A thermal desorption modulator is made from a short segment of thin tubing containing an adsorbent or a chromatographic stationary phase. A carrier gas containing the analyte is introduced into the analytical column through the modulator which acts as a sample trap. Rapid electrical heating of the modulator releases a "concentration pulse" of the analyte and this serves as an injection similar to that from an injection valve. The modulator also acts as a sample preconcentrator and can be used to make repetitive injections every few seconds. In this research, the mechanism of thermal desorption modulator was studied and was used in continuously monitoring volatile organic pollutants.