Influence of the Soret effect on the analytical signal in cw-laser thermallens spectrometry of micellar solutions

The influence of the Soret effect in cw-laser thermal lens spectrometry of micellar solutions has been investigated. The temperature gradient produced subsequently to nonradiative relaxation of the sample may induce migration of micelles in the hotter region of the solution. Migration of micelles wo uld result in a local enhancement of the thermo-optical properties of the s olution, leading to an additional signal with a risetime of some seconds in stead of some hundred milliseconds for the thermal lens signal. The Soret e ffect depends on the concentration of both the analyte and the surfactant. This can lead to erroneous interpretation of experimental data, especially when steady-state measurements are made. On the contrary, the Soret effect seems to be inoperative when the thermal lens signal is derived from time-r esolved measurements or when using a continuously chopped irradiation with lock-in detection. (C) 1999 Elsevier Science B.V. All rights reserved.