Determination of mercury by continuous flow cold vapor atomic fluorescence spectrometry using micromolar concentration of sodium tetrahydroborate as reductant solution

Abstract

Systematic experiments were conducted to evaluate and compare the analytical figures of merit of two reducing agents (SnCl₂ and NaBH₄) in a continuous flow cold vapor atomic fluorescence mercury analyzer. It was found that sodium tetrahydroborate can efficiently reduce Hg²⁺ in various environmental samples at a concentration as low as 10 μM (ca. 3.8 × 10⁻⁵ % w/v). Most commonly encountered transition metals (Fe²⁺, Fe³⁺, Zn²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Cr³⁺) did not interfere with total Hg determination. No interference from hydride-forming elements (Se⁴⁺, Sb³⁺ and As³⁺) was observed. Interference caused by Mn²⁺ and Ag⁺ could be readily removed by dilution and by using appropriate modification of the reaction matrix. A higher concentration of NaBH₄ (0.1 M) is stable for 1 month when stored in the NaOH matrix (0.2 M) and at low temperature (4 °C). A working solution of NaBH₄ can be freshly prepared by dilution. With NaBH₄, the whole continuous flow system is kept clean much more easily as no precipitate is formed, which in turn considerably reduces memory effects, simplifies analytical operation and reduces the chemical cost six-fold.