Klssel, T. R.; Sandifer, J. R.; Zumbuiyadis, N. Clln. Chem. (WlnstonSalem, N.C.) 1082, 28, 449-452. (58g) Koch, D. D.; Parrish, D.; Ladenson, J. H. Clln. Chem. (Winston-Salem, N . C . ) 1083, 29, 1090-1092. (59g) Koch, D. D.; Ladenson, J, H. Anal. Chem. 1983, 55, 1807-1809. (60g) Korenaga, T. J. Autom. Chem. 1981, 3, 191-195. (61g) KruseJarres, J. D.; Schott, F. J.; Trendelenburg, C. R o c - I n t . Congr. Clh. Chem. 1082. 11. 1143-1147. (62g) Ladenson, J. H:; Apple, F: S.; Aguanno, J. J.; Koch, D.D. Clln. Cbem. ( Winston-Salem, N.C.) 1082, 28, 2383-2386. (63g) Landry, J. C.; Cupelin, F.; Michal, C. Analyst (London) 1981, 106, 1275-1280. (64g) Langhoff, E.; Stelness, F. Clln. Chem. (Wlnston-Salem, N . C . ) 1082, 28, 170-172. (65g) Loshoo, C. A.; Bowers, G. N. Clln. Chem. (Winston-Salem, N.C.) 1982, 28, 1577. (66s) Maas, A. H. J.; Weisberg, H. F.; Zijlstra, W. 0.; Durst, R. A.; SlggaardAndersen, 0. J. Clln. Chem. Clln. Elochem. 1083, 21, 313-321. (67g) Malekpour, A.f; Taylor, D.; Klng, M. E. Clin. Chem. (Wlnston-Salem, N . C . ) 1982, 28, 1576. (68g) Manz, A.f; Simon, W. J. Chromatogr. Sci. 1083, 21, 326-329. (69g) Margules, G. S.; Hunter, C. M.; MacGregor, D. C. Med. Blol. Eng. Comput. 1983, 21, 1-8. (70g) Marshall, 0. B.; Midgley, D. Analyst (London) 1983, 708, 701-711. (71g) Martin, R.; Jones, C.; Le Llevre, K.; Magnantl, D.; Mulholland, L.; Pelosi, M.; St. Andre, J.; Young, C. C. Clln. Chem. (Winston-Salem, N . C . ) 1983, 29,1188. (72g) Masclni, M.; Palleschi, G.; D’Ottavio, 0.; Mazzella, G. Ann. Chlm. 1083, 73, 29-46. (73g) Mascini, M.; Palleschi, 0. Anal. Cblm. Acta 1982, 136, 69-76. (74g) Mascini, M.; Palleschi, G.; Anal. Cblm. Acta 1083, 745, 213-217. (75g) Matsushha Electric Industrial Co., Ltd. (Patent) Jpn. Kokal Tokkyo Koho 58 83,259 May 19, 1963; Appl. 811182,822, Nov 13, 1981; Chem. Abstr. 1083, 99, 99495g. (76g) Meyerhoff, M. E.;Kovach, P. M. J. Chem. Educ. 1083, 80, 766-768. (77g) Monastyrenko, E. S.; Sukhorukova, L. S.; Dolya, A. M. Zavod. Lab.
1982, 48, 21-23 (Russ.); Chem. Abstr. 1982, 97,187990r. (78g) Montalvo, J. G.; Truxlllo, L. A.; Wawro, R. A.; Watkins, T. A,; Phillips, A.; Jenevin, R. M. Clln. Chem. (Wlnston-Salem, N . C . ) 1982, 28, 655-658. (79g) Mueller, H. Anal. Chem. Symp. Ser. 1981, 8, 279-286. (8Og) North, J. W. Clln. Chem. (Winston-Salem, N . C . ) 1082, 28, 1248. (8lg) North, J.; Chlttenden, C.; Kariicek, T.; Happe, T. Clln. Cbem. (Wlnston-Salem, N.C.) 1082, 28, 1632. (82g) Ortolano, G. A.; Stuart, R. C.; Wunschel, K. R., Jr.; Kaiser, E. A,; Hammond, R. P.; Swonger, A. K. Microchim. J. 1983, 28, 409-430. (839) Payne, R. B. Ann. Clln. Eiochem. 1982, 79, 233-237. (849) Plant, S. B.; McCarron, D. A. Clin. Chem. (Winston-Salem, N . C . ) 1982, 28, 1362-1363. (85g) Rehak, N. N.; Elin, R. J. Chesler, R. A.; Chiang, B. T. Johnson, E. E. Clin. Cbem. (Winston-Salem, N . C . ) 1982, 28, 1630. (86g) Rocks, B.; Riley, C.; Clln. Chem. (Wlnston-Salem, N.C.) 1082, 28, 409-42 1. (87g) Sandhu, R. S.; Fischman, S. J. Clln. Chem. (Winston-Salem, N . C . ) 1982, 28, 1576. (88s) Seddon, R. M.; Parker, P. H.; Wlnton, M. R.; Lansdell, A. W. Clin. Cbem. (Wlnston-Salem, N.C.) 1083, 29, 212. (89g) Seshimoto, 0.; Sakaguchi, S.; Takayama, T.; Sato, A. Ger. Offen. DE (Patent) 3,222,464, Dec 30, 1982; Appl. 81192,887, June 15, 1981; Cbem. Abstr. 1083, 98, 85779g. (90g) Sherwin, J. E.;Bruegger, B. B.; Belisie, T.; Flores, 0. Clln. Chem. (Wlnston-Salem, N.C.) 1082, 28, 1830-1631. (91g) Siggaard-Andersen, 0.; Fogh-Andersen, N.; Thode, J. Scand. J. Clin. Lab. Invest., Suppl. 1083, 43, 43-46. (92g) Simpson, S. F.; Holler, F. J. Anal. Cbem. 1982, 54, 43-46. (93g) St. Andre, J.; Jones, C.; Le Lievre, K.; Magnanti, D.; Martin, R.; Mulholland, L.; Pelosl, M.; Young, C. C. Clln. Chem. (Wlnston-Salem, N . C . ) 1083, 29, 1188. (94g) Suzuki, K.; Aruga, H.; Shlrai, T. Anal. Chem. 1983, 55, 2011-2013. (95g) Taki, M.; Yamauchi, I.Rlnsho Kensa 1081, 25, 1661-1664 (Jpn.); Chem. Abstr. 1982, 96, 118417b. (96g) Thode, J.; Wandrup, T.; Aas, F.; Siggaard-Andersen, 0. Scand. J. Clln. Lab. Invest. 1082, 42, 407-415. (97g) Truchaud, A.; Bolgne, J. M. Clln. Chem. (Wlnston-Salem, N.C.) 1983, 29, 1188. (98s) Trojanowicz, M.; Matuszewski, W. Anal. Chim. Acta 1983, 151, 77-84. (99g) Trojanowicz, M.; Matuszewski, W. Anal. Cbim. Acta 1082, 138, 71-79. (IOOg) Van Den Winkel, P.; De Backer, G.; Vandepatte, M.; Mertens, N.; Dryon, L.; Massart, D. L. Anal. Chlm. Acta 1983, 745, 207-212. (IOlg) Virtanen, R. Anal. Cbem. Symp. Ser. 1081, 8, 375-385. (102g) Waiter, B. Anal. Chem. 1083, 55, 498-514. (103g) Wilcox, A. Clln. Chem. (Winston-Salem, N.C.) 1982, 28* 1631. (104g) Willis, J. P.; Young, C. C.; Martln, R.; Stearns, P.; Pelosi, M.; Magnanti, D. Clln. Chem. (Winston-Salem, N . C . ) 1083, 29, 1193. (105g) Wlmberley, J. W.; Carel, A. B.; Cabblness, D. K. Anal. Lett. 1082, 75, 89-100. (106g) Wolf, B.; Ishii, S.; Flores, 0. Clln. Chem. (Wlnston-Salem, N . C . ) 1982, 28, 1630. (1079) Xue, X.; Lu, C.; Qao, S.; Wu, G. Fenxl Huaxue 1983, 7 7 , 548-551 (Ch.); Chem. Abstr. 1083, 99, 1 3 6 2 5 5 ~ .
Thin-Layer and Paper Chromatography Joseph Sherma* Department of Chemistry, Lafayette College, Easton, Pennsylvania 18042
Bernard Fried Department of Biology, Lafayette College, Easton, Pennsylvania 18042
This review covers the literature of thin-layer chromatography (TLC) and paper chromatography (PC) cited in Chemical Abstracts from December 14,1981, to November 28,1983, and Analytical Abstracts from November 1981 to November 1983. Also searched directly were the most important journals ublishing papers on TLC and PC, namely, the Journal of C romatography (including its biblio raphy sections), Journal of High Resolution Chromatograp8,y and Chromatography Communications, Journal of Chromato-
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4a R
graphic Science, Chromatographia, Analytical Chemistry, and the two yearly special TLC issues of the Journal of Liquid Chromatography. Papers reporting important advances in PC continued to decline in the last 2 years while those on TLC again increased significantly. Macek reported in his chapter in the latest revision of Heftmann’s Chromatography (151A)that between 1970 and 1979 TLC held second place behind liquid column chromatography, with a total of 27% of all chromatography
0003-2700/84/0356-40R~O6.5~l0 0 1904 Amerlcan Chemical Society
