Cell design, electron-capture detector

The choice of carrier gas is sometimes dependent on the detector in use, as is the case, for example, with the thermal conductivity cell, the electron capture detector, or in GC/MS combination. For an optimum performance of certain detectors (dependent on either the principle or a particular design), an extra gas must be added at some point between the column end and the point of detection. Such make-up gas arrangements are particularly common in capillary GC. 

A diagram of an electron capture detector is shown in figure 4.11. There are a large number of different detector designs, but the basic electron capture detector consists of a small chamber 1 or 2 ml in volume with two metal electrodes. The electrodes may be formed by concentric cylinders or by metal discs separated by a suitable insulator. The cell contains the radioactive source, usually electrically connected to the conduit through which the carrier gas enters and to the negative side of the power supply. A gauze "diffuser" is sometimes connected to the exit of the cell and to the positive side of the power supply. The electrode current is monitored by a suitable amplifier. 

For closed-cell detectors, including the photoionization detector, the electron-capture detector and the TCD, extracolumn band broadening can be excessive unless specially designed devices with small cell volume are used or the detector is operated at subambient pressure. At reduced column outlet pressure, the carrier-gas velocity in the detector is increased, and the cell is swept out more quickly. Extra gas, called makeup gas, can be introduced into the detector cell to sweep the cell more rapidly and reduce peak broadening and distortion. 

FIGURE 6.14 ECD cell designs (a) modified chamber of a coaxial ECD geometry (pin-cup detector) and (b) plane-parallel electron-capture detector (adapted from Reference 44 with permission, copyright 1974 Elsevier). 

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