Investigations of and performance improvements in the mercury thiourea complex ion chromatography method for mercury speciation analysis

Olsen, Todd

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Description

Title

Investigations of and performance improvements in the mercury thiourea complex ion chromatography method for mercury speciation analysis

Author(s)

Olsen, Todd

Issue Date

2014-05-30T16:42:13Z

Director of Research (if dissertation) or Advisor (if thesis)

• Werth, Charles J.
• Hudson, Robert J.

Department of Study

Civil & Environmental Eng

Discipline

Environ Engr in Civil Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• MeHg (monomethyl mercury)
• EtHg (ethylmercury)

Abstract

The Hg thiourea (TU) complex ion chromatography (HgTU-IC) method for mercury speciation analysis, first developed at UIUC, was the subject of an in depth investigation aimed at i) understanding the causes of system performance problems, ii) optimizing its chemistry, and iii) coupling it to ICP-MS detection. The new system chemistry described herein is capable of reliably, accurately, and sensitively quantifying both monomethyl (MeHg) and mercuric (HgII ) mercury species, and unexpectedly demonstrates the presence in freshwater samples of previously unknown Hg species with low net charge that are chemically different from both MeHg and HgII. In addition, it is shown that the system’s preconcentrator effectively traps both MeHg and HgII in samples containing strong Hg-binding ligands − 1 mM glutathione, thiosalicylic acid, and 0.15 M thiourea – giving it capabilities for rapid analysis of samples containing such ligands that the standard methods for low-level Hg speciation do not have. Although previous HgTU-IC system chemistries had high sensitivity and effectively separated MeHg from HgII, there were three problems that became apparent with their use over an extended period of time. The first was random, narrow spikes in the signal caused by particle formation in the post-column reaction coils. This issue was solved here by optimizing the oxidation chemistry and replacing SnII with BH4- as the reducing agent. A second arose when the manufacturer ceased production of the base gel used in synthesis of resin for the system’s preconcentrator. A replacement product, a commercially-available thiol resin, was shown to effectively trap MeHg and HgII in samples of at least 5-mL volume even when the matrix includes components known to interfere with MeHg analysis, i.e., thiol compounds. However, the strongest commonly-occurring ligand, hydrogen sulfide, did inhibit trapping of HgII at 1 mM. The third and arguably most important problem was identified and corrected while investigating the cause of inter-method differences in MeHg measured in samples from freshwater systems. Analyzing such samples using the original separation chemistry along with ICP-MS detection and isotopically-labeled internal standards revealed that the ambient Hg in the “MeHg” peak had a distinctly different shape from the internal standard. As the ion chromatographic separation is dependent on the [H+] in the mobile phase, it was shown that the misshapen peak could be resolved into true MeHg and one or more previously unknown inert Hg species that are at least partially trapped by the preconcentrator. Results obtained with this chemistry agree with distillation/ethylation-GC, but for some freshwater samples, results obtained using the previous HgTU-IC chemistries were biased high because they combined this inert Hg species with MeHg. Other tests with isotopic tracers showed that the TU-catalyzed SPE and HgTU-IC do not create a MeHg artifact from HgII during sample preparation or analysis and that with the addition of a longer IC column, the system is capable of measuring MeHg and EtHg in a single sample preparation/analysis. Using a slightly different mobile phase composition, the method can quantitate HgII directly. Method detection limit (MDL) studies were performed for MeHg and HgII resulting in a MeHg MDL of 0.003 ng/L and a HgII MDL of 0.01 ng/L for 40 mL samples. The MeHg MDL rivals that of ethylation/GC and exceeds those of other HPLC methods. The HgII MDL rivals the best HPLC methods in use.

Graduation Semester

2014-05

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Copyright and License Information

Copyright 2014 Todd Olsen

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