Graduate Thesis Or Dissertation
 

Development and application of an analytical technique for the determination of methylmercury compounds in environmental samples based on isolation by distillation, followed by purge-and-trap sample concentration and GC/MS separation and detection, after aqueous phase ethylation with sodium tetraethylborate

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  • An analytical technique has been developed for the determination of methylmercury compounds in environmental samples. The technique is based on a two-stage procedure. In the first stage, methylmercury compounds are isolated from the sample matrix as methylmercury chloride (MeRgCl) by distillation. The distillation procedure is based on a series of published methods for the determination of methylmercury compounds in sediment and natural waters. In the second stage, MeRgCl is converted to the more volatile methylethylmercury (MeRgEt) by derivatization with an aqueous solution of sodium tetraethylborate. The volatile species is then determined by purge-and-trap sample concentration and gas chromatography/mass spectrometry (GC/MS) separation and detection. Initial work focused on the development of the method with optimization of the experimental parameters and operating conditions associated with the distillation and final measurement steps. The concentration of MeHgCl in standard solutions (concentration range from 0-30 ng/L as Hg), with and without nitric acid preservation, was monitored as a function of storage time to evaluate storage losses. Aqueous solutions of MeHgCl at the ng/L level are very stable for at least three months if stored (1) in the dark at 4 DC, (2) in acid-cleaned polypropylene flasks, (3) and without nitric acid preservation. Rapid loss of MeHgCl was observed for solutions acidified with nitric acid and stored under identical conditions (25% and 40% decrease in signal response after 3 and 96 days, respectively). The analytical technique was applied to the determination of methylmercury compounds in lake-bottom sediment and surface-water samples collected from Cottage Grove Reservoir, located in Lane County, Oregon. The concentration of methylmercury in a surface-water sample was 2.1 ± 0.11 ng/L as Hg(II). Recoveries of approximately 100 percent were observed for surface-water samples spiked with MeHgCl to a level of 4 ng as Hg(II). Concentrations of methylmercury in lake-bottom sediment ranged from 0.143 ± 0.008 to 35 ± 3.1 ng/g sediment as Hg(II) (wet weight). Recoveries of approximately 90 percent were observed for sediment samples spiked with MeHgCl to a level of 15 ng as Hg(II). The reproducibility of the entire analytical technique and the measurement step alone were evaluated through the analysis of replicate sediment samples. The percent relative standard deviation (RSD) of the entire analytical procedure was 2.6 percent, while the percent RSD of the measurement step alone was determined to be 1.5 percent. The absolute detection limit for MeHgCl was determined to be 4 pg as Hg(II) for the analysis of a 40-mL sample volume.
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