Mercury Speciation refers to a suite of techniques employing ligand exchange and ion-exchange reactions and solid phase extractions to effect both matrix removal and analyte preconcentration for separations and individual determination of monomethyl and inorganic mercury (MeHg and Hg(II), respectively). The heart of the system is the HPLC-based analyzer developed by Quicksilver Scientific founder Christopher Shade, during his Ph.D. work at the University of Illinois at Urbana-Champaign. The patent-pending system uses preconcentration onto a proprietary thiolic resin, followed by elution with a highly acidic thiourea solution and separation across an ion-exchange resin (Hg-Thiourea Ion Chromatography) and atomic fluorescence spectrometric quantification of generated on-line cold-vapor (CVAFS).

Efficient Use of Metals Speciation:
The cornerstone of the system is the use of thiourea as a complexing agent. Thiourea, [(NH2)2C=S]0, is a unique sulfur-based ligand (a thione) that combines the strong complexation properties of thiols (is a strong Lewis base; log K1 for Hg(2+) = 11.34 ) with the protonation-resistant properties of halides (is a weak Brønsted base; log KH = -1.34). Thus, the pH scale can be very effectively used to establish either thiol complexation or thiourea complexation (Figure 2), giving incredible flexibility to the system. Since essentially all environmental compartments retain mercury forms via sulfhydryl bonds, this means that samples can be efficiently and effectively leached with acidic thiourea solutions (Table 1), AND, if necessary, those sample solutions can be preconcentrated after pH adjustment onto an on-line thiol resin trap before injection.

Sample Preconcentration:
The thiol resin trapping of prepared samples is much less sensitive to solution chemistry than is the ethylation procedure of EPA Method 1630. This yields two important results: 1) it allows for a wider range of sample preparation chemistries, and, very importantly, 2) it allows the sample introduction to be automated, resulting in vastly lower labor costs over EPA Method 1630.

Charged Complex Formation:
Thiourea is a water soluble polar molecule but has a neutral charge, thus resulting complexes retain the net charge of the central metal cation (i.e. CH3Hg+ is +1 and HgII is +2). This yields both a simple ion-exchange separation strategy and an efficient method for extracting ionic mercury from non-polar solvents such as toluene (CH3Hg+ in sediment solvent extractions) or petroleum hydrocarbons such as methane condensates and napthas (both HgII and CH3Hg+).

The accuracy and precision of system has been validated with reference materials using two different sample introduction methods. The table to the right shows results from sample loop injections (SL) and thiol trap preconcentration (TT) of solvent extracted reference materials. RSD’s are typically less than 5%, and are quite often below 3%.

Applications/Matrixes
Sediments:
Acid Leaching/Solvent Extraction: Quicksilver uses HNO3/CuSO4 leaching and a solid phase extraction resin (replaces the methylene chloride or toluene typically used for solvent extraction). This extraction method is more powerful than often-used H2SO4/KBr/CuSO4, which has demonstrated low recoveries on some difficult sediment types, such as organic wetland soils2. For example, our comparison of the two leaching methods on wetland sediments from near Gary, IN showed a 95% spike recovery using nitric acid, but only 65-80% recovery from H2SO4/KBr/CuSO4, depending on the sample site. The extraction can be performed on wet or dried sediments, and creates virtually no artifactual methylmercury from native inorganic mercury, making it ideal for high Hg sites. LOD ~ 20 pg/g

Biota (Simultaneous MeHg and HgII):
Thiourea Leaching: Quicksilver uses a direct acidic thiourea leaching of biotic solids (biotic tissues, or human hair or nails) to provide a simultaneous direct analysis of both methyl and mercuric mercury, eliminating the need for HgT measurements. The leaching chemistry does not solubilize biotic proteins, thus allowing the simplicity of sample loop injections for most sample types. Thiol preconcentration allows for very small sample sizes—as little as a single insect or a few mg of tissue—or very low level samples.
LOD ~ 0.5ng/g or 5pg/g, depending on sample introduction method and needs.

Total Mercury (HgT)

Quicksilver Scientific offers full Total Hg support, by either combustion/amalgamation/CVAA (NIC MA-2000 Direct Hg Analyzer) or by acid digestion/amalgamation/CVAA, with resulting compliance with EPA Methods 1631, 7473, 247.1, 247.2 and 247.5. Combustion has been shown to be more effective on sediments and geologic materials than has acid-digestion3.

References:

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qs-technical-brief.pdf
qs-technical-brief-2.pdf