Anal. Chem. 1885, 57, 1373-1376
LITERATURE CITED Kissinger, P. T.; Refshauge, C.; Dreiling, R.; Adams, R. N. Anal. Lett. 1973, 6,465-477.
Kissinger, P. T. Anal. Chem. 1977, 4 9 , 447A-456A. Bibliography of Recent Reports on Electrochemical Detection”; Bioanalytical Systems, Inc.; West Lafayette, IN, 1982. Anderson, J. L.; Chesney, D. J. Anal. Chem. 1980, 52, 2156-2161. Weisshaar, D. E.; Tallman, D. E.; Anderson, J. L. Anal. Chem. 1981, 5 3 , 1809-1813. Anderson, J. L.; Weisshaar, D. E.; Tallman, D. E. Anal. Chem. 1981, 53, 906-908. Lanouette, M.; Pike, R. K. J. Chromatogr. 1980, 190, 208-211. Mayer, W. J.; Greenberg, M. S.J. Chromatogr. 1981, 208, 295-304.
King, W. P. “Current Separations”; Bioanalytical Systems, Inc.: West Lafayette, IN, 1980; Vol. 2, Part 1, pp 6-8. Kissinger, P. T.; Bratin, K.; King, W. P.; Rice, J. R. ACS Symp. Ser. 1980,No. 136, 57-88. Armentrout, D. N.; McLean, J. D.; Long, M. W. Anal. Chem. 1979, 51, 1039-1045.
Anderson, J. E.;Tallman, D. E.; Chesney, D.J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051-1056. Chesney, D. J.; Anderson, J. L.; Weisshaar, D. E.; Taiiman, D. E. Anal. Chim. Acta 1981, 124, 321-331. Welsshaar, D. E.; Tallman, D. E. Anal. Chem. 1983, 55, 1148-1151. Tallman, D. E.; Weisshaar, D. E. J. Llq. Chromatogr. 1983, 8 , 2157-2172.
Fiiinovsky, V. Yu. Nectrochim. Acta 1980, 25, 309-314. Moidoveanu, S.; Anderson, J. L. J. Electroanal. Chem., in press. Lawrence, J. F.; Lewis, D. A.; McLeod, H. A. J. Chromatogr. 1977, 738, 143-150.
Tjan, G. H.; Jansen, J. T. A. J . Assoc. Off. Anal. Chem. 1979, 62, 789-773.
Dorough, H. W.; Thorstenson, J. H. J. Chromatogr. Scl. 1975, 13,
(27) (28) (29) (30)
2 12-224. Hall, R. C.; Harris, D. E. J. Chromatogr. 1979, 169, 245-259. Sparacino, C. M.; Hines, J. W. J. Chromatogr. Scl. 1976, 14, 549-556. Aten. C. F.; Bourke. J. B. J. Agric. Food Chem. 1977, 25, 1428- 1430. Stoeber, I.; Reupert, R. Vom Wasser 1978, 5 1 , 273-283. Lawrence, J. F.; Turton, D. J. Chrornatogr. 1978, 759, 207-226. Frei, R. W.; Lawrence, J. F.; Hope, J.; Cassidy, R. M. J. Chromatogr. Sci. 1974, 12, 40-44. Moye, H. A. J. Chromatogr. Scl. 1975, 13, 268-279. Cochrane. W. P. J. Chromatogr. Scl. 1979, 17, 124-137. Muth, J.; Giles, J. Altex Chromatogram 1980, 3(2), 5-6. Barton. M. E.; Brewster, J. D.; Anderson, J. L., unpublished results, University of Georgia, Athens, 1981.
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Weber, S. G.; Purdy, W. C. Anal. Chim. Acta 1978, 100, 531-544. Moldoveanu, S.;Anderson, J. L. J . Electroanal. Chem. 1984, 175, 67-77.
Anderson, J. L.; Moldoveanu, S . J. Electroanal. Chem. 1984, 179, 107- 117. Weber, S . J. Nectroanal. Chem. 1983, 145, 1-7. Anderson, J. L., unpublished results, University of Georgia, Athens, 1983.
Nagy, F.; Horanyi, G.; Vertes, G. Acta Chim. Hung. 1982, 3 4 , 35-49. Caudili, W. L.; Howell, J. 0.; Wightman, R. M. Anal. Chem. 1983, 54. 2532-2535. Elbicki, J. M.; Morgan, D. M.; Weber, S. 0. Anal. Chem. 1984, 56, 978-985. Lores, E. M.; Bristoi, D. W.; Moseman, R. F. J. Chromatogr. Scl. 1978, 16, 358-362. Batley, G. E.; Afghan, B. K. J. Electroanal. Chem. 1981, 725, 437-446. MacDougaii, D.; Crummett, W. Anal. Chem. 1982, 5 2 , 2242-2249. van Rooijen, H.; Poppe, H. J. Llq. Chromatogr. 1983, 6 , 2231-2254. Morgan, D. M.; Weber, S . G. Anal. Chem. 1984, 56, 2560-2567. Gueshi, T.; Tokuda, K.; Matsuda, H. J. Nectroanal. Chem. 1978, 8 9 , 247-260. Gueshi, T.; Tokuda, K.; Matsuda, H. J. Nectroanal. Chem. 1979, 101, 29-38.
RECEIVED for review July 5,1983. Resubmitted December 26, 1984. Accepted February 7,1985. We acknowledge gratefully primary funding for this project by the Office for Research and Development, U S . Environmental Protection Agency, under Grant R-808084-01and additional funding by the U.S. Department of the Interior, Office of Water Research and Technology, under Grant A-090-GA. The Environmental Protection Agency does not necessarily endorse any commercial products used in the study. The conclusions represent our views and do not necessarily represent the opinions, policies, or recommendations of the Environmental Protection Agency. Presented in part at the Symposium on LCEC and Voltammetry, Indianapolis, IN, May 16, 1983.
Pulsed Amperometric Detection of Electroinactive Adsorbates at Platinum Electrodes in a Flow Injection System John A. Polta and Dennis C. Johnson* Department of Chemistry, Iowa State University, Ames, Iowa 50011 Adsorbed electroinactlve species on Pt electrodes, as Illustrated here by CI- and CN-, alter the rate of surface oxide formation followlng a posltive potential step. Hence, triplestep potential wave forms similar to those used successfully for pulsed amperometrlc detectlon (PAD) of electroactive adsorbates (e.g., alcohols, carbohydrates, and amino acids) can be applied successfully for electroinactlve adsorbates injected into a stream of electrolyte. Depending on the wave form, the total anodic current at the detectlon peak can be greater than or less than the base-line signal which corresponds to oxide formatlon in the absence of the adsorbate. Sensitivity is hlgh (lod = ca. 1 X lo-’ M In a 50-pL sample, i.e., 0.2 ng of Cl-1. Calibration curves are consistent wlth adsorptlon accordlng to the Langmuir isotherm.
Pulsed amperometric detection (PAD) using triple-step potential wave forms has been applied successfully to the anodic determination of alcohols, polyalcohols, and carbohydrates at Pt and Au flow-through electrodes (1-6) and amino
acids (7)and organic sulfur compds. (6) at Pt electrodes. Severe loss of the activity of noble metal electrodes is associated normally with anodic detection of organic compounds by dc amperometryor by voltammetry at low rates of potential scan, resulting from adsorption of the carbonaceousproducts of the anodic reactions (e.g., radicals). Loss of response is avoided in PAD by measurement of the anodic signal a short time (e.g., 50-250 ms) after application of the detection potential (E,) followed by sequential application of large positive (E,) and negative (E3)values for oxidative and reductive cleaning, respectively, of the electrode surface prior to the next detection cycle. The frequency of the wave form can be sufficiently high (ca. 1-2 Hz)to allow application in chromatographic and flow-injection systems. The oxidation of amino acids occurs at potential values positive of the value at which platinum oxide formation begins. Thus, when PAD is applied to amino acids (7),the total current measured in the detection pulse results from the formation of surface oxide as well as the faradiac reaction of the adsorbed amino acids. The large background is sufficiently stable to permit dc offset at the recorder input, and sensitive
0003-2700/85/0357-1373$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57,NO. 7,JUNE 1985
determinations of very small analytical signals are still possible. The investigation of the application of PAD to the detection of amino acids led to the observation of “negative” peaks (i.e., the anodic peak current is less than the background current) if the current is sampled after a very short delay (e.g., 0.3 V. Hence, io, - l / t plots, obtained by the pulse technique in the presence of CN-, are not expected to be linear. This was observed both for CN- and GI-. The magnitude of the negative detection peak, as defined by eq 3, results from oxide suppression and is proposed to be
C
Zox,aup,p
n
-- lox - lox,e.up
(3)
a measure of surface coverage by the adsorbate according to eq 4 where Bad corresponds to the peak value. The relative (I
+--t
2.0 min
ZOX,SUP,P
- c [ E- Eo] - c [ E - Eo][1 - 8adl = t t
time-
Flgure 3. Pulsed amperometric detection (PAD) of CN- in 0.25 M NaOH. Conditions: (A) direct (“positive”) detection, E, = 625 mV, E, = 700 mV, E, = -900 mV, t , = 400 ms, t , = 50 ms, t, = 425 ms; (B) indirect (“negative”) detection, E, = 100 mV, E, = 700 mV, E, = -900 mV, t, = 400 ms, t, = 250 ms, t, = 425 ms. Con(c) 1.0 X (d) 7.5 centration (M): (a) 1.0 X lo-,, (b) 7.5 X X (e) 2.5 X
tions to the observed current resulting from an anodic potential step are the charging of the double layer and formation
peak surface coverage is defined as the ratio of iox,sup,p to io, for a fixed t, eq 5,
-~OX,SUP,P - 60,
c[E - E01 leadl t = dad c[E - E01
(5)
L
under the assumption that the proportionality constant ( c )
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
l i O
o.+
T
I
I
I
I
A
\
I
0.01
I
1
I
I
I
1
I
I
I
150
100
50
1
20 0
'd ( m s )
Figure 5. Plot of Sad (accordlng to eq 5)vs. f d for CI- In 0.5 M HPS04. Conditions: (A) E, = 700 mV, remainder as in Flgure 4, (B) E, = 800 M CI- in mV, remainder as In Figure 4. Concentration: 5.0 X 0.5 M H,SO,.
- 0O 2
50 L
2
20
30 40 - l o g [Cl-] (Mi
Figure 6. Plot of log [Sa,l(l
- Sa,)]
vs. log [CI-1. Conditions as in
Figure 4.
is independent of Sad. The maximum suppression possible for oxide formation for a given adsorbate corresponds to the maximum surface coverage attainable for that adsorbate, Sadsnax. In effect, a limiting suppression current (iox,supmmax) will be observed for a surface saturated with adsorbate (Sad = 1.0); this has been vefified experimentally. The decrease in Sad for C1- was found to be approximately linear with time (Figure 5) which we interpret to represent zero-order kinetics with respect to Sad, over the range of t represented in the figure, as given by eq 6 where Sad$ is the
(6) = 6ad,0 - Kt value of Sad for t = 0. The slope of the plot of Sad vs. t (i.e., -k) was determined also to increase with increasing positive value of E,. This dependence on E , is expected since desorption is influenced by Box which is, in turn, a potentialdependent quantity. This dependency will be considered in detail in a future publication. Plots of iox,aup,p and Sad vs. C have a distinct Langmuirian appearance. Figure 6 displays a plot of log [ead/(1 - Sad)] vs. log [Cl-] which is linear with a slope of 1.0, as expected for control by the Langmuir isotherm. Deviation from linearity at large concentrations occurs due to lateral interactions of the adsorbate for Bad 1. The initial rate of adsorption for a species with fast adsorption kinetics at a uniformly accessibleelectrode (e.g., single crystal) is expected to be limited by mass transport. Likewise, Sad (eq 5), measured for short adsorption times (i.e., the duration of E3,t3),is expected to be mass transport limited, hence proportional to the bulk concentration (Cb) and t8. This Oad
-
proportionality was not observed for the C1- system; the increase of t 3 beyond 50 ms had no effect to increase the surface coverage. The integrated transport-limited flux to the electrode surface, calculated on the basis of measurements for an electroactive analyte, requires t3 > 1 s to achieve the peak value of Oad observed for C1- with t3 = 50 ms. Hence, we conclude that the majority of C1- adsorbed at the reduced Pt surface remains at the oxide-covered surface even for E2 in the region of 0 2 evolution for short t2. This had not been anticipated. Evidence for the adsorption of C1- on PtO in anodic regions exists from radiochemical studies ( I I , 1 2 ) . If the oxidative cleaning process at E2 is not 100% effective, the adsorbed C1- will carry over into the next detection cycle. Also, Clp produced by the oxidative cleaning that does not diffuse from the electrode surface is reduced to Cl- during the cathodic potential step (E3)and is available for adsorption. In FIA experiments, peak tailing is still present after many equivalent sample volumes of carrier stream have passed through the detector. Such tailing (Figure 4) is not a result of the sample diffusion profile and is attributed to the carry-over of C1-. Positive absolute errors occur if repetitive sample injections are closely spaced in time. We conclude that it is by carry-over that Langmuir adsorption control of the current response for detection of C1- is observed for small t3 for both the negative and positive detection techniques. For the case in which C1- is detected at potentials at which oxide formation and C1- oxidation occur simultaneously, plots of iOx,,vs. C are not linear. However, plots of log [L&/(l- Oad)] vs. log C are linear, and we conclude again that detection control is by Langmuir-type adsorption. As expected, a plot of l/i, vs. 1/C is linear. This is significant in light of the fact that calibration plots for the PAD detection of carbohydrates, amino acids, and sulfur compounds are nonlinear, whereas plots of l/i, vs. 1/C are linear (6). We conclude that the current response of these compounds is governed in the same manner as the responses for C1-.
CONCLUSION We conclude that any species adsorbed a t a reduced Pt surface will affect the i-t curve for formation of the surface oxide following a positive potential step. Hence, pulsed amperometric detection (PAD) at Pt electrodes is useful for sensitive detection of electroactive and electroinactive adsorbates. We propose that PAD is useful not only for detection in liquid chromatographic analysis where sufficient resolution of adsorbable species is provided, but also for fundamental studies of adsorption on the noble metal electrode. Registry No. C1-, 16887-00-6;CN-, 57-12-5;Pt, 7440-06-4. LITERATURE CITED (1) Hughes, S.; Meschl, P. L.; Johnson, D. C. Anal. Chim. Acta 1981, 132, 1. (2) Hughes, S.; Johnson, D. C. Anal. Chlm. Acta 1981, 132. 11; 1983. 149, 1. (3) Hughes, S.; Johnson, D. C. J . Agric. Food Chem. 1982, 30, 712. (4) Edwards, P.; Haak, K. K. Am. Lab. (Fairfield, Conn.) 1983, Apr, 78. (5) Rocklln, R. D.;Pohl, C. A. J . Li9. Chromatogr. 1983, 6 , 1577. (6) Austin, D. S.; Polta, J. A,; Polta, T. 2 . ; Tang, A. P.-C.; Cabelka, T. D ; Johnson, D. C. J . Electroanal. Chem. 1984, 168, 227. (7) Polta, J. A,: Johnson, D. C. J:
