The addition of a thermally-desorbed, small-volume, solid-sorbent preconcentrator prior to real-time chemical sensor measurement of organic vapors can improve sensitivity and the initiation of the heating defines when analytes are delivered to the analytical system. Systems using preconcentrator can provide detection levels that are 10-1000 times lower than systems using direct sampling and analysis. During operation, a small volume of solid sorbent material collects chemicals from a large gas sample (e.g., at a given flow rate for a fixed period of time) and then releases the chemical(s) into a small gas volume during thermal desorption. This results in a concentrated chemical pulse that generates a rapid peak in the detector response. The signal before and after this peak is used as the baseline. Thus the process provides preconcentration, sample injection, and signal modulation functions. This signal modulation overcomes difficulties with baseline drift and sensor re-zeroing, and facilitates automated feature extraction, i.e., determining the magnitude of the response from the temporal data stream. These features are particularly useful for continuous unattended monitoring applications.