Thin layer cells, liquid chromatography

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.)... 

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

Liquid chromatography with sandwich-type thin-layer cell 275... 

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. 

Figures 2 through 9 are infrared spectra of fractions collected from partition columns, gas chromatography, thin-layer chromatography, or a combination of these separation techniques. Figure 10 is the infrared spectrum of a compound isolated by gas chromatography after hydrolysis of a pyrethrum concentrate. In this case the compound is a long-chain ester. All the infrared spectra were made with a Perkin-Elmer Model 221 instrument. The following operating parameters were used. A liquid demountable cell with a 0.01-mm path length was employed.

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],...

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. 

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. 

Microcystins were first purified by Botes et al in 1982, and, since then, many different approaches " have been adopted for the isolation of microcystins from cyanobacterial cells. The most widely used proce-dures are as follows The lyophilized cyanobacterial cells which contain microcystins are extracted with organic solvents several times, and then the extracts are applied to multistep column chromatography and thin-layer chromatography.For example, Harada et al. established an effective analysis method for microcystins RR and LR. They used 5% aqueous acetic acid solution as an extracting solvent and isolated microcystins by using preparative or semipreparative liquid chromatography with ODS, silica gel, or gel permeation columns. 

A glucocerebroside from the yeast form of the dimorphic human pathogen S. schenckii has been isolated and its components characterized by thin layer and gas liquid chromatography and mass spectrometry (72). It was found to contain glucose, sphingosine and a-hydroxy stearic acid (1 1 1). No role has been attributed to this compound in association with infection and cell surface reactivity. 

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. 

Phenol can be detected electrochemically by oxidation at a carbon paste electrode (Wehmeyer et al., 1983). A convenient means of determining a low concentration of phenol in a small volume of sample is by liquid chromatography with electrochemical detection (LCEC). A diagram of the LCEC system is shown in Fig. 2. The sample is injected by means of 20-pl sample loop into a 5-cm column slurry-packed with lO-pm Cjg stationary phase. The column serves to separate the peak for phenol from other assay constituents in order to achieve a better detection limit. The phenol is detected by oxidation in a thin-layer electrochemical cell with a carbon paste working electrode. 

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