Gas Chromatography Detectors - Pressure Sensitivity > Page 17


Modern sensors and electronic systems employ fast solid state sensors and solid state electronic components. Thus, the majority of detector systems commercially available are sufficiently fast for the vast majority of chromatography applications. In general, the overall time constant of the detecting system should be less than 50 milliseconds. For special applications involving very fast separations, this value may need to be reduced to around 15 milliseconds. Sensors and electronics, with very small time constants, unfortunately, will also readily respond to high frequency noise. Consequently, the chromatographic system must be carefully designed to reduce short term noise, which, asalreadystated, is not normallyaproblemingeneralchromatographicanalysis. Thismay involve careful magnetic screening to reduce the effect of stray, low-frequency electromagnetic fields from nearby power supplies and any high energy consuming laboratory equipment.

Pressure Sensitivity

The pressure sensitivity of a detector can be very important as it is one parameter that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector sensitivity or minimum detectable concentration. Some detectors are more sensitive to pressure changes than others. The katharometer detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive. Careful design can minimize the effect of pressure. It should be noted that all bulk property detectors will tend to be pressure sensitive. The pressure sensitivity (DP) should be given as the output in millivolts for unit pressure change in the detector (e.g. as mV/p.s.i or mV/kg/m2). The pressure sensitivity can be used to calculate the pressure change (NP) that would provide a signal equivalent to the detector noise (ND),