Photo-induced chemical vapor generation with formic acid for ultrasensitive atomic fluorescence spectrometric determination of mercury: potential application to mercury speciation in water

Abstract

A new photochemical reaction for mercury chemical/cold vapor generation (CVG) coupled to atomic fluorescence spectrometry (AFS) is described for the speciation analysis of inorganic mercury ion (Hg²⁺) and organic methylmercury (MeHg) in aqueous solution. The new CVG simply uses one reagent, formic acid only, to react with Hg²⁺ or MeHg in aqueous solution, under room natural light (Vis) or ultraviolet irradiation (UV), for the generation of cold mercury vapor, which is subsequently detected by AFS. In the presence of the UV, both Hg²⁺ and MeHg can be converted to Hg⁰ for the determination of total mercury; and only Hg²⁺ can be reduced to Hg⁰ with the Vis, thus determining Hg²⁺ only. Then, the concentration of MeHg can be calculated by subtracting the Hg²⁺ concentration from the total mercury concentration. The optimal conditions for the best CVG efficiency are discussed, together with the interference from transition metals. There exists no significant interference from as high as 100 mg L⁻¹ Co²⁺ or Ni²⁺, and 50 mg L⁻¹ Cu²⁺ for the determination of as low as 5 μg L⁻¹ Hg²⁺. The new CVG minimizes the contamination sources and avoids off-line pre-oxidation of MeHg. A simple Hg²⁺ standard series can be used for the calibration of both Hg²⁺ and MeHg, eliminating the use of more toxic and more expensive MeHg standard series. The linear dynamic ranges of the calibration curves are up to 25 μg L⁻¹ with the UV and 300 μg L⁻¹ with the Vis. The limit of detection is 0.003 or 0.2 μg L⁻¹ for total mercury with the UV or Hg²⁺ with the Vis, respectively. The accuracy of this method was validated by determination of mercury in one certified reference water sample. The new CVG is a simple, fast, green, highly selective, and ultrasensitive yet inexpensive method for the speciation analysis of Hg²⁺ and MeHg. It is expected to have similar applications in other analytical atomic spectrometric techniques.