A method of determining the analyte concentration of a test sample is described.
A temperature gradient is introduced in the test sample and infrared radiation
detectors measure radiation at selected analyte absorbance peak and reference wavelengths.
Reference and analytical signals are detected. In the presence of the selected
analyte, parameter differences between reference and analytical signals are detectable.
These parameter differences, having a relationship to analyte concentration, are
measured, correlated, and processed to determine analyte concentration in the test
sample. Accuracy is enhanced by inducing a periodically modulated temperature gradient
in the test sample. The analytical and reference signals may be measured continuously
and the parameter difference integrated over the measurement period to determine
analyte concentration.