Liquid detector cell

The reduction of dimensions also reduces volumes which are accessible to the detector. Thus, detection principles related to geometric dimensions of the detector cell ai e not ideally suited for coupling to microsystems, whereas surface sensitive principles, such as electrochemical methods or optical methods utilizing the evanescent field of a waveguide, or methods which can be focussed on a small amount of liquid, such as electrochemiluminescence (ECE), ai e better suited. This is why electrochemiluminescence detectors ai e combined to microsystems. Moreover ECE has found wide applications in biochemistry because of its high sensitivity, relatively simplicity and feasibility under mild conditions. 

Svensson, L. M. and Markides, K. E., Fiber optic-based UV-absorption detector cell for high-temperature open tubular column liquid chromatography,... 

A simphstic view of a FIA analytical system is outfined in Fig. 2.19. The liquid flow can be obtained in a number of ways, most commonly by using a peristaltic pump. In addition, gravity-feed systems, overpressure systems on liquid vessels and simple and double piston pumps normally associated with HPLC systems, are often used. The interrelation of the pumping system and the bore size of the transport tubing to a great extent modify the theoretical considerations involved and the operation of the FIA regime. In common with CFA, the minimization of dead volumes—in detector cells and between T pieces for example—is particularly important. Injections of the sample into... 

The first experimental investigation and performance demonstration of an integrated liquid chromatography chip was carried out by Ocvirk et al. [84]. The device is shown schematically in Fig. 15 and comprises a split injector, a smallbore separation column, a frit, and a detector cell, all integrated in a monolithic manner. An electron micrograph of the silicon chip is also depicted in Fig. 15. The whole device was composed of two 350 pm Si chips and a 50 pm interme-... 

This kind of amperometry is the most widely used electrochemical detection method in liquid chromatography. A constant DC potential is continuously applied to the electrodes of the detector cell. The theory of amperometry with constant working potential does not differ from the theory of hydrodynamic voltammetry, even though the applied potential remains constant. 

Non-suppressed ion chromatography employs a conventional liquid chromatographic system with a conductivity detector cell connected directly to the outlet end of an ion-exchange separation column. No suppressor unit is required. The successful development of this method was made possible by three principal innovations (1) the use of an anion- or cation-exchange resin of very low capacity (initially 0.007 to... 

The liquid adsorption and diffusion measurements were carried out in a BECKMAN HPLC system, which consists of one model 421 system controller, two model 110 solvent metering pumps, one solvent mixer and one model 210 sample injector with a 20 yl sample loop. A Hitachi model 100-40 UV-Vis Spectrometer was used as the detector. To increase the pressure in the UV-VIS detector cell, a back pressure regulator was connected to the effluent stream from the detector to avoid formation of air bubbles due to vaporization in the detector cell. Figure 1 shows the schematic of the HPLC system used in the experiments. 

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. 

The detector itself consists of a small liquid flow cell through which the eluent from the column flows. UV light passes through the cell and hits the UV photodetector. The cell is usually made of quartz which is UV-transparent. To avoid band broadening, the flow cell volume is minimised about 10-15 pL in the case of a standard flowcell or 6-8 pL in the case of a semimicro flowcell, the choice of which depends, among other factors on the column and the application. 

Amperometric detectors are easily miniaturized with preservation of performance, since their operation is based on reactions at the electrode surface. Using a single carbon fiber or microelectrode as a working electrode allows detector cells of very small volume and in-column detectors to be constructed for use in open tubular and packed capillary column liquid chromatography [189-192]. These microcolumn separation techniques combined with amperometric detection are exploited for the quantitative analysis of volume-limited samples such as the contents of single cells [193,194]. 

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