Electrochemical detectors carbon fiber

The need to measure concentrations in very small volumes is not restricted to biological systems. For example, open tubular columns for liquid chromatographic separations offer the advantage of increased resolution, but because their internal diameters may be as small as 15 pm, the amount of material in the eluted peaks is very small. Thus, the use of these columns requires detectors that can be used with low concentrations in small volumes. Jorgenson and co-workers showed that this could be accomplished by the insertion of a 10-pm-diameter, cylindrical electrode made from a carbon fiber into the end of the column [4]. The close fit between the column wall and the fiber ensured that a large fraction of the eluting molecules were electrolyzed. When the electrochemical data were collected in a voltammetric mode, the resolved compounds could be classi-... 

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

An electrochemical reaction, the reduction of benzoquinone, is exemplarily described. An electrochemical micro structured reactor is divided into a cathode and anode chamber by a Nafion hollow-fiber tube. The anode chamber is equipped with a platinum electrode and the cathode chamber contains the analyte and carbon or zinc electrodes. Current density and flow rate are controlled to maximize current efficiency as determined by analysis of the formed hydroquinone by an electrochemical detector. Hydroquinone is extracted subsequently in a micro extractor from the resulting product stream [84],... 

Electrochemical measurements can be readily adapted for on-line monitoring. An electrochemical detector uses the electrochemical properties of target analytes for their determination in a flowing stream. An electrochemical flow system, based on an SWV operation at a carbon-fiber-based detector, for use in the on-line continuous monitoring of trace TNT in marine environments was developed [16]. Such flow detector offers selective measurements of sub-part-per-million concentrations of TNT in untreated natural water samples with a detection limit of 25ppb. It responds rapidly to sudden changes in the TNT concentration with no apparent carryover. About 600 runs can be made every hour with high reproducibility and stability (e.g., relative standard deviation (RSD) = 2.3%, n = 40). The system lends itself to full automation and to possible deployment onto various stationary mobile platforms (e.g., buoys and underwater vehicles). 

Karube and Yokoyama presented an overview on the developments in the biosensor technology [60]. The overview describes the use of micromachining fabrication techniques for the construction of detection units for FIA, electrochemical flow cells and chemiluminescence detectors. Acetylcholine microsensors using carbon fiber electrodes and glutamate microsensors for neuroscience were discussed. 

The system shown in Figure 12.9 was used with a 26-pm i.d. capillary for the detection of catecholamines, and detection limits of 200-500 amol were obtained (for S/N = 2). When the capillary diameter was reduced to 12.7 pm, the same 5-pm carbon fiber working electrode yielded detection limits of 5-25 amol, almost two orders of magnitude lower. The improvement results from the collection efficiency of the working electrode. The smaller i.d. capillary allows less analyte to exit the capillary without being electrochemically converted.7 This result contrasts directly with those obtained with optical detectors, where detection limits are worse for smaller capillaries because less detectable analyte is present in the light path. 

This technique has increased rapidly in popularity over the past several years. In certain situations an electrochemical detector can offer picogram limits of detection. Furthermore, it is one of the few detectors that is easily adaptable for use with microcolumns. White et al. have shown the feasibility of using a single carbon fiber as the working electrode inserted into the end of a 15-p.m-i.d. capillary column (62,63). Slais has reviewed the use of electrochemical detectors with low-dispersion (microbore) columns (64). 

Besides cell designs that employ metals or other electrode materials in disc shape for external reflection spectroscopy, Robinson and McCreery have successfully employed cylindrical carbon fibers of 12 pm diameter [58, 59]. The carbon fiber was illuminated with a tunable dye laser. Scattered light was collected with fiber optics and guided to a photomultiplier detector. Because no thin layer arrangement and consequently poor electrochemical cell response were involved, fast experiments on a microsecond time scale were possible. Studies of polyaniline films deposited on platinum discs have been described [60]. 

Figure 1 Illustrations of electrochemical detector configurations coupled to capillary liquid chromatography columns. (A) End-column or wall jet electrode configuration with the working electrode positioned proximal to the fused silica capillary outlet. (B) On-column electrode showing a cylindrical carbon fiber positioned in the fused silica capillary outlet forming a thin annular layer between the fiber surface and the inner capillary wall.

Fig. 14.2 illustrates the determination of an equimolar low concentration (0.1 pM) of three neurotransmitters with the diamond-based CE detector. The very low and stable noise levels in the background current enabled us to attain the lowest detection limits from the diamond-based electrochemical detector. Under the same CE separation conditions, the diamond microhne electrode showed lower noise levels (0.5 - 1 pA), and a more stable background current, than that of the carbon fiber electrode (whose minimum noise level was 2 pA), even though its surface area was 25 times larger. Variations in the background current (low-frequency noise) were also much smaller and less irregular than those for the carbon fiber microelectrodes. In addition, the diamond microelectrodes exhibited greater stability in the amperometric response compared to that for the carbon fiber microelectrodes. 

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