Obliquely crossed, differential thermal lens measurements under conditions of high background absorbance

A differential thermal lens spectrometer based on the oblique crossing of separate pump and probe beams is described. This arrangement utilizes the differential response of the thermal lens to compensate for the intensity noise in the pump source while maintaining the intrinsic sensitivity of photothermal measurements. Thus, this design is ideally suited for absorbance measurements in the presence of a large background, such as might be encountered in circular dichroism or indirect photometry. The theory describing the thermal lens signal observed with this configuration will be developed and experimentally verified. The experimental conditions mandating differential TL measurements will also be investigated. Absorbance changes of 2 × 10−7 are demonstrated. Application of this arrangement to indirect HPLC detection is also discussed.