Anal. Chem. 1987, 59, 203-204
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Comparison of the Pulsed Amperometric Detection of Carbohydrates at Gold and Platinum Electrodes for Flow Injection and Liquid Chromatographic Systems Sir: Since 1981 our laboratory has been successfully developing pulsed amperometric detection (PAD) a t Pt electrodes for compounds considered to be electroinactive under conditions of dc detection. The intended application of the PAD technique is for detection in high-performance liquid chromatography (LC-PAD). However, application to onecomponent samples in flow injection systems (FI-PAD) is immediately obvious and is, in fact, used to test the reliability of the detector technology. We have focused our attention on classes of organic compounds without extensive ?r bonding for which W-vis photometric detection in LC and FI systems does not offer sufficient sensitivity. Applications of LC-PAD with Pt electrodes in our laboratory have included amino acids ( I ) , aminoglycosides (2), carbohydrates (3-5),and recently, sulfur-containing compounds (6, 7). The highest sensitivity for PAD is observed in alkaline media, and robust chromatographic columns are preferred which utilize the alkaline supporting electrolyte as the mobile chromatographic phase. Instrumentation for LC-PAD is now commercially available based on separator columns designed for inorganic ion chromatography (8,9). In the commercialized version, a Au detector has been recommended over a Pt detector (10, 11). General comparisons of electrode materials for amperometric detection in LC have appeared (12); however, no specific comparison has emerged related to the use of PAD for carbohydrates at Au and Pt electrodes. Here we describe briefly some results for glucose and fructose, taken to be representative of simple carbohydrates, and conclude that PAD at Au electrodes has the advantages of high sensitivity and lower detection limits in comparison to Pt electrodes. The triple-step potential waveform described previously for detection of carbohydrates at Pt electrodes (3) is based on a detection potential corresponding to the surface-controlled anodic oxidation of adsorbed carbohydrate in a potential region where dissolved O2 can be cathodically reduced (e.g., ca.-0.2 to -0.8 V vs. SCE in 0.2 M NaOH, see Figure 1). This can lead to serious analytical interferences in FI-PAD if the presence of O2 results in larger base-line signals with an expected decrease in the signal-to-noise ratio (SIN). Deaeration of the mobile phase in LC-PAD improves the situation; however, since the chromatographic systems commonly used in ion chromatography are constructed with 02-permeable plastic tubing, complete elimination of dissolved O2 from the effluent stream is not practical. Detection of carbohydrates at Pt electrodes can be achieved without O2 interference using a detection potential in the waveform which is more positive than recommended above (e.g., 0.00 V vs. SCE in 0.20 M NaOH). This corresponds to the potential region where the anodic formation of surface oxide is rapid at the Pt electrode and the resulting increase in base-line signal (and noise) more than eliminates any advantage expected from removal of O2 interference. Carbohydrates can be detected anodically at Au electrodes in a potential region where O2 reduction does not occur and the formation of surface oxide is not significant (e.g., 0.00-0.20 V vs. SCE in 0.2 M NaOH, see Figure 2). The resulting increase in the S I N for LC-PAD with Au electrodes as compared to Pt electrodes has been verified with detection limits decreased by a factor of -5, for glucose, fructose, sorbitol, and sucrose. The linear region of the calibration curve (i-C) is also more extensive for Au as compared to Pt.
E ! V v s SCE)
Figure 1. Pulsed voltammagrams for glucose obtained at a Pt rotated disk electrode (0.16 cm2; 900 rpm) using a threastep potential waveform: (waveform) E , = varied (200 ms), E, = 0.60 V (200 ms), E, = -1.0 V (600 ms); (A) air satd 0.20 M NaOH; (B) N, satd 0.20 M NaOH; (C) air satd 2.0 mM glucose in 0.20 M NaOH; (D) N2 satd 2.0 mM glucose in 0.20 M NaOH. E ! V vs S C E i
c I
0.9
06
03
00
-03
I
I
I
I
I
-0.6 I
-0.9 I
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Figure 2. Pulsed voltammagrams for glucose obtained at a Au rotated disk electrode (0.005 cm2; 900 rpm) using a three-step potential waveform: (waveform) E , = varied {200 ms), E, = 1.0 V (200 ms), E, = -1.0 V (600 ms); (A) air satd 0.20 M NaOH; (B) N, satd 0.20 M NaOH; (C) air satd 2.0 mM glucose in 0.20 M NaOH; (D) N, satd 2.0 mM glucose In 0.20 M NaOH.
A comparison of the observed signals obtained utilizing the best waveforms for Au and Pt is shown in Figure 3 for 1.0 mM fructose. In addition, the peak signals are also shown which are obtained by dc detection. The base-line currents (FA) for each set of peaks are given in parentheses. Selection of potentials other than ca. -0.36 V for detection with Pt electrodes results in much greater noise levels, as demonstrated for E , = -0.73 V. The dispersion constant for the FI system (DJ, defined by the ratio of the peak or maximum concentration at the detector and the bulk analytical concentration of the sample injected (Cp/Cb) was 0.56. As a final note we observe that mechanical polishing of electrode surfaces used in PAD can result in an unstable base line. Because the alternate anodic formation and cathodic
0003-2700/87/0359-0203$01.50/0 0 1986 American Chemical Society
Anal. Chem. 1987. 59, 204-206
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of maintaining surface activity. The waveform in PAD is designed to remove efficiently all surface-adsorbed reaction products and solution impurities; hence, mechanical polishing is seldom needed. Registry No. Au, 7440-57-5; Pt, 7440-06-4; 02, 7782-44-1; glucose, 50-99-7; fructose, 57-48-7. LITERATURE CITED
Flgure 3. Comparison of flow injection detection peaks for fructose using dc and pulsed amperometric detection: injection, 50 pL of 1.O mM fructose (50 nmol) in 0.20 M NaOH; carrier stream, 0.20 M NaOH at 0.50 mL min-I; (A) gold electrode (PAD) E , = 0.15 V (250 ms), E, = 0.75 V (100 ms), E, = -1.00 V (100 ms) and (dc) E = 0.15 V; (B) platinum eiectrode (PAD) E l = given in figure (500 ms), E = 0.48 V (500 ms), E, = -0.92 V (250 ms) and (dc) E = -0.73 V.
,
dissolution of surface oxide in the pulsed method results in gradual microscopic roughening of the electrode surfaces (13, 14), the base-line signal of newly polished electrodes may require a long time to reach a steady-state value, i.e., a constant surface area. For example, we observe drift of the base line for Pt over a several-hour period, whereas the base line for a newly polished Au electrode usually reaches a steady value in 5-10 min. We recommend that users of PAD resist the traditional urge to polish electrodes regularly for the purpose
(1) Polta. J. A.; Johnson, D. C. J . Liq. Chromatogr. 1983. 6 , 1727. (2) Polta, J. A.; Johnson, D. C.; Merkel, K. E. J . Chromatogr. 1985, 324, 407. (3) Hughes, S.; Johnson, D. C. Anal. Chim. Acta 1981, 732, 11. (4) Hughes, S.; Johnson, D. C. J . Agrlc. Food Chem. 1982, 30, 712. (5) Hughes, S.; Johnson, D. C. Anal. Chim. Acta 1983, 149, 1. (6) Polta, T. Z., Johnson, D. C. J . Nectroanal. Chem. 1988, 209, 159. (7) Polta, T. 2 . ; Luecke, G. R.; Johnson, D. C. J . Nectroanal. Chem. 1988, 209, 171. (8) Rocklln, R. J . Liq. Chromatogr. 1983, 7(8), 504. (9) Edwards, P.; Haak, K. Am. Lab. (FalffieM, Conn.) 1983, (April), 78. (10) Rocklln, D.; Pohl, C. A. J . Liq. Chromatogr. 1983, 6(9), 1577. (1 1) Dionex Corp. Technical Note 7 7 Feb 1983. (12) Rocklin, R. LC Mag., LC Lip. Chromatogr. HPLC Mag. 1984, 2(8), 588. (13) Chiako, A. C.;Triaca, W. E.; AN^, A. J. J . Necfroanal. Chem. 1983, 746, 93. (14) Chiaivo, A. C.; Triaca, W. E.; A ~ i a A. , J. J . Elecfroanal. Chem. 1984, 171, 303.
Glen G. Neuburger Dennis C. Johnson* Department of Chemistry Iowa State University Ames, Iowa 50011
RECEIVED for review June 2,1986. Accepted August 25,1986. This work was supported by the National Science Foundation through Contract CHE-8212032.
AIDS FOR ANALYTICAL CHEMISTS 32-Gold-Electrode-Array Thin-Layer Flow Cell Michael DeAbreu* a n d W. C. P u r d y
Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada With few exceptions, machinable glass ceramics have not been widely used in the fabrication of electrodes despite having several desirable characteristics. Macor is a machinable glass ceramic produced by Corning Glass Works (Corning, NY).It is a hard, mechanically stable material that does not deform under stress as do other ductile materials such as Teflon or Kel-F. It can be easily polished. I t is machinable with ordinary metalworking tools, does not require postfiring, and retains its properties over a wide temperature range. Also Macor can accept thin-film and thick-film metal coatings. Roe and Aparicio-Razo (1)constructed ring-disk electrodes by using Macor coated with gold by thermal evaporation or by sputtering. By use of these methods, a large number of metal coatings are possible. The main drawbacks are the thickness of the coating (10-50 nm), the limited adhesive strength of the coating, the necessity of electroplating the metalization to increase its thickness, and the requirement of specialized thermal evaporation or dc sputtering vacuum chambers. A thick-film metal coating can overcome some of these problems. In our laboratory, gold-metal ink and a simple firing
procedure have been used to produce rugged thick-film gold electrodes which are incorporated into a 32-electrode thin-layer flow cell. The electrode array is inlayed to allow the surface to be polished flat thereby maintaining laminar flow through the cell. The gold strips are 400 pm wide and 200 pm apart. Although Macor can be machined to a tolerance of 15 pm, it is likely that it will chip when the grooves are so tightly spaced. A photolithographic process for the fabrication of copper circuit boards was adapted for use with Macor ceramic. EXPERIMENTAL SECTION The thin-layer cell (see Figure 1)is based on designs previously used in this laboratory (2, 3). The upper cell body is made of Kel-F. It has two tapped holes for the solution inlet and outlet and a piece of sheet platinum inlayed into the surface to serve as a counter electrode. The lower cell body is a 2.5-in.-long hemicylindrical piece of Macor that is cut with a diamond grit saw from 1-in.-diameter rod stock. An array of 32 strip electrodes of gold is inlayed into the surface. A rectangular flow channel is defined by an opening cut into a piece of film made of Teflon, which is then clamped between the cell pieces. The yoke, made of PVC, holds the cell and the external connector. The connector
0003-2700/87/0359-0204$01.50/00 1986 American Chemical Society
