A thermal desorption modulator is a short section of column whose temperature can be rapidly and precisely controlled. Heating the modulator drives a pulse of retained substances out of the modulator and into the following column. This concentration pulse is equivalent to a small-volume injection, which, because it is generated within the column itself, is of the appropriate volume for the column. Multiple pulses applied during an extended sample introduction period result in a multiplexed detector output signal, from which the chromatogram may be computed. Chromatograms may be obtained from 50μm-diameter columns less than 1 m long without the need to manipulate small sample volumes.
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Phillips et al. (1986) studied this question.