Photothermal lensing spectroscopy, such as thermal lens spectroscopy and photothermal refraction, has been the subject of much interest as a new analytical technique for ultrasensitive absorption measurements. However, the limit of detection of this technique is often dominated by the background signal caused by the absorption of solvent itself or by impurities in the solvent. A simple method for diminishing the background signal is to use a solvent with low absorptivity, like carbon dioxide in supercritical fluid chromatography. Another method is to use a differential technique. In thermal lens spectroscopy (TLS), both single-beam and dual-beam configurations have been used. A dual-beam configuration with a single laser has also been reported. In a single-beam configuration, a differential response of the system was obtained by the use of the antisymmetric dependence of the thermal lens effect on the position of the lens relative to the waist in a Gaussian beam. This technique was applied to the detection of liquid chromatographic effluents to compensate for solvent absorbance. On the other hand, no study has been made on a double-beam system with a dual-beam optical configuration. In this paper, a new double-beam system based on a dual-beam configuration is proposed to compensate for the background signal due to solvent absorption.

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