Ultrasound-assisted vapor generation of mercury

Cold vapor generation arising from reduction of both Hg2+ and CH3Hg+ occurs using ultrasonic (US) fields of sufficient density to achieve both localized heating as well as radical-based attack in solutions of formic and acetic acids and tetramethylammonium hydroxide (TMAH). A batch sonoreactor utilizing an ultrasonic probe as an energy source and a flow through system based on a US bath were optimized for this purpose. Reduction of CH3Hg+ to Hg0 occurs only at relatively high US field density (>10 W cm−3 of sample solution) and is thus not observed when a conventional US bath is used for cold vapor generation. Speciation of mercury is thus possible by altering the power density during the measurement process. Thermal reduction of Hg2+ is efficient in formic acid and TMAH at 70 °C and occurs in the absence of the US field. Room temperature studies with the batch sonoreactor reveal a slow reduction process, producing temporally broad signals having an efficiency of approximately 68% of that arising from use of a conventional SnCl2 reduction system. Molecular species of mercury are generated at high concentrations of formic and acetic acid. Factors affecting the generation of Hg0 were optimized and the batch sonoreactor used for the determination of total mercury in SLRS-4 river water reference material.