Development and Applications of Thermal Lens Techniques for Chemical Analysis and Characterization

Thermal lens spectrometry is based on the measurement of the temperature rise that is produced in an illuminated sample by nonradiative relaxation of the energy absorbed from a laser. It is one of the most sensitive techniques for trace determination. The main objective of this work is to extend the scope and applicability of the technique so that it can be used in the area of general trace chemical analysis and characterization. This was accomplished by improvements in the theoretical model, sensitivity and applicability through instrumentation and methodology. Instrumentally, the selectivity of the technique was substantially improved by the development dual wavelength double beam thermal lens spectrometer. This apparatus is based on sequentially exciting the sample with two different wavelengths. It thus, facilitates the correction of solvent background absorption and the identification of unknown samples as well as the determination of two component mixtures. For example, 4.7 ⨯ 10-5 M Pr3+was measured in the presence of nickel glycinate whose concentrations were as high as 0.01 M. In addition this spectrometer also facilitates the determination of solution pHs with indicator concentrations as low as 1 ⨯ 10-9 M. For aqueous solutions which are generally considered to be the worst medium for thermal lens measurements. The sensitivity of the technique was improved by simply increasing the solution temperature and/or by adding strong electrolytes or surfactants into the solution. In addition, the sensitivity was also improved by the synergistic use of bimodal behavior and temperature dependence of thermal lens effect in water. New relations for the dependence of thermal lens signal on changing concentration were derived. These relationships enable this ultrasensitive thermal lens technique to be used for the measurements of reaction rates of reagent concentrations 100 times lower than those used conventional