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 fo r the analysis of weak absorbances. Recently, a convenient mathematica l expression has been derived for steady-state and time-resolved measu rements, using a mode-mismatched dual-beam optical configuration. This work discusses the validity of the theoretical predictions, over a wi de range of concentrations including high absorbance solutions, when a chopped continuous wave laser is used to irradiate the sample. Depend ing 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 th e chopped excitation beam on the sample. However, a good agreement is achieved when the chopping disc is arranged in such a way that the dar k period following each irradiation period is long enough to allow the sample to relax to its initial thermal state before being excited aga in.