Thermal lensing and its applications to liquid chromatography

This dissertation presents the research work for determination of the optimal optical configuration for dual-beam coaxial thermal lensing spectrometry with the excitation beam tightly focused. In the study, the excitation beam is focused to a waist size of 1.2 mum and the path length of the sample cell is 100 mum. The thermal lens signal depends not only on the distance between the waist positions of the excitation beam and the probe beam but also on the probe beam waist size. It is found that the maximum thermal lensing signal occurs when the distance between the two foci is about 5 Rayleigh ranges of the probe beam and the waist size of the probe beam is about 1.5 times that of the excitation beam. When the optical configuration is optimized, the thermal lensing detection has an enhancement factor of 13 compared to the conventional UV-Vis detection with the same path length. The dependence of the thermal lensing signal on the path length of the sample cell has also been studied.; The thermal lensing technique has been successfully applied to HPLC detection with a 200-mum-path-length flow cell. The root mean square of the baseline noise for the system is 9.3x10-9 AU, and the peak-to-peak baseline noise is 3.5x10-8 AU. The detection of a small absorbance on the order of 10-8 AU has been achieved and a linear dynamic range of 3.5 orders of magnitude has been demonstrated. More than 140 times improvement in terms of minimum detectable absorbance has been achieved in the thermal lensing detection, compared with the standard single-pass absorption spectroscopy.