Eco-scale non-chromatographic method for mercury speciation in fish using formic acid extraction and UV–Vis photochemical vapor generation capacitively coupled plasma microtorch optical emission spectrometry

Abstract

An eco-scale method based on UV–Vis photochemical vapor generation and detection in a capacitively coupled plasma microtorch by optical emission measurement with a low resolution microspectrometer was developed for Hg speciation as Hg2+ and CH3Hg+ in seafood. Our proposal provides a simpler, greener analytical method in terms of sample preparation and derivatization, and use of small-scale instrumentation. Coupling of UV–Vis photo-reactor with plasma microtorch is a very promising approach to develop eco-scale methods for Hg speciation in seafood with major advantages to classical methods. The most distinctive characteristic of the method is the use of a single reagent, formic acid, for the extraction of Hg species from solid sample and as medium for the photo-induced cold vapor generation. An amount of 0.2 g lyophilized sample was subjected to ultrasound assisted extraction in 10 ml HCOOH 98–100% at 50 °C for 3 h then on line UV–Vis photo-induced derivatization in 0.6 mol l−1 HCOOH was achieved. Total Hg and Hg2+ species were quantified against Hg2+ external standard solutions by measuring emission at 253.652 nm in a low power/low Ar consumption (15 W/100 ml min−1) plasma source with Maya2000 Pro (Ocean Optics) microspectrometer. Under UV exposure both Hg2+ and CH3Hg+ species were converted with same efficiency to cold vapor, while natural room light (Vis) achieved selectively derivatization of Hg2+. Concentration of CH3Hg+ was obtained by subtracting Hg2+ amount from total. The proposed method was compared with Thermal Decomposition Atomic Absorption Spectrometry eco-scale method developed for the determination of total Hg from solid sample and CH3Hg+ according to the procedure recommended by the European Commission. The limits of detection/quantification provided by the proposed method were 9/27 μg kg−1 total Hg and 4.8/14.4 μg kg−1 Hg2+. Study of accuracy against certified reference materials gave recovery of 99 ± 6% total Hg, 99 ± 9% Hg2+ and 99 ± 10% CH3Hg+, while precisions assessed from measurements on real sample were 2.6–10.2%, 2.0–13.4% and 5.3–14.5% respectively, similar to those in the reference method.