Thermal lens spectrometry in trace metal analysis

Abstract

A dual-beam thermal lens spectrometer was used for the determination of trace amounts of Co, Bi, Fe, Ni and Mo. Its characteristics have been tested on model systems and the geometry of the optical scheme optimized to give increased sensitivity and a wider linear calibration range. The systematic methodological approach to the adaptation of conventional spectrophotometric procedures to thermal lens spectrometry has been elaborated. It is based on the specific demands of thermal lens spectrometry and includes an additional study of the original technique, the step-by-step elaboration of the procedure for thermal lens spectrometry on the model system and then on the spectrophotometric reaction with final testing on real samples. Various liquids were studied to ascertain their suitability as thermo-optic media. The mixed water–organic solvents proved to be the most suitable because they combine the convenience of water at the sampling stage with the high thermo-optic parameters of organic liquids. Thermal lens techniques are based on well known spectrophotometric routines (Co with 2-nitroso-1-naphthol, Ni with α-dioximes, Mo with thiocyanate ions, Fe with 1,10-phenanthroline and Bi with iodide ions). The application of thermal lens spectrometry allowed us to increase the sensitivity of these analytical procedures by up to 10³–10⁵ times. The limits of detection (3 × S/N) are: Bi, 10 pg cm^–3; Co, 0.17 pg cm^–3; Fe, 50 pg cm^–3; Mo, 10 pg cm^–3, and Ni, 9 pg cm^–3.