Prediction of the calibration curves for the analysis of high absorbances using mode-mismatched dual-beam thermal lens spectrometry with chopped continuous wave laser excitation

Thermal lens spectrometry is a sensitive method especially designed for the analysis of weak absorbances. Recently, a convenient mathematical expression has been derived for steady-state and time-resolved measurements, using a mode-mismatched dual-beam optical configuration. This work discusses the validity of the theoretical predictions, over a wide range of concentrations including high absorbance solutions, when a chopped continuous wave laser is used to irradiate the sample. Depending on the chopping frequency and on the thermo-optical properties of the solvent, experimental calibration curves can differ significantly from the theoretical ones because of a cumulative effect induced by the chopped excitation beam on the sample. However, a good agreement is achieved when the chopping disc is arranged in such a way that the dark period following each irradiation period is long enough to allow the sample to relax to its initial thermal state before being excited again.