Factors affecting the determination of total mercury in biological samples by continuous-flow cold vapor atomic absorption spectrophotometry

Abstract

The acidic reduction of Hg using a continuous-flow analytical system was evaluated. With 25% SnCl₂ as the reductant, characteristic concentrations (sensitivities) of 0.44 μg/L (open cell) and 0.29 μg/L (flow-through cell) were obtained using inorganic Hg²⁺ standards in 1.5% HCl. When CH₃Hg⁺ standards were used, absorption signals were an order of magnitude lower, indicating that Sn(II) is incapable of producing Hg° from organic Hg in this acidic reduction system. Addition of CdCl₂ to the SnCl₂ reductant, as suggested by Magos (1) for the reduction of organomercurials under alkaline conditions, was without beneficial effect. Similarly, combining Sn, with another reducing agent (hydroxylamine hydrochloride), or a strong alkaline solution (40% NaOH), in the reaction coil of the flow-through system did not significantly enhance the Hg absorption signal for either inorganic or organic Hg. Recovery of Hg from spiked liver homogenates digested at 70–80°C using a HNO₃/H₂SO₄/HCl procedure and stabilized with 0.5 mM K₂Cr₂O₇ was >85%, using either inorganic Hg²⁺ or CH₃Hg⁺, indicating that this digestion procedure successfully breaks the C-Hg bond to form readily reducible Hg species. Usingl-cysteine to stabilize standards of inorganic Hg²⁺ in HCl caused significant depressions of the Hg absorption signal atl-cysteine concentrations >0.001% (≈0.5 mM); 0.1%l-cysteine caused total suppression of the Hg signal. These results indicate that: (1) acidic reduction of Hg by Sn in this continuous-flow system requires breakdown of organomercurials prior to analysis; (2) tissue digestion using HNO₃/H₂SO₄/HCl followed by the addition of K₂Cr₂O₇ to stabilize Hg²⁺ achieves this breakdown and allows good recovery of total Hg; and (3) use ofl-cysteine to complex and prevent losses of Hg should be avoided in systems using acidic reduction of Hg. Concentrations of endogenous tissue sulfhydryls are generally lower than those associated with depressed absorbance signals during the acidic reduction of Hg.