Picomolar mercury measurements in seawater and other materials using stannous chloride reduction and two-stage gold amalgamation with gas phase detection

Abstract

Sampling and analytical methodologies were developed and tested which are non-contaminating, accurate, and sensitive, permitting the reliable determination of picomolar levels of Hg in natural waters. Mercury was isolated from solution using SnCl 2 reduction and gas phase stripping with collection and concentration onto Au utilizing Class 100 clean laboratory conditions and practices. Mercury detection was conducted using a two-stage Au amalgamation gas train to introduce elemental Hg 0 vapor into the gas cell of a flameless atomic absorption spectrophotometer. By carefully controlling and precisely estimating the procedural blank, a detection limit of 0.21 pM was achieved using a 2-1 sample volume for analysis. An analytical precision of about 10% was obtained for solutions with Hg contents between 2 and 20 pM using 500-ml aliquots for sample analysis. Verification of the analytical accuracy and precision of the method was demonstrated using aqueous laboratory and NBS standard reference materials spiked into acidified natural water samples at picomolar levels. Sample exposure to laboratory air containing elevated Hg was identified as a potentially serious source of Hg contamination to acidified natural water collections containing picomolar levels of Hg. Additional studies revealed that the bulk of Hg in open ocean and coastal seawater (≥88%) consists of labile species which are immediately reactive to SnCl 2 reduction under acidic conditions.