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The main types of analytical procedures used in thermal-lens spectroscopy and microscopy, which are based on photometric reactions in (i) aqueous solutions, (ii) organo-aqueous mixtures, (iii) polymer-containing (nonionic surfactants or polyethylene glycol) aqueous solutions, (iv) water-organic extraction systems, and (v) two-phase aqueous extraction systems, were compared from the viewpoint of both reproducibility and sensitivity. This comparison was made by examples of the determination of cobalt and iron for batch conditions, flow determination, and detection in HPLC, flow-injection analysis (FIA), and μFIA. It was revealed that for all five types, the real analytical efficiency (a decrease in the limit of detection (LOD) as compared to spectrophotometry) is primarily determined by the reaction conditions, provided that excitation of the thermal lens is the same. Aqueous solutions provide more efficient optimization of reaction conditions than do those in organo-aqueous solutions and solvent-extraction water-organic mixtures. The best results are achieved when shifting to polymer-containing aqueous solutions and two-phase aqueous extraction systems, which decreases in the LODs by a factor of 20 -100%.

Abstract. This work considers the application of thermal-lens spectrometry to the monitoring of the crystallization processes in aqueous solutions. Formation of a crystalline germ in a previously homogeneous medium drastically changes the heat and mass transfer along the laser beam path; thus, affecting both equilibrium and time-resolved thermal-lens phenomena. It is shown that long-term oscillations of the steady-state thermal-lens signal could be associated with the formation and growth of the crystal germ. The preliminary estimation of the minimal germ size detectable from this data gives the limit at the level of 300 nm.

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