Tubular detector cell

The Golay equation [9] for open tubular columns has been discussed in the previous chapter. It differs from the other equations by the absence of a multi-path term that can only be present in packed columns. The Golay equation can also be used to examine the dispersion that takes place in connecting tubes, detector cells and other sources of extra-column dispersion. Extra-column dispersion will be considered in another chapter but the use of the Golay equation for this purpose will be briefly considered here. Reiterating the Golay equation from the previous chapter. 

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

The detector cell was a three-electrode system consisting of a flow-through nickel working electrode, a saturated calomel reference electrode (SCE), and a stainless steel outlet tubing counter electrode. The tubular-type electrode cell housing was constructed of molded Teflon, which was machined to provide the channels and to accommodate the fittings. The working electrode area was... 

The miniaturization of detector cells is also extremely difficult and the technological problems have not yet been solved because the detection limit should also be decreased or, at least, kept constant [2-6]. Some progress in the design of very small cells for UVD and FD has been reported [3-7], but the miniaturization of RD and RID seems much more difficult in spite of some suggestions [8]. Similarly, the development of open tubular columns is plagued by the lack of a suitable detector with a small contribution to band broadening. 

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... 

Loop-based injection is used for sample insertion and a similar procedure is used for displacing the tubular detector D between monitoring sites a and b (Figs 7.9 and 7.10, lower). Consequently, this strategy has usually been accomplished using two loop-based injectors or a double injection valve/commutator [81], where a flow cell with two small transmission lines replaces one of the sampling loops. Although the flow system can be manually operated, computer-controlled valves or commutators have been preferred. 

Consequently, the strategy is prone to Schlieren effects. Moreover, the available time for monitoring at a specific site is lower relative to ordinary flow systems. Consequently, a small inner volume tubular flow cell and a detector with a fast response time are preferred. 

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]. 

The wire electrode is advantageous when miniaturization is necessary. Knecht et al. (1984) have described the use of single carbon fibers (diameter < 10/open tubular liquid chromatography. The fiber working electrode was plac directly into the outlet of the column by means of a microscope. The rest of the detector cell was situated outside the tube (Fig. 25). 

An original route is that proposed by Ter-Minassian and Million in 1983 [44] with their pneumatic compensation calorimeter, represented in Fig 10. The tubular sample cell 4 is in good thermal contact with four metallic bulbs. Two of them operate like bulb 1 in the figure, Le. as pneumatic thermal detectors. They are filled with gas, say around 1 bar, and their pressure is compared, by means of a differential manometer, with the constant pressure of a reference reservoir 3 immersed in the surrounding thermostat block 5. Therefore, they detect any temperature change of the sample. The two oflier bulbs operate like bulb 2, i.e. as pneumatic energy-compensating devices. They are also filled with gas, say around 1 bar, but they are connected to flie piston-cylinder 7 which enables the heat of compression (or decompression) necessary to cancel the temperature difference between the sample and thermostat (as detected with the first set of bulbs) to be produced in the bulb. More recently, Zimmermaim and Keller built a comparable pneumatic compensation calorimeter whose calorimetric performances were carefully examined [45]. 

Tubular Planar (parallel flow) Thin-layer cell Planar (perpendicular) Wall-jet detector 1= 1.61 nFC(DA/r)2/3U123 i = 0.68 nFCD2l3v- 6(A/b)V2UV1 i = 1.47 nFC(DA/b)2/3U123 i = 0.903nFCD2f3v- 6A3/4u 2 i = 0. mnFCD2/3v-5/ua-V2A3/tU3/4... 

Contactless conductivity detection mode, based on an alternating voltage capacitively coupled into the detection cell, is the practical and robust arrangement nowadays employed in commercially available detectors that has been independently developed in 1998 by Zemann et al. [54] and by Freacassi da Silva and do Lago [55]. This detection mode is based on two tubular electrodes. 

Fig. 8. Typical tubular electrode assemblies, (a) Integral construction. A, Generator electrode B, detector electrode C, reference electrode D, counter electrode E, porous frits F, ball and socket joints G, epoxy resin, (b) Demountable type. A, Generator electrode B, counter electrode C, Teflon spacers D, reference electrode E, Teflon cell body F, brass thread. (From ref. 128.)...

The following table provides guidance in the selection of hot wires for use in thermal conductivity detectors (TCDs).13 This information is applicable to the operation of packed and open tubular columns. Some of the entries in this table deal with analytes, and others deal with solutions that might be used to clean the TCD cell. 

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