Photo- and thermo-chemical vapor generation of mercury

Abstract

Photochemical vapor generation of both inorganic and methylmercury species can be achieved with equal efficiency when a sample reaction medium containing 2–10% formic acid is irradiated by low power (0.3 mW) deep UV LED sources with output in the range 245–260 nm. Whereas pseudo first order kinetics is evident for reduction of methylmercury, inorganic mercury does not conform to either first or second order models and the overall reaction rate is proportional to the concentration of formic acid but independent of that of mercury. Noteworthy, is that at room temperature, no reduction of either mercury species is achieved when radiation from 360 nm LEDs is used. A thermal reduction system operating at nominally 85 °C can also be used to reduce both mercury species in a 2% formic acid medium with the initial stages of the reaction being dominated by a first order kinetic process that is approximately 7-fold slower for methylmercury. Despite this disparity in reaction rates, direct speciation is not possible. Using a 10 cm quartz tube cell for cold vapor AAS measurement, a limit of detection of 0.68 ng absolute was achieved for mercury using a 9 min irradiation time with the deep UV LED sources. Precision of replicate measurement was 2.4% RSD for a sampled volume of 480 μl of a 200 ng ml⁻¹ solution of Hg²⁺ (96 ng absolute) in 2% formic acid. The efficacy of the low power deep UV LED photoreactor was demonstrated by quantitation of total mercury in certified reference materials PACS-2 (marine sediment) and DOLT-4 (fish liver tissue) using the method of additions to compensate for matrix interferences.