Ultratrace Determination of Metals with Dithizone by Thermal Lens Spectrophotometry

A pump-probe thermal lens instrument was constructed, evaluated, and applied to the determination of metals after extraction with dithizone in carbon tetrachloride. An argon-ion laser operated at 514.5 nm, with a power of just 22 mW at the cuvette position and chopped at 2 Hz, and a He-Ne laser were used as a pump and probe, respectively. The thermal lens effect was measured by a pinhole-photodiode and a lock-in amplifier. Evaluation of the setup with an iodine solution in carbon tetrachloride gave a minimum detectable absorbance of 5 × 10⁻⁶ (S/N = 3), the upper limit of the dynamic range being 5.7 × 10⁻³. Relative standard deviation was about 3%. Calibration curves and limits of detection for several metal dithizonates were obtained. From extractions of cadmium dithizonate at concentrations lower than 2 ng mL⁻¹, a LOD of 8 pg mL⁻¹, corresponding to an absolute LOD of 4 fg of cadmium in the probe volume, was achieved.