A modification of the wavelength modulation diode laser (DL) absorption technique for detection of broad-band molecular absorption lines is proposed. The method based on the detection of differential absorption of two narrow-band DLs at specially selected wavelengths within the molecular absorption band enables the measurement of the temperature changes of a sample. These cause shifts and deformations of the absorption band. The potentials and limitations of the proposed technique are shown using contactless detection of the temperature of isotachophoretic zones on a polymer microchip as an example. A variation in the temperature of an ITP-zone provides a difference in the absorption of two diode laser beams with their wavelengths tuned to the opposite slopes of a broad molecular absorption band of water. The 970 nm absorption band of water was used in this work. The minimum detectable temperature increment of about 0.4 °C is realized in a capillary filled with a typical ITP buffer. The drifts and fluctuations of the base line increase 10-20 fold if a buffer front propagates along the capillary. The propagation of the temperature front was numerically simulated using the tabulated values for heat capacity and electric and thermal conductivity of ITP-buffers and plastic material. The simulated values of temperature variations caused by on/off switching of the capillary current fit very well the experimentally measured ones. It is expected that the sensitivity and temporal resolution of the technique can be improved by modification of the microchip and by use of the strongest absorption band of water at 1.44 μm.
V. Krivtzun, B. Graß, R. Hergenröder, M. Bolshov, K. Niemax, and A. Zybin, "Temperature Measurement of Liquids by Differential Absorption of Two Diode Lasers: Application of Contactless Optical Detection in Isotachophoresis," Appl. Spectrosc. 55, 1251-1258 (2001)