Thin-layer electrochemical detector

Weber, S. G., The dependence of current on flow rate in thin-layer electrochemical detectors used in liquid chromatography. A clarification, /. Electroanal. Chem., 145, 1, 1983. 

A. Yildiz, P. T. Kissinger, and C. N. Reilley, Anal. Chem. 40 1018 (1968). (A host of intriguing thin-layer cell applications are suggested, including the possibility of thin-layer electrochemical detectors for liquid chromatography, realized several years later by Kissinger and Adams.)... 

HPLC-based electrochemical detection (HPLC-ECD) is very sensitive for those compounds that can be oxidized or reduced at low voltage potentials. Spectrophotometric-based HPLC techniques (UV absorption, fluorescence) measure a physical property of the molecule. Electrochemical detection, however, measures a compound by actually changing it chemically. The electrochemical detector (ECD) is becoming increasingly important for the determination of very small amounts of phenolics, for it provides enhanced sensitivity and selectivity. It has been applied in the detection of phenolic compounds in beer (28-30), wine (31), beverages (32), and olive oils (33). This procedure involves the separation of sample constituents by liquid chromatography prior to their oxidation at a glassy carbon electrode in a thin-layer electrochemical cell. 

The electronic tongue system based on flow injection analysis (FIA) with two amperometric detectors was set up. The FLA apparatus consisted of a Jasco (Tokyo, Japan) model 880 PU pump and two EG G Princeton Applied Research (Princeton, NJ, USA) Model 400 thin-layer electrochemical detector connected in series. Each detector was equipped with a working electrode (a dual glassy carbon electrode and a gold... 

Figure 1. Thin-layer electrochemical detector ( (A) auxiliary electrode (R) reference electrode (W) working electrode)...

Acetylcholineesterase and choline oxidase The detector fabrication of glutaral-dehyde co-crosslinking of AChE and ChO with bovine serum albumin on the Pt working electrode of a conventional thin layer electrochemical flow cell. A mobile phase of phosphate buffer 0.1 M pH 6.5 containing 5 mM-sodium hexane sulfonate and 10 mM-tetra-methylammonium phosphate was used in the ion-pair, reversed phase liquid chromatography. 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

Figure 11.6.7 Different geometries for thin-layer electrochemical detector cells involving different placements of the working (W), auxiliary (A) and reference (R) electrodes. [Reprinted from S. M. Lunte, C. E. Lunte, and P. T. Kissinger, in Laboratory Techniques in Electroanalytical Chemistry, ...

Neurochemicals in tissue samples (subpicomole levels) HPLC strong cation exchangers thin layer electrochemical detector Christensen and Le Roy Blank [417]... 

J. Wang and L. D. Hutchins, Thin-Layer Electrochemical Detector with a Glassy Carbon Electrode Coated with a Base-Hydrolyzed Cellulosic Film. Anal. Chem.y 57 (1985) 1536. 

Bergstrom et al. [63] used HPLC for determination of penicillamine in body fluids. Proteins were precipitated from plasma and hemolyzed blood with trichloroacetic acid and metaphosphoric acid, respectively, and, after centrifugation, the supernatant solution was injected into the HPLC system via a 20-pL loop valve. Urine samples were directly injected after dilution with 0.4 M citric acid. Two columns (5 cm x 0.41 cm and 30 cm x 0.41 cm) packed with Zipax SCX (30 pm) were used as the guard and analytical columns, respectively. The mobile phase (2.5 mL/min) was deoxygenated 0.03 M citric acid-0.01 M Na2HP04 buffer, and use was made of an electrochemical detector equipped with a three-electrode thin-layer cell. The method was selective and sensitive for mercapto-compounds. Recoveries of penicillamine averaged 101% from plasma and 107% from urine, with coefficients of variation equal to 3.68 and 4.25%, respectively. The limits of detection for penicillamine were 0.5 pm and 3 pm in plasma and in urine, respectively. This method is selective and sensitive for sulfhydryl compounds. 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

Following extraction and cleanup, nitrofurans are separated by thin-layer or liquid chromatography and measured by spectrophotometric, fluorometric, electrochemical, or mass spectrometric detectors (Table 29.5). Thin-layer chromatography has been carried out on commercially available silica gel plates. Nitrofurans were separated using various solvent mixtures as mobile phases and subsequently detected after spraying with pyridine, using spectrophotometric (29) or fluorometric (158, 159) detectors. 

The most popular electrochemical detectors to date have been based on the amperometric conversion of analyte in a cross-flow thin-layer cell. The basic functioning of this mode of detection is depicted schematically in Figure 27.2. 

The portable instrumentation and low power demands of stripping analysis satisfy many of the requirements for on-site and in situ measurements of trace metals. Stripping-based automated flow analyzers were developed for continuous on-line monitoring of trace metals since the mid-1970s [16,17]. These flow systems involve an electrochemical flow detector based on a wall-jet or thin-layer configuration along with a mercury-coated working electrode, and downstream reference and counter electrodes. 

Notes LOD, limit of detection MeOH, methanol EtOH, ethanol ACN, acetonitrile EtAC, ethyl acetate SPE, solid phase extraction HLB (hydrophilic lipophilic balanced) TFA, trifluoroacetic acid GC, gas chromatography TMS, trimethylsilyl MS, mass spectrometry HPLC, high-performance liquid chromatography DAD, diode array detector NMR, nuclear magnetic resonance ESI, electrospray ionization APCI, atmospheric pressure chemical ionization CE, capillary electrophoresis ECD, electrochemical detector CD, conductivity detector TLC, thin layer chromatography PDA, photodiode array detector. 

Fig. 14.8 A thin-layer cell for use as a high pressure liquid chromatography electrochemical detector (courtesy of Bioanalytical Systems).

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