A multi-thermal-lens approach to evaluation of multi-pass probe beam configuration in thermal lens spectrometry

In this work, a recently proposed thermal lens instrument based on multi-pass probe beam concept is investigated and described as a multi-thermal-lens equivalent system. A simulation of the photothermal lens signal formation in a multi-thermal-lens equivalent configuration of the system is performed and validated by comparing the experimental signals of single, dual and ten-pass configurations to theoretically calculated values. The theoretically predicted enhancement of the signal is 9 to 10-fold for a weak thermal lens when comparing the ten-pass configuration with the conventional single-pass thermal lens system. Experimentally achieved signal enhancement in the ten-pass system is 8.3 for pure ethanol sample and between 8 and 9 for solutions with different concentrations of the Fe(II) - 1,10-Phenanthroline complex. Additionally, a value of 9.1 was calculated as the ratio of the slopes of the calibration lines obtained using the ten-pass and single-pass configurations. The achieved limit of detection for determination of Fe(II), in the ten-pass configuration, was 0.4 µgL-1, with a relative standard deviation around 4.5%, which compares favorably with previously reported results for TLS determination of Fe(II) in thin samples using low excitation power. For the multi-pass configuration the linear range of measurement is reduced when compared to the single-pass configuration. This is explained by the theoretical analysis of the photothermal signal under multi-pass condition, which shows the important contribution of the nonlinear term in the theoretical expression for the photothermal signal. The ten-pass configuration, which is presented and validated experimentally for the first time, offers important signal enhancement needed in recently developed TLS instruments with tunable, low power excitation sources