SENSITIVITY AND DETECTION LIMIT ENHANCEMENT OF GAS-CHROMATOGRAPHIC DETECTION BY THERMAL MODULATION

A thermal desorption modulator placed between the outlet of a column and the inlet of a detector enhances chromatographic sensitivity, signal-to-noise ratio and detection limit. The modulator operates at a selectable frequency up to 10 Hz, chopping the effluent from the chromatographic column into a series of concentration pulses whose envelope follows the form of the chromatogram. Measuring the difference between the top of each concentration pulse and the following valley demodulates the detector output signal. Thermal modulation increases the recorded signal amplitude because the modulator concentrates substances emerging from the column over a relatively long time into concentration pulses of much shorter duration. Thermal modulation also attenuates low frequency drift. Using this technique enhanced the signal amplitude of a flame ionization detector by a factor of 15 and the signal-to-noise ratio by a factor of 5.