Thin-layer amperometric detector

Figure 5.4 Common amperometric detectors thin-layer (A) and wall jet (B) flow cells. 5.2.2 Practical considerations...

The most widely used amperometric detectors are based on the thin-layer and wall-jet configurations (Figure 3-22). The thin-layer cell relies on a thin layer of solution that flows parallel to the planar electrode surface, which is imbedded in a... 

FIGURE 3-26 Dual-electrode thin-layer detector configurations for operation in the series (a) and parallel (b) amperometric modes. 

Example 3-6 Flow analysis of a urine sample at a thin-layer amperometric detector, with a flow rate of 1.25mLmin yielded a limiting current value of 1.6 pA for its unknown uric acid content. A larger current of 2.4 pA was observed for a sample containing 1 x 10 4 M uric acid and flowing at a rate of 0.9 mL min. Calculate the original concentration of uric acid in the sample. 

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. 

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 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... 

In one report, a thick-film C electrode (by screen printing) was constructed. Carbon ink (10 p.m thick) was first printed on an alumina plate, and cured thermally. Then the silver ink (28 p,m thick) was printed and cured to partially overlap with and hence connect to the C layer. The thick-film C electrode was found to enhance the detection sensitivity, as compared to the thin-film amperometric detector [753]. 

K. Bratin, P. T. Kissinger, and C. S. Brunlett, Reductive mode thin-layer amperometric detector for liquid chromatography, J. Liquid Chromatogr., 4-.Y111 (1981). 

Three working electrode materials are commonly used in thin-layer amperometric detectors glassy (or "vitreous") carbon, carbon paste, and gold amalgamated with a thin film of mercury. 

With respect to chromatography, electrochemical detection means amperometric detection. Amper-ometry is the measurement of electrolysis current versus time at a controlled electrode potential. It has a relationship to voltammetry similar to the relationship of an ultraviolet (UV) detector to spectroscopy. Whereas conductometric detection is used in ion chromatography, potentiometric detection is never used in routine practice. Electrochemical detection has even been used in gas chromatography in a few unusual circumstances. It has even been attempted with thin-layer chromatography (TLC). Its practical success has only been with liquid chromatography (LC) and that will be the focus here. 

Reversed-phase liquid chromatography with thin-layer amperometric detector operated 271 at +0.720 V vs. Ag/AgCI... 

Cells are classified according to how the working electrode is positioned relative to the flow stream. There are three major configurations tubular, thin layer, and wall jet. The tubular cell (open or packed) with its greater working electrode surface area is used for coulometric detection. The thin layer and wall jet designs are used for amperometric detector cells. In thin layer cells, the eluent flow is in the same plane as... 

Detection electrochemical amperometric detector LC-4B (Bioanalytical Systems Inc.) mode single electrode cell geometry thin layer, 2 pm working electrode glassy carbon reference electrode silver / silver chloride range 10 nA. 

Amperometric detection included a BAS (Bioanalytical Systems, Inc.) LC-4B amperometric detector, a BAS TL-3 thin-layer amperometric flow cell (5 mil spacer), and an RC-2A reference compartment. The flow cell contained a carbon paste-paraffin oil working electrode, a silver/silver chloride reference electrode (RE-1), and a stainless steel auxiliary electrode. A BAS RYT strip-chart recorder was used. 

Using a packed bed of Hg-coated Ag particles approximately the size of a precolumn, Eggli and Asper (1978) developed a novel scheme for the detection of thiols and disulfides. The packed reactor was inserted between the column and an amperometric Hg pool electrode detector. Incoming disulfides were effectively reduced in the reactor to the parent thiol, followed by the latter s downstream detection at the Hg pool, llie approach was considerably miniaturized using a dual Hg/Au thin-layer cell, with concomitant improvement in SNR (Allison and Shoup, 1983). 

The Kissinger type twin electrode thin layer cell is a widely used tool in the everyday analytical practice especially in the field of flow injection techniques and as an amperometric detector in liquid chromatography. Less attention is paid to the possibilities offered by this cell 2LS a microanalytical tool when it is filled with a quiescent solution sample. In this way the determination of electroactive components in a volume of about 50-100 pi can be carried out by applying a proper excitation potential program. 

TL, thin-layer cell equipped with a glassy carbon electrode WJ, wall-jet cell equipped with a glassy carbon electrode CO-screen coulometric cell with dual-electrodes of porous graphite used in screen mode PAD, pulsed amperometric detector equipped with a gold electrode. 

Mao, L., G. Shi, Y. Tian, H. Liu, L. Jin, K. Yamamoto, S. Tao, and J. Jin (1998). A novel thin-layer amperometric detector based on chemically modified ring-disc electrode and its application for simultaneous measurements of nitric oxide and nitrite in rat brain combined with in vivo microdialysis. TalarUa 46, 1547—1556. 

Of the various EC detector cell designs (Figure 3.4), the thin-layer and wall-jet configurations are commonly used in commercially available amperometric... 

Modem amperometric detectors possess a number of useful features. These include rapid response time, low cell volume, ease of access to the electrode surface for cleaning, ability to be used in series with other detectors, good sensitivity with suitable analytes, in-built facilities for scanning the detection potential and minimal mnning costs. Since amperometric electrodes are small it is possible to incorporate more than one into a thin-layer electrode block. Most commercial cells usually contain two electrodes with their necessary connections. At the simplest level this allows the rapid connection of the second electrode when the first becomes contaminated. The electrode connections are simply transferred to the other electrode pin without having to dismantle the cell. A discussion of the use of two or more amperometric electrodes for differential analysis is given below (Section 7). 

Some of the comparisons are described below, but the results must be interpreted with due caution particularly in view of continuing improvements in detector design. One report compared two coulometric detectors for catecholamine analysis and concluded that they were equivalent to current amperometric detectors. Another study compared wall-jet and thin-layer cell configurations. Other more comprehensive studies have been controversial. Forzy et al compared 11 detectors for the analysis of 5-hydroxyindoleactic acid (5-HIAA). They used the same HPLC system with each detector to determine linearity, repeatability, absolute sensitivity, limit of detection and stabilisation time. Driebergen and Benders evaluated 10 detectors with respect to their suitability for routine use in a pharmaceutical company using tetramethylbenzidine as the test compound. Both of these reports found similar relative results with respect to sensitivity and ease of use but stressed the importance of matching instrument to application. 

Figure 18. Schematics of the most common amperometric detector cells A) Wall-jet cell B) Thin-layer cell a) Working electrode...

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