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were examined. Vacuum deposition of a AgC1-AgBr mixture with 26 mol % AgC1, which exhibits a minimum melting point (2, 3) provided a film on the substrate with a ratio of 35 mol 0'9 AgCl to 65 mol % AgBr. This composition was determined through analysis of the surface by ESCA and comparison to samples of AgBr, AgC1, and an AgBr-AgC1 mixture. Data reduction was through procedures outlined elsewhere ( 4 ) . Substrates were Si3N, on silicon, manufactured by thermal nitridation (5).
RESULTS Systems using only AgCl as a gate material responded in Nernstian fashion to uAg+even when the AgCl film had apparently dissolved. However, chloride response, initially Nernstian, became significantly sub-Nernstian, perhaps about 10 mV/decade as AgCl was lost. Figure 1 is a representation of the responses of the mixed AgC1-AgBr sensors which shows agreement of experimental and the theoretical slopes for Ag+, C1-, and Br- activities. The intercepts of these mixture electrodes are always shifted negatively from the measured responses of commercial membrane electrodes in the same solutions ( I ) . Reproducibilities of successive electrodes were within k1 mV for Br- and f 5 mV for C1-. This experiment indicates the use of vacuum deposited mixed salts on ISFETs as a possible method to improve longevity of the ion sensor. Sensitivity to more than one anion is provided without sacrifice of ionic selectivity.
-100'
2
4
5
I
Figure 1. Folded response plots of potential (body vs. reference) as a function of silver ion, bromide ion, and chloride ion activities. Closed symbols represent pressed pellet comparison electrodes, open symbols are the ion selective field effect potentiometric sensors. (H) Ag' responses, ( 0 )CI- responses, and (A)Br- responses
(5) Hackleman, D. E.; Vhsov, Yu. G.; Buck, R. P. J . Electrochem. SOC.1978, 125, 1875.
' Present address:
Department of Chemistry, Leningrad State University, Leningrad, USSR. Present address: Hewlett-Packard Corporation, 1000 N.E. Circle Blvd., Corvallis, Ore. 97330.
Yu. G . Vlasov' D. E. Hackleman' R. P. Buck*
LITERATURE CITED Buck, R. P.; Hackleman. D. E. Anal. Chem. 1977, 4 9 , 2315. Janz, G. J. "Molten Satt Handbook"; Academic Press: New York, 1967; p 33. Arabadzhan, A. S.; Bergman, A. G. Russ. d . Inorg. Chem. 1964, 9 , 958. .... Moses, P. R.; Wier, L. M.; Lennox, J. C.; Finklea, H. 0.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978, 50, 576.
1
1 3 P(0
Department of Chemistry William R. Kenan Laboratories of Chemistry University of North Carolina a t Chapel Hill Chapel Hill, North Carolina 27514 RECEIVED for review April 4,1979. Accepted May 18, 1979.
Complication in the Determination of Nitrite by Ion Chromatography Sir: A serious problem in the determination of nitrite by ion chromatography ( 1 , 2 )has recently come to our attention. Upon elution of a freshly prepared solution of nitrite (-7 ppm) under standard conditions (Table I), two peaks were observed. T h e possibility of an impurity in the nitrite was considered but discounted after noticing that the heights of the two peaks changed as a function of depletion of the suppressor column (Figures l a and Ib). It was suspected that the suppressor column (cation-exchangeresin) was responsible for the splitting. T o confirm this, the analytical separator columns (anion-exchange resin) were bypassed and another sample of the nitrite solution was analyzed, the only column in stream being the suppressor column. Again two peaks were observed (Figure IC),the first peak coincident with the solvent front as determined by injection of a fluoride/chloride standard under identical conditions. Further supportive evidence that the suppressor was to blame was obtained by substituting the standard suppressor column with a smaller column (3 X 250 mm), all other chromatographic parameters remaining standard. With the analytical separator columns in stream (Figure 2a), some splitting is observed; with the analytical columns bypassed, only one peak is apparent (under the specified conditions, insufficient residence time is afforded the nitrite to effect or detect any splitting). It is our contention
t h a t the first peak is due to nitrate ions formed by the oxidation of nitrous acid on the suppressor columr. and that the second peak is due to nitrous acid/nitrite ion broadened and delayed by ion exclusion on the separator column. The following explanation is offered. The function of the suppressor column is to exchange the cations of the eluent and of the sample with hydrogen ions, thus reducing the background conductance of the eluent (by conversion to the weakly ionized carbonic acid (2)) and converting the sample anions to their acid form. For the strong acids this conversion to the acid form (e.g., Na'C1H+Cl-) is inconsequential since the species are still totally dissociated, and hence these anions move unimpeded through the suppressor column. For the weak acids, such as nitrous acid, this conversion to the acid form can affect its elution from the column by retention of the undissociated acid on the column by ion exclusion ( I , 3 ) . Since the exchange front of the suppressor column is well-defined and moves from top to bottom as the column is depleted of H + ion, the point a t which the anion is converted to the acid form is dependent on the degree of depletion of the column. Consequently, the amount of ion exclusion (resulting in longer retention times and broader peaks) is inversely proportional to the degree of depletion of the suppressor column. This shifting and broadening is easily seen
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This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
1 I 2 4 6 minutes
2 4 6 minutes
minutes 2
Flgure 1. Chromatograms of 7 ppm nitrite. Standard conditions. (a) Standard suppressor column approximately 5 % depleted. (b) Standard suppressor column approximately 75 % depleted. Analytical separator
column by-bypassed. Standard suppressor column approximately 5 YO depleted
4
6
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2
4
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Figure 3. Chromatograms of 7 ppm nitrite. Standard conditions. 3 X 250 mm suppressor column substituted for standard suppressor column. (a) With analytical separator column and suppressor column. (b) Analytical separator column by-passed
Table I. Standard Ion-Chromatographic Conditions at the National Bureau of Standards eluant: 0.003 mol/L NaHCO, 10.0018 mol/L Na,CO, flow rate: 2.0 mL/min separator column: 3 x 150 mm anion pre-column plus 3 x 500 mm anion separator column suppressor column: 6 x 250 mm anion suppressor column injection volume: 100 U L
_L_ii 2 4 6 minutes
2 2 4 minutes
Figure 2. Chromatograms of 7 ppm nitrite. Standard conditions. 3 X 250 mm suppressor column substituted for standard suppressor column. (a) With analytical separator column and suppressor column. (b) Analytical separator column by-passed.
by comparing the second peak on Figures l a and l b . T o further complicate the matter, although stable in alkaline solution, nitrite in acid solution is easily oxidized to nitrate. Hence, as soon as nitrite reaches the exchange front of the suppressor column it is subject to oxidation. The amount of oxidation, then, is also inversely proportional to the degree of depletion of the suppressor column, as indicated by the first peak in Figures l a and 1b. Evidence that the first peak is indeed an oxidation product (nitrate) of nitrite is shown in Figure 3. T h e eluant used for chromatogram 3a was standard eluant in equilibrium with air; the eluant used for chromatogram 3b was standard eluant which had been purged with nitrogen. T h e first peak was dramatically diminished, while the second peak increased slightly. Complete elimination of the oxidation product was not achieved, probably owing to residual oxygen in the Teflon tubing and the polystyrene resin itself. In summary, nitrite determination by ion chromatography is complicated by both ion exclusion and oxidation occurring on the suppressor column. Partial remedy can be achieved by deaeration of the eluant. It is strongly advised that frequent calibration with standard nitrite solutions be made, preferably run so as to bracket the unknown, if accurate
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
quantitation is desired. T h e same complications, no doubt, also exist for all weak acids and/or easilv oxidizable anions, such as sulfite.
LITERATURE CITED (1) H. Small, T . s. Stevens, and W. C. Bauman, Anal. Cbem.,47, 1801 (1975). (2) C. Anderson, Clin. Chem. (Winston-Sabm, N.C.), 22, 1424 (1976). (3) F. Smith, Jr., W. Rich, and T. Sidebottom, "Ion Exclusion Coupled to Ion Chromatography: Instrumentation and Application", Second National
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Symposium on Ion Chromatographic Analysis of EnvironmentalPollutants, Research Triangle Park, N.C., October 1978.
William F. Koch Center for Analytical Chemistry National Bureau of Standards Washington, D.C. 20234 RECEIVED for review March 13, 1979. Accepted May 7, 1979.
AIDS FOR ANALYTICAL CHEMISTS Pre-Column Electrochemical Cell for High-Performance Liquid Chromatography Gary W. Schieffer Pharmaceutical Research Division, Norwich-Eaton Pharmaceuticals, Division of Morton-Norwich Products, Inc., Norwich, New York 138 15
Although post-column flow-through electrodes have been used extensively as electrochemical detectors in high-performance liquid chromatography (HPLC) ( I ) , the usefulness of pre-column electrochemical cells apparently has not been explored. Such a cell might provide a means for studying the chromatographic properties of electrochemically generated labile organic compounds or compounds otherwise difficult to obtain or prepare. For example, in HPLC methods requiring the monitoring of the stability of organic compounds (such as active ingredients in pharmaceutical preparations). it is generally desirable t o test the specificity of the method with respect to postulated degradation products before the method is qualified for use. However, since it is sometimes difficult, time consuming, and expensive to purchase and/or synthesize all of the known or postulated degradation products, a quick, simple method for generating some of the products would be useful. Fortunately, the electrochemical oxidation or reduction of organic compounds in many cases yields the postulated degradation products that might be expected to occur during storage through electron transfer with atmospheric oxygen or oxidizing and reducing agents present in the environment (as impurities or matrix constituents). An example is the electrochemical oxidation of catecholamines which yields o-quinones and indoles (2,3),products expected to be formed through chemical oxidative degradation ( 4 ) . Thus, electrochemically degrading the analyte in a sample might result in a useful test of the specificity of a chromatographic method for t h e analyte in the presence of the degradant. With this in mind, an electrochemical cell was designed and constructed to be inserted between the injection port and column of an HPLC apparatus. The major requirements for the cell were a low dead volume to prevent band broadening and a capability for withstanding moderately high pressures (1000-1500 psi) associated with microparticulate packings.
EXPERIMENTAL Cell Design and Construction. A schematic of the cell, which 0003-2700/79/0351-1573$01.0010
is based on an earlier design (51, is shown in Figure 1. The body consisted of two 2-inch (5.1-cm) diameter Plexiglas cylinders both 3/,-inch (1.9-cm)thick, held together with four stainless steel bolts (not shown). The silversilver chloride reference electrode (SSCE) was wound around a Plexiglas post (6.4 mm long and 1.02 cm in diameter) which extended from the upper cylinder into a cavity (2.74 cm in diameter, 8.3 mm deep) in the lower cylinder. Epo-Tek 349 epoxy (Epoxy Technolgy Inc., Watertown, Mass.) was used to seal a 3.07-mm thick glassy carbon disk (Continental Ore Corporation, New York, N.Y.) into the cavity of the lower cylinder, and the SSCE and platinum counter electrode leads into the upper cylinder. Epo-Tek 410E silver-filled conducting epoxy was used to make the electrical contact between the glassy carbon and a copper lead. A 0,025-inch (0.635-mm) diameter channel was drilled through the Plexiglas body with a twist drill bit and continued through the glassy carbon with a stainless steel hypodermic tube attached to an ultrasonic milling machine (Sonipak, Bullen Ultrasonics, Inc., Eaton, Ohio), forming the tubular electrode in the configuration shown in Figure 1. Bolting the two cylinders together exerted pressure on eight cation-exchange membrane washers (0.25 mm thick, 0.63-mm i.d., 2.72-cm o.d., Nafion XR-170, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.) located in the cavity of the lower cylinder. This provided a leak-free solution bridge between the working electrode and reference and counter electrodes. Cheminert fittings and '/ 16-inch(1.6-mm)Teflon tubing were used for the inlet and outlet. The 0.1 M KC1 inlet and outlet consisted of Tygon tubing press fitted, and sealed with cyclohexanone. Instrumentation. All potentials reported in this paper are given with respect to the SSCE (0.1 M in KCl). Potentials were applied with a Princeton Applied Research Model 364 Polarographic Analyzer, the current output being monitored with a strip chart recorder. A high-performance liquid chromatograph (ALC/GPC 202, Waters Associates, Milford, Mass.) equipped with a U6K injection valve and Model 6000 reciprocating pump (Waters Associates), a variable wavelength detector (Model 770, Schoeffel Instrument Corp., Westwood, N.J.), and a prepacked microparticulate reverse-phase column (p-Bondapak CI8,Waters Associates) was used. D 1979 American Chemical Society
