Anal. Chem. 1989, 6 1 , 787-789
LITERATURE C I T E D Oka. K.; Minagawa, K.; Hara, S.; Noguchi, M.; Matsuoka, Y.; Kohno, M.; Irimajiri, S. Anal. Chem. 1984, 56, 24-27. Oka, K.; Ohki, N.; Noguchi, M.; Matsuoka, Y.; Irimajiri, S.; Abe, M.; Takizawa, T. Anal. Chem. 1984, 5 6 , 2614-2617. Oka. K.; Ijitsu, T.; Mlnagawa, K. Hara, S.; Noguchi, M. J. Chromatogr. Biomed. Appl. 1985, 339, 253-261. Oka, K.; Noguchi, M.; Kitamura, T.; Shima, S. Ciin. Chem. 1987, 3 3 ,
1639-1642. Oka, K.; Hirano, T.; Noguchi, M. J. Chromatogr. Biomed. Appl. 1987, 423, 285-291. Oka, K.; Noguchi, M.; Hirano, T. Clin. Chem. 1988, 34, 557-560. Oka, K.; Aoshirna, S . ; Noguchi, M. J. Chromatogr. Biomed. Appl. 1985, 345, 419-424. Pinkerton, T. C.; Miiier, T. D.; Cook, S.E.; Perry, J. D.; Rateike, J. D.; Szczerba, T. J. Biomed., Chromatogr. 1988, 1 , 96-105. Pinkerton, T. C.; Hagestarn, I.H. Anal. Chem. 1985, 5 7 , 1757-1763.
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(10)Pinkerton, T. C.; Perry, J. A.; Rateike, J. D. J. Chromatogr. 1986, 367, 412-418. (I 1) Nakagawa, T.; Shibukawa, A.; Shimono, N.; Kawashima, T.; Tanaka, H.; Haginaka, J. J. Chromatogr. Biomed. Appl. 1987, 420, 297-311. (12)Fasco, M. J.; Cashine. M. J.; Kaminsky, L. S . J. Liq. Chromatogr. 1979, 2, 565-575. (13)Schweizer, K.; Wick, H.; Brechbuhler, T. Clin. Chim. Acta 1978, 9 0 , 203-208. (14)Brelter, J. J. Clin. Chem. Biochem. 1976, 14, 46. (15)Joiiey. M. E.; Stroupe, S. D.; Schwenzer, K. S . ; Wang, C. J.; LuSteffes, M.; Hili, H. D.; Popeika, S. R.;Holen, J. T.; Keiso, D. M. Clin. Chem. 1981, 2 7 , 1575-1579. (16)Ogilvie, R. I. Clin. Pharmacokinet. 1978, 3 , 267-293.
RECEIVED for review May 13, 1988. Accepted December 1, 1988.
Potentiometric pH Detection in Suppressed Ion Chromatography M a r e k Trojanowicz Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland Mark
E. MeyerhofPr
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055 The development of modern high-performance ion chromatography was based on two fundamental achievements presented in the original pioneering work of Small et al. (1). These advances were the preparation of new ion-exchange resins that featured high efficiencies yet very low exchange capacities and the ingenious use of a stripper (suppressor) column in conjunction with conductivity detection. Addition of a stripper column (loaded with appropriate ions) immediately after the separating column effectively neutralizes the high concentration of hydrogen or hydroxyl ions in desired eluents, and this results in a dramatic decrease in the background conductivity of the effluent (i.e. suppressed ion chromatography). As a result of the ion-exchange process in the original packed-bed stripper columns or, more recently, in hollow ion-exchange suppressor fibers, separated cations are present as hydroxides in the effluent while separated anions elute with counter protons. Although the counter hydrogen and hydroxide ions have large limiting conductances that aid in the sensitive detection of the separated ions by conductivity, variations in the equivalent conductances of the individual separated analyte ions influence the ultimate sensitivity toward each. Moreover, conductivity detection is extremely sensitive to temperature variations, and this can result in severe base line drift problems unless the cells are well thermostated or some type of reference cell compensation is used. Recent technological advances in the field of ion-selective membrane electrodes have enabled the fabrication of very sensitive and reliable potentiometric sensors selective for hydrogen ions based on the use of plastic membranes doped with appropriate neutral carrier molecules (2,3). Such sensors have been applied successfully to monitor the pH of flowing systems. Under certain conditions, these polymer membrane type pH sensors can exhibit better dynamic response than conventional pH glass electrodes (2). Surprisingly, despite numerous applications of potentiometric detection in ion chromatography (4),only one report has appeared in connection with using a hydrogen ion electrode as a detector (5). This work focused on monitoring the separation of organic
* Author to whom correspondence should be addressed.
carboxylate anion species with a glass membrane p H sensor by detecting small changes in the pH of a postcolumn reagent buffer continuously added to the effluent stream (due to the basicity of the carboxylate species). Since in modern suppressed ion chromatography separated ions are present in the effluent with equimolar amounts of hydrogen or hydroxyl ions, it seemed worthwhile to examine whether a simple polymeric pH electrode could be used in place of a conventional conductivity detector to monitor the separation of both cations and anions. For this purpose, a poly(viny1chloride) (PVC) membrane electrode prepared with tri-n-dodecylamine (2) was used in a flow-through wall-jet arrangement, and the detection capabilities of this system were directly compared to those obtained with conductivity detection. EXPERIMENTAL SECTION Apparatus. The instrumental setup used in this study (see Figure 1)consisted of a Spectra Physics SP 8700 solvent delivery system (San Jose, CA), injection valve 7124 (Rheodyne, Cotati, CA) with a 1OO-pLsample loop, and a conductivity detector, Model 213 (Wescam, Santa Clara, CA). The potentiometric detector, placed downstream from the conductivity detector, was connected to an Accumet Model 910 pH/mV meter (Fisher Scientific, Pittsburgh, PA). Analog outputs of both signal transducers were connected to a Fisher Recordall Model D5117-5AQ strip chart recorder. Columns. Separation of cations was performed with an Interaction Chemicals ION-210 cation-exchange column (Mountain View, CA), whereas separation of anions was achieved on a Hamilton PRP-X100 column (Reno, NV). In both cases, an ion Guard polymeric column from Interaction Chemicals was placed between the injector and the analytical column. The suppressor used for the chromatography of anions was a 240-cm length of Nafion 811X tubing (Perma Pure Products, Toms River, NJ), while for cations, a 90-cm length of Raipore T-1030 strong anion-exchange tubing (RAI Research Corp., Hauppauge, NY) was employed. The ion-exchange tubings used were coiled (4-cm diameter) and immersed in a 1-L beaker containing the appropriate regenerant solution and a magnetic stir bar. The regenerant solution was changed periodically, as determined from observing the base-line drift of the detectors. Potentiometric Detector. Potentiometric detection was carried out by using a large-volume wall-jet style membrane
0003-2700/69/0361-0767$01.50/0 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
Table I. Comparison of Detection Limits Obtained in Chromatography of inorganic Anions Using Conductivity and Potentiometric Detectiona conductivity
detection
t
eluent concn, mmol, L pH of effluent* bkgd conductivity of effluent, g S bkgd electrode potential of effluent, ml'
Flgure 1. Schematic diagram of instrumentation used for ion chromatography measurements.
electrode arrangement analogous to that described previously (6). An exchangeable cap made of Delrin resin was fitted over the end of the indicator electrode body (Phillips, Model IS-561, Glasblaserei Moller, Zurich). The outlet nozzle of the cap was 0.56 mm in diameter. The distance between the exit orifice of the connection tubing and the surface of the polymer membrane held in the electrode body was 4 mm. The entire indicating electrode with the wall-jet assembly, together with a saturated calomel reference electrode (SCE),was placed in a 100-mL beaker, where the level of effluent solution was kept 1 cm above the membrane of the indicator electrode. The indicator electrode contained a PVC-type hydrogen ion selective membrane mounted in the Phillips electrode body. The pH membrane was prepared by evaporating a mixture containing 24 gL of tri-n-dodecylamine (Eastman Kodak, Rochester, NY), 51 mg of high molecular weight poly(viny1chloride) (Polysciences, Warrington, PA), 132 g L of dibutyl sebacate (Fluka, Ronkonkoma, NY), 1.4 mg of sodium tetraphenylborate (Sigma,St. Louis, MO), and 1.5 mL of distilled tetrahydrofuran (Aldrich, Milwaukee, WI) from a flat glass surface of 3.8 cm2. Smaller disks were cut from this larger membrane and mounted in the electrode body. The internal solution of the polymer pH electrode was 0.25 mol/L KH2P04, 0.25 mol/L Na2HP04,and 0.1 mol/L NaCl. Such polymer pH electrodes function effectively for at least 1 month without significant changes in potentiometric pH response. Reagents. Solutions of nitric acid used as eluents in the chromatography of cations were prepared by diluting concentrated nitric acid (Mallinckrodt,Paris, KY). Sodium hydroxide solutions used as eluents in anion chromatography and as a regenerant for the suppressor tubing in cation chromatography were prepared from solid reagent grade material (Aldrich). Hydrochloric acid solutions used for suppression in anion chromatography were prepared by diluting concentrated reagent grade acid (Mallinckrodt). Solution mixtures of separated cations were prepared from analytical grade ammonium, lithium, potassium, and sodium chlorides. Solution mixtures of anions were prepared from analytical grade sodium salts, except in the case of bromide where potassium bromide was used. All solutions were made with deionized water obtained from a Milli-Q system (Millipore, Bedford, MA).
RESULTS AND DISCUSSION The main goal of this study was to compare, under similar experimental conditions, results obtained when conductivity and potentiometric p H detection schemes are used in suppressed ion chromatography of anions and cations. Measurements were performed in a system containing a simple coiled ion-exchange suppressor tubing immersed in stirred regenerant solution, and connected in series with a 2-pLvolume conductivity detector and the wall-jet potentiometric cell. Experiments reported here were performed by using conditions optimized earlier for the same analytical columns used in replacement ion chromatography employing a potassium ion selective electrode as the detector (7). For the chromatography of anions, the optimal detection limits for both types of detectors were obtained under slightly different experimental conditions (see Table I). The most sensitive response with conductivity detection was achieved
conductivit)r' noise, m\' detection limit, Gmol/Ld
FCINO*-
Br-
potentiometric
PH detection
15 5.37 0.27 113
50 5.90
0.29 82
detection pH conductivityC pH
0.015
0.1
0.015
0.1
1.5 1.9 9.4 5.2
2.3 2.8 8.3 5.3
2.4 3.0 12 32
1.1 1.3 3.3 6.5
"Conditions: flow rate, 1 mL/min; sample volume, 100 fiL; suppressor, 250-cm Nafion 811X tubing; column, Hamilton PRPX100; regenerant, 12 mmol/L HCI. bMeasured in collected effluent with conventional pH glass electrode. ( 1 m\' = 0.1 p S . Determined on the basis of S/.V = 3.
with 15 mmol/L NaOH. Increasing the concentration of this eluent results in a slight increase in background conductivity and worse detection limits, particularly for the more strongly retained anions (nitrite and bromide, see Table I). The optimal response of the potentiometric p H detection system was obtained with 40 mmol/L NaOH as the eluent, where the p H of the effluent was closer to a neutral value. Ideally, absolute neutralization of the effluent by the suppressor tube would be desirable for both types of detectors. However, in practice, this is difficult to achieve (with the homemade suppressor system used here) owing to variations in the kinetics of the ion-exchange process in the suppressor tubing, impurities in the eluent reagents, and some slight leakage of counterions through the walls of the suppressor tubing. With improved commercial suppressor materials and designs (e.g. Dionex system), some of these problems may be eliminated, yielding even better detection limits for both detectors. Nonetheless, under given system conditions, Table I clearly shows that for anion chromatography, potentiometric p H detection competes quite favorably with conductivity. Typical anion chromatograms recorded with both detectors are shown in Figure 2. Similar results were obtained for the separation and detection of cations. In this case, optimal response for both types of detectors was achieved with the same eluent, 15 mmol/L nitric acid, with background effluent conductivity of 0.18 NUS and a p H of 6.70. As shown in Table 11, despite even larger noise in the potentiometric signal (compared to that observed in the case of anion chromatography), estimated detection limits for all of the separated cations are approximately 2 times lower with potentiometric p H detection than with conductivity. Typical cation chromatograms recorded with both detectors are shown in Figure 3. Estimates of the detection limits obtainable with the p H detection method (Tables I and 11) were made by assuming a linear relation between peak height and concentration of injected sample for potential changes not exceeding 20 mV. Previous efforts (8-11) have established that in this restricted region, logarithmic potentiometric detectors exhibit a nearly linear response toward concentration of injected ions. Such a relationship was also confirmed in this study, where calibration curves for the separated cations and anions were in fact linear below AI3 values of 20-25 mV (not shown). Re-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
1
A
r
B
I
789
1
IIOm"
B
r10mV I
!
, 0
20
0 2 0
I
I
Figure 2. Anion chromatograms obtained in suppressed system with conductivity detection (A) and potentiometric detection using hydrogen ion selective electrode (B). Conditions: 15 (A) and 50 mmol/L (B) NaOH solutions at a flow rate of 1 mL/min were used as eluents, and 12 mmol/L HCI as regenerant for the suppressor. Injected: 100 pL of 0.2 mmol/L F- (I],CI- (2). NOz- (3), and Br- (4).
Table 11. Comparison of Detection Limits Obtained in Chromatography of Inorganic Cations with Conductivity and Potentiometric Detectiona
potentiometconductivity ric pH detectionb detection noise, mV recording peak height, mVc Li+ Na+ NH4' K+
detection limit, pmol/Ld Lit Na+ NH,' K+ (1
Na
LI
t , rnin
0.015
0.7
0.54 0.54 0.28 0.28
44.0 42.8 28.4 24.0
8.3 8.3 16 16
4.7 4.9 7.4 7.8
Conditions: Column, Interaction Chemicals ION-210; eluent,
10 mmol/L HN03;flow rate, 0.8 mL/min; sample volume, 100 pL; suppressor, 90-cm Raipore anion-exchange T-1030 tubing in 10 mmol NaOH. b l mV = 0.1 pS. cFor 100 mmol/L standards. dBased on S I N = 3.
strictions on the range of linearity with the pH detector represent the only observed disadvantage of this monitoring scheme relative to conductivity detection. It should be noted that the large-volume wall-jet electrode detector arrangement used here may not be the best design for monitoring small pH changes in the flowing effluent of suppressed ion chromatography systems. Indeed, some dispersion in this type of detector arrangement is likely, causing smaller net signals than might be observed with zero dead volume tubular polymer or capillary glass membrane pH electrodes. Moreover, by use of higher impedance amplifiers,
0
I
I
I
30
t,min Flgure 3. Cation chromatograms obtained in suppressed system with conductivity detection (A) and potentiometric detection using hydrogen ion selective electrode (B). Conditions: 10 mmol/L HNO, was used as eluent at a flow rate of 0.8 mL/min, and 10 mmol/L NaOH as regenerant for the suppressor. Injected: 100 pL of each component at 0.1 mmol/L.
better shielding, and pulse suppressing techniques, it should be possible to greatly reduce the noise levels in our potentiometric pH detector, and this should further improve the observed detection limits of the proposed system. In view of the preliminary results obtained to date and the simplicity, low cost, and accessibility of potentiometric detection equipment, we believe that pH detection in suppressed ion chromatography represents an attractive alternative to the conventional conductivity method. Registry No. F, 16984-48-8; C1, 16887-00-6; NO2, 14797-65-0; Br, 24959-67-9; Li, 7439-93-2; Na, 7440-23-5; NH4+,14798-03-9;
K, 7440-09-7. LITERATURE CITED ( 1 ) Small, H.; Stevens, T.; Bauman, W. Anal. Chem. 1975, 4 7 , 1801-1 809. (2) Schulthess, P.; Shijo, Y.; Pham, H. V.; Pretsch, E.; Ammann, D.;Simon, W. Anal. Chim. Acta 1981, 131, 111-116. (3) Oesch, U.; Brzozka, 2.:Xu, A,; Rusterholz, 8.: Sutter, G.: Pham, H. V.; Weiti. D. H.: Ammann, D.: Pretsch, E.: Simon, W. Anal. Chem. 1986, 58, 2285-2289. (4) Trojanowicz, M. Proceedings of EiectroFinn Analysis Conference, Turku, Finland, June 1988. (5) Egashlra, S.J . Chromatogr. Ig80, 202, 37-43. (6) Ilcheva, L.; Trojanowicz, M.: Krewczyviski vel Krawczyk. T. Fresenlus' 2.Anal. Chem. 1987, 328, 27-32, (7) Meyerhoff, M. E.: Trojanowicz, M., submitted for publication In Anal. Chlm. Acta. ( 8 ) Franks, M. C.; Pullen, D.J. Analyst 1974, 9 9 , 503-514. (9) Bardin, V. V.; Ivanov. Y. M.; Shartukov, 0. F. Zh. Anal. Khim. 1978, 33, 1732-1737. (10) Wang, W.; Chen, Y.; Wu. M. Analyst 1984, 109, 281-286. (11) Haddad, P. R.; Alexander, P. W.: Trojanowicz, M. J . Chromtogr. 1985, 321, 383-374.
RECEIVED for review September 13,1988. Accepted December 15, 1988.
