Anal. Chem. 1988, 6 0 , 1977-1979
solute simultaneously. Thus, it is suggested that the IR light acts through the excited neutral parent solute as well as through its geminate cation-electron pair. In conclusion, this study shows that a considerable improvement in the photoionization detection sensitivity is possible by the simultaneous action of a UV and an IR nanosecond laser at room temperature. Further physicochemical and analytical studies are now in progress. ACKNOWLEDGMENT
I thank Professors T. Ogawa and I. Shinno of Kyushu University for their encouragement and for the use of their electronic instruments and optical components. Registry No. 9-Phenylanthracene,602-55-1; pyrene, 129-00-0; perylene, 198-55-0.
(8) Yamada, S.;Ogawa, T.; Zhang, P.-H. Anal. Chim. Acta 1986, 783, 251-256. (7) Sato, N.; Yamada, S.; Ogawa, T. Anal. Sci. 1987, 3 , 109-111. (8) Yamada, S.;Sato, N.; Kawazumi, H.; Ogawa, T. Anal. Chem. 1987, 59, 2719-2721. (9) Yamada, S.; Ogawa, T. frog. Anal. Spectrosc. 1986, 9 , 429-453. (IO) Speiser, S.; Jorlner, J. Chem. fhys. Lett. 1976, 4 4 , 399-403. (11) Letokhov, V. S. Laser fhotoionlzation Spectroscopy; Academic: Orlando, FL, 1986; Chapters 4 and 5. (12) Yakoviev, B. S.; Lukin, L. V. I n Photodissociation and photoionization; Lowley, K. P., Ed.; Why: New York, 1985; pp 99-160. (13) Lukin, L. V.; Toimachev, A. V.; Yakovlev, B. S. Chem. fhys. Lett. 1981, 87, 595-598. (14) Lukin, L. V.; Tolmachev, A. V.; Yakovlev, B. S. Chem. fhys. Lett. 1983, 99, 16-21. (15) Braun, C. L.; Scott, T. W. J. fhys. Chem. 1983, 8 7 , 4776-4778. (16) Braun, C. L.: Scott, T. W. J. fhys. Chem. 1987, 97, 4436-4438. (17) Scott, T. W.; Braun, C. L. Can. J. Chem. 1985, 63, 228-231. (18) Scott, T. W.; Braun, C. L. Chem. fhys. Lett. 1986, 727, 501-504. (19) Scott, T. W.; Braun, C. L. J. fhys. Chem. 1986, 90, 1739-1741. (20) Miyasaka, H.; Mataga, N. Chem. fhys. Lett. 1986, 126, 219-224.
LITERATURE CITED (1) Voigtman, E.; Jurgensen, A,; Winefordner, J. D. Anal. Chem. 1981, 53, 1921-1923. (2) Yamada, S.; Kano, K.; Ogawa, T. Bunsekl Kagaku 1982, 37. E247E250. (3) Voigtman, E.; Wlnefordner, J. D. Anal. Chem. 1982, 5 4 , 1834-1839. (4) Yamada, S.;Hino, A.; Kano, K.; Ogawa, T. Anal. Chem. 1983, 55, 1914- 19 17. (5) Fujiwara, K.; Voigtman, E.: Winefordner, J. D. Spechosc. Lett. 1984, 77, 9-20.
1977
Sunao Yamada Laboratory of Chemistry College of General Education Kyushu University Ropponmatsu, Fukuoka 810, Japan RECEIVED for review February 2,1988. Accepted May 6,1988.
Gradient Elution of Anions in Single Column Ion Chromatography Sir: Traditionally, gradient elution has been considered difficult or impossible to accomplish with liquid chromatographic systems utilizing bulk property detectors ( I , 2). Changes in composition of mobile phases required during the development of a gradient were thought to cause under all circumstances too large a change of the response by bulk property detectors. Relatively small deflections due to the zones of analytes passing through the detector cell were expected to remain undetected at the crude sensitivity setting imposed by the simultaneously occurring change in the bulk concentration of the eluent. In recent years there have been several reports (3-6) describing a successful utilization of conductivity detection in conjunction with the gradient elution of ions. Initial steps toward gradient elution followed by refractive index detection have also been reported (7). In all published work so far,conductivity detection is made compatible with gradient elution by the employment of suppressors connected between the separator column and the conductivity detector. The main function of suppressors is the conversion of the high conductivity signal produced by the eluent into a low level background reading (8). In the same fashion a pronounced change in conductivity-as observed during the gradient elution-can be reduced considerably. A computer-aided base-line subtraction of prerecorded blank gradients is then usually employed to improve the appearance of chromatographic recordings and to enable a reliable quantitation. In a new technique which we call isoconductive gradient, the background conductivity changes can be minimized by a judicious choice of countercations in the mobile phases employed for gradient elution of anions. This approach enables gradient separations under the conditions of single column (nonsuppressed) ion chromatography. EXPERIMENTAL SECTION Preparation of Eluents. All chemicals were utilized as obtained from commercial sources. Milli-&water (Millipore Corp.) was used for all aqueous solutions mentioned in this report. Unadjusted Eluents (“Conventional Gradient”). A1 (background conductivity ca. 321 &): 11 mM boric acid, 1.48 mM 0003-2700/88/0360-1977$01.50/0
gluconic acid, 3.49 mM potassium hydroxide, 0.65 mM glycerin, and 12% acetonitrile. B1 (backgroundconductivity ca. 395 wS): 13.75 mM boric acid, 1.85 mM gluconic acid, 4.36 mM potassium hydroxide, 0.81 mM glycerin, and 12% acetonitrile. Increased cohcentration of glycerin in B1 did not contribute to the eluting strength. This increase was derived from the fact that 25 mL of a 50-fold concentrate of A1 (without acetonitrile) was used to prepare B1. Eluents Adjusted t o Equal Conductivity (“Zsoconductive Gradient”). A2: Eluent A2 was identical with Al. B2 (background conductivity ca. 322 WS). Lithium hydroxide monohydrate (5.13 mM), concentrations of boric acid, gluconic acid, glycerin, and acetonitrile were the same as in B1. A3 (background conductivity ca. 344 &. 8.25 mM boric acid, 1.11mM gluconic acid, 3.08 mM cesium hydroxide, 0.48 mM glycerin, and 12% acetonitrile. B3 (background conductivity ca. 341 &): 12.65 mM boric acid, 1.70 mM gluconic acid, 4.72 mM lithium hydroxide, 0.75 mM glycerin, and 12% acetonitrile. Lithium hydroxide monohydrate (99%),potassium hydroxide (86.2%),cesium hydroxide monohydrate (99%),boric acid (99% A.C.S. reagent), and D-gluconic acid (50% wt in water) were supplied by Aldrich Chemical Co. Glycerin (U.S.P.-F.C.C.) and acetonitrile (HPLC grade) were obtained from J. T. Baker Chemical Co. Close match of conductances is an overriding concern in the preparation of the isoconductive eluents. Generally it was found to be feasible to match a pair of gradient eluents within 3 p S simply by weighing in and by pipetting the amounts and volumes specified for A2,B2 or A3,B3 into a 1000-mL volumetric flask. Because of the fluctuating purity of hydroxides due to the changes occurring during their storage, small variations in the actual molar ratios of alkaline hydroxide to other more stable components of isoconductive eluents had to be expected. Corrections were carried out by adding small volumes of water to the eluent of higher conductivity. Such f i e adjustments could be calculated by assuming a linear relationship between the background conductance and the volume of an eluent. Conductivity mismatch of 10-15 p S could still be successfully compensated by the employed base-line subtraction routine (Waters M 840 Expert Software). However, the precision data presented in Table I were obtained with eluents A2,B2 differing by no more than 1 fiS. Preparation of Standards. All standard solutions of inorganic 0 1988 American Chemical Society
1978
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
Table I. Reproducibility of the Isoconductive Gradient % RSD
anion
peak area
fluoride chloride nitrite bromide nitrate phosphate sulfate oxalate chromate molvbdenum
3.1 0.9 1.6 3.5 2.4 3.4 2.0 1.7 0.5 1.1
for six runs retention time 0.1 0.3 0.2 0.2 0.2 0.2 0.3 0.2 0.4 0.4
and organic anions used in this study were prepared from sodium or potassium salts of reagent grade purity. Chromatographic Apparatus. All components of the chromatographic system were supplied by Waters Chromatography Division of Millipore Corp. A programmable M590 highpressure pump controls a solvent switching valve connected between the pump on one side and solvent reservoirs with eluents of different eluting strength on the other side. An automatic high-pressure switching valve was used as an injector. This was accomplished by connecting an appropriate sample loop (20 or 100 pL) and an injection port to such a valve. A signal connection was made between the injector and the M590 pump. Solvent changes resulting in step gradients and also the data acquisition by the M840 chromatographic data station were then triggered by manual actuation of the injector. Due to the delay volume of 3 mL and the flow rate of 1.2 mL/min for all chromatograms, the changed composition of the eluent reached the column approximately 2.5 min after an injection. The same delay volume made it possible to change back to the eluent B even before the current gradient run was completed, i.e. 2.5 min before the elution of the last peak. Two different columns were used for the gradient experiments. The first of which, the IC-Pak Anion, had the dimensions of 0.46 X 5.0 cm and was packed with polyacrylatebased anion exchange material. The particle size was 10 pm and the ion exchange capacity was specified as 30 pequiv/mL. The second column (IC-Pak Anion HR) had a different size (0.46 X 7.5 cm) and was packed with smaller particles (7 pm) of the polyacrylate resin. The ion exchange capacity of the resin in the second column was the same as in the first column. A M430 detector was utilized to measure conductivity changes in the column eluates and to evaluate background conductivities of the eluents. The detector cell (nominal cell constant = 1) was thermostated at 35 “C. Conductivity Measurements. Extech Model 590 conductivity meter was chosen for the evaluation of conductivities in solutions of borate and gluconate. Value of the cell constant (1.0cm-’) was verified by measuring the conductance of a 0.01 N KC1 solution. Equivalent conductance of the borate gluconate complex was calculated from the conductivity reading obtained from a solution containing 2.6 mM sodium gluconate (Eastman Kodak P8626), 1.3 mM boric acid, and 1.3 mM sodium hydroxide. These concentrations were chosen in the view of recent report (9) postulating the formation of the borate gluconate complex from two molecules of gluconate and one molecule of borate. The value of equivalent conductance of the complex carrying three negative charges was determined to be 25.5 cm2equiv-l 0-l. This value was confirmed by additional measurements in which the borate gluconate ratio was held constant and the concentration of sodium hydroxide was increased in several steps. All measurements were carried out at 25 “ C . RESULTS A N D DISCUSSION The background conductance signal caused by a fully dissociated ionic mobile phase is given by (8)
where A, and ha are cationic and anionic equivalent conduc-
0
2
4
6
8
10
12
14
Minutes
Figure 1. Conductivity changes during the development of a gradient of an anion exchange column: (A) unadjusted eluents A1 and 81 both with potassium as a countercation; (8)eluents adjusted to the same level of conductance using potassium for the weaker mobile phase (A2) and lithium for the stronger (82). The compositions of mobile phases as well as the description of the gradient are given in the Experimental Section.
tances in cm2equiv-l W. The concentrations in equiv L-’ and cell constant in cm-* are given by c and K , respectively. In our approach to gradient elution of anions eluent cations are varied from one of high ionic equivalent conductance in the “weak” eluent to another of a low ionic equivalent conductance in the “strong” eluent, thereby allowing the anionic eluting strength of the eluent to increase without changing the background conductance significantly. For any two isoconductive mobile phases (G, = G,) with a common anion A” and two different cations C1 and C2, it is possible to derive expression 2 linking the ratio of the weaker (cl) and stronger (e2) concentrations with tkie corresponding equivalent conductances
Application of eq 2 facilitates the search for optimum ionic combinations in isoconductive eluents. As determined by this relationship, large concentration ratios are possible only with eluents containing anions of low ionic equivalent conductance. = 198) and borate/gluconate For example with hydroxide (kH (ABC = 25.5 see Experimental Section), the calculated values of c2/c1 for a potassium to lithium gradient step are 1.1and 1.5, respectively. Results obtained with the eq 2 are found to be in a good agreement with the experimental data. For example, the practical value for an isoconductive increase of the gluconic acid concentration in the presence of an appropriate concentration of boric acid, using potassium and lithium hydroxides for conductivity adjustments was determined to be 2.10/1.48 = 1.5. In Figure 1conductivity changes in an unadjusted gradient are compared with those obtained in isoconductive eluents. The originally overwhelming change of the background signal is efficiently removed by the technique discussed in this report. The size and shape of the two small base-line disturbances of the profile B in Figure 1 are largely column independent. They remain unchanged even if the anion exchange column is removed from the system. The first fluctuation occurs between 2.5 and 4 min as the changing composition of the eluent begins to reach the detector cell. The larger of the two base-line deflections having the amplitude of about 7 p S is then observed at the changeover back to the initial eluent, 9.5-11 min in Figure 1. The chromatograms in Figure 2A,B are included to illustrate a frequently reoccurring problem in chromatographic separations of anions. Compounds with relatively close, strong retentions (for example chromate and molybdate) cannot be separated efficiently under isocratic conditions, if an efficient resolution of weakly retained anions is also required in the same run. Stepwise gradients with the first gradient step followed by an isocratic elution with the
Anal. Chem. 1988, 60, 1979-1982
10
5
0
l5
Minutes
T 611 - ?
1. Fluoride 1 ppm 2. Carbonate 2 ppm R Chloride 2 DDm
6
Nitrate 4 ppm 7 Phosphate 6 ppm
8 Sulfate 4 ppm 9 Oxalate 4 ppm 10 Chromate 10 ppm 11 Molybdate 10ppm 0
5
0
5
10
15
10
15
Minutes
20
20
Minutes
Flgure 2. Optimization of the separation of 11 anions with the help of an isconductive gradient: (A) isocratic separation using the weaker of the two eluents (A2); (B) isocratlc separation with the strong eluent (82); (C) step gradient from A2 to 8 2 initiated simultaneously with the injection of 100 pL of the standard mixture. For a more detailed description of the eluents A2,B2 and of the ICPak anion column utilized for the above separations, see Experimental Section. I,
4
l I
1. Fluoride 5 ppm 2 . Carbonate 20 ppm 3. Chloride 15 DDm
5. Bromide 2O'ppm 6. Nitrate 20 ppm 7. Phosphate 30 ppm E . Phosphite 20 ppm 9. Sulfate 20 ppm
10. Oxalate 20 ppm 11. Tunastate 25 ppm 12. Chromate 25 ppm Molybdale 25 ppm Thiocyanate 25 ppm
13
2: 0
' 10
'5
' 15
Mlwtcs
Figure 3. Gradient separation of 14 anions using a high-resolution column (IGPak Anion HR). A step gradient from A3 to 8 3 was initiated in the moment of injection. At the eighth minute, the flow rate was increased from 1.2 to 1.7 mL/min. Complete description of the column as well as of the eluents is provided in the Experimental Section. Twenty microliters of the standard mixture was injected. help of the stronger of the two eluents are recommended on the basis of the general theory for such mixtures (2). It is also relatively easy to convert most isocratic ion chromatographs to step gradient instruments by an addition of appropriate
1979
solvent select valves. After an evaluation of a delay volume for each particular pump-solvent valve configuration in an experiment similar to that in Figure 1,the weak and strong eluents can be combined to obtain a gradient chromatogram such as the one in Figure 2C. Relative standard deviations of the retention times and of the integrated areas for each of the 11peaks in this separation are given in Table I. Stepwise gradient elution with the borate gluconate anion and the selected countercations proves to be highly reproducible, achieving better than *0.5% precision of retention times for the majority of the analyzed anions. The observed values of percent relative standard deviation for peak areas, 0.8-3.870 for the concentration range of 1-6 ppm and 0.5-1.7% for 10 ppm, make the discussed technique acceptable for many practical applications. Employment of a more efficient anion exchange column based on a smaller diameter of the particles constituting the stationary phase leads to further improvements of the gradient separation. With the additional help of a flow rate gradient, carried out at a time when the stronger eluent determines the elution behavior on the column, 14 different anions can now be separated within a run time of about 14 min, Figure 3. Further work is currently under way involving new eluent compositions for isoconductive gradients and investigations of the influence of various shapes of gradient profiles. LITERATURE C I T E D (1) Engelhardt, H. H@h Performance Liquid Chromatography; Springer: New York, 1979; p 63. (2) Jandera, P.; Churacek, J. Gradient Nution in Column Liquid Chromatography; Eisevier: Amsterdam, 1985; p 243. (3) Sunden. T.; Lindgren, M.; Cedergren, A.; Siemer, D. D. Anal. Chem. 1983, 55, 2-4. (4) Dasgupta, P. K. Anal. Chem. 1984. 56, 769-772. (5) Tarter, J. G. Anal. Chem. 1984, 56, 1264-1268. (6) Shintani, H.; Dasgupta, P. K. Anal. Chem. 1987, 5 9 , 802-808. (7) Berry, V. V.; Waldron, T. Am. Lab. (FairfieM, Conn.) 1988, 18, 57-66. (8) Gjerde, D. T.; Fritz, J. S. Ion Chromatography, 2nd ed.; Alfred Huethig Verlag: New York, 1987; pp 93-128. (9) Schmuckler, G.; Jagoe, A. L.; Girard, J. E.; Buell, P. E. J . Chromatogr. 1986, 356, 413-419.
William R. Jones P e t r Jandik* Allan L. Heckenberg Waters Chromatography Division Millipore Corp. 34 Maple Street Milford, Massachusetts 01757
RECEIVED for review March 18,1988. Accepted May 24,1988.
Flow Injection Analysis of Electroinactive Anions at a Polyaniline Electrode Sir: Recently, Ikariyama and Heineman (I) reported a novel chemically modified electrode (CME) sensor capable of quantitating electroinactive anions in flow injection analysis. The sensor, constructed by electropolymerizing a coating of polypyrrole onto a platinum electrode surface, utilized the repetitive doping and undoping of the polymer by anioncontaining plugs injected into the carrier phase. Because the electrode was poised at a potential sufficiently high to cause oxidation of the polypyrrole to a positively charged form, the presence of anions possessing high enough mobility to penetrate the film facilitated oxidation of the polymer and resulted 0003-2700/88/0360-1979$01.50/0
in the flow of transient, concentration-dependent anodic currents. By this approach, acetate, phcephate, and carbonate could be detected conveniently and reproducibly at concentrations in the 0.01-1 mM range. The most novel aspect of this system is, of course, that at conventional electrodes none of these anions is itself able to be directly oxidized or reduced-and therefore detected-at commonly accessible potentials. In our laboratory, we have observed analogous currents for a variety of electroinactive anions at CMEs where the deposited polymer is formed from aniline monomer units. While 0 1988 American Chemical Soclety
