Coating of a thin layer of NaBH4 solution for mercury vapor generation coupled to atomic fluorescence spectrometry

Abstract

Sodium tetrahydroborate (NaBH4) was, for the first time, coated onto PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene propylene), Excelon (microline) and PEEK (polytetrafluoroethylene) surfaces and used for mercury vapor generation-atomic fluorescence spectrometric determination. A lab-on-valve (LOV) incorporating a miniature membrane phase separation chamber was employed to facilitate the operation. A 600–1200 nm thin film of NaBH4 solution was coated onto the interior surface of tubes by sweeping through 30 μl of NaBH4 solution with an argon stream. Mercury cold vapor was generated as sample solution flowing through the pre-coated tubing carried by argon. The film coating was renewed for each operating run. The vapor was either immediately excited in the outlet of the separator in the LOV and detected by directing the fluorescence to a photomultiplier with optical fibers, or directed to a commercial AFS-920 atomic fluorescence spectrometer. NaBH4 coating not only minimized fluorescence quenching by concomitant hydrogen encountered in conventional AFS, but the coating process also effectively eliminated mercury memory effects. In addition, the coating substantially simplified the operation of vapor generation using a single syringe pump for fluidic delivery. When using PTFE tubing as a coating medium, limits of detection of 0.06 and 0.005 μg l−1 were derived by monitoring with a LOV-AFS system and an AFS-920 spectrometer, respectively, with a sample volume of 500 μl. A sampling frequency of 60 h−1 along with RSDs of 2.9% and 2.0% at 2.0 μg l−1 were achieved. Mercury contents in certified reference materials and a local coastal seawater were measured.