ABSTRACT
The flame-ionization detector (FID) has become the most widely used detector in gas chromatography. The design of suitable high-performance electrometers requires careful consideration of many parameters not normally encountered at such low levels. This chapter describes electrometers in which the capacitive current noise has been reduced by locating the electrometer within a few inches of the detector, eliminating the cable capacitance and permitting the use of a small-geometry field-effect transistor with its somewhat higher voltage noise. Several new electrometers based on transistor feedback have been designed to circumvent many of the problems of the conventional approach. Designing circuits to detect the very-low-level signals from the FID and electron-capture detector requires fairly careful assessment of the sources of noise in the detector, electrometer amplifying semiconductor devices, feedback components used in ranging, bias supplies, cables, connectors, and mounting hardware. Reducing the bandwidth does reduce the noise, but a point is reached where further bandwidth reduction seriously distorts the peak shapes.
