A photothermal signal from a liquid sample in a capillary channel flow is detected and analyzed to optimize experimental conditions of microfluidic devices used for separation. A theoretical model for photoinduced temperature increase and photothermal signal intensity generated by intensity-modulated cw excitation beams at a crossed-beam configuration is proposed. Four experimental parameters (probe beam offset, excitation beam chopping frequency, linear flow velocity, and excitation beam size) and physical properties of solvent mainly dominate the signal. The model well explains the photothermal signal obtained experimentally under low velocity and high chopping frequency conditions. The obtained results make it possible to optimize the experimental conditions for the highly sensitive detection of chemicals under flow conditions.
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