Detection of Mercury and Methylmercury Using IC-ICP/MS Method

Alkylated mercury (CH₃Hg⁺), inorganic Hg(II) and elemental Hg(0) are some of the various forms of the element mercury. Methylmercury is the most toxic of the most common mercury species existing in the environment. Methylmercury is categorized as a neurotoxin; it bio-accumulates quickly and even small amounts can lead to serious health issues or death. According to the US Food and Drug Administration (FDA), the consumption of contaminated fish is the major exposure pathway to methylmercury in humans and wildlife.

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In addition to fish, mercury can also be tested in end consumers. It can be tested for in human hair, for example, or in potential polluters such as crude oil. The US Environmental Protection Agency (EPA) has set a reference dose for methylmercury of 0.1 μg/kg of body weight per day and the World Health Organization (WHO) has set the dose at 1.6 μg/kg of body weight per week (Levenson, C. W., 2006).

Mercury and arsenic speciation test using IC-ICP/ MS

Specific isotope dilution mass spectrometry (SIDMS) is a technique used to analyse the species transformations of mercury that occur during several sample preparation methods. This paper explains the firm advantage that SIDMS (as detailed in Environmental Protection Agency [EPA] Method 6800) provides in the analysis of mercury species transformations that occur during the preparation of fish tissue samples. The use of SIDMS is anticipated to rise, given the unique features and advantages of the EPA Method 6800. This tool is valuable for validating and optimizing trace-metals-speciated sample preparation and it is expected to become widely accepted by analytical chemists.

Results

It took less than five minutes to achieve separation and the retention times were 1.87 ± 0.02 and 2.98 ± 0.03 minutes, as shown in Figure 1. The linear calibration curves for CH₃Hg⁺ and Hg(II) were achieved in the 1 to 20 μ g/L range. Detection limits for Hg(II) and CH₃Hg⁺ were 0.46 ± 0.02 and 0.78 ± 0.08 μg/L, respectively.

Non-quantitative recoveries and species transformations observed during extraction can be overcome using the SIDMS tool. SIDMS enables mercury species to be determined with high accuracy, precision and correction for species transformations. Inorganic mercury contamination in the extracting reagent could be observed, depending on the extraction method used, although alkaline extraction methods produced results that agreed well with the 95% confidence levels of the certified values.

Figure 1. 10 μg/L Hg(II) and CH₃Hg⁺. Chromatograms obtained at different masses were shifted for clarity. Column: DVB-C18 column, 150 × 4.6 mm, 2 μm; eluent: 50 mmol/L pyridine, 0.5% (w/v) mmol/L cysteine, 5% (v/v) methanol (pH = 3); flow rate: 1 mL/min, isocratic; m/z 199, 200, 201, 202.

Mercury species in crude oil by speciated isotope dilution LC-ICP/MS

Mercury, which arises from many anthropogenic and natural sources, is present in various physical and chemical forms in the environment. One of the processes that releases mercury into the environment is the burning of fossil fuels, which is a major concern. Sources include petroleum, crude oil, gas condensate, and natural gas and these contain significant amounts of mercury. Depending one where it is produced, crude oil can contain mercury ranging from 0.1 to 20,000 μg/kg.

Results

By following ccf-SIDMS in accordance with the EPA Method 6800, HPLC-ICP/MS analysis of mercury species can be applied successfully for crude oil samples (Figure 2). The method detection limit for C₂H₅Hg⁺, Hg²⁺, and CH₃Hg⁺ was below 0.1 ng/mL and therefore below 1ng/g for crude oil samples.

Figure 2. Three mercury species (10 μg/L each). Column: Supelcosil LC-18 (300 mm × 4.0 mm, 5 μm); eluent: 0.4% (w/v) L-Cysteine, 0.05% (v/v) 2-mercaptoethanol, 0.06 mol/L ammonium acetate, 5% (v/v) methanol; flow rate: 1 mL/min; m/z 199, 200, 201, 202.

Mercury species transformations in human hair using speciated isotope dilution mass spectrometry

By testing hair certified reference material (IAEA-085) approved for the content of monomethyl mercury and total mercury, nine different analytical extraction procedures commonly used for mercury speciation in human hair were assessed. For each procedure, the extraction efficiencies, recoveries, and potential for species transformations during sample processing were assessed using SIDMS as a quantitative and diagnostic tool. The extraction methods evaluated were based on acid leaching with HNO₃ and HCl. ICP/MS and high-performance liquid chromatography-ICP/MS were used to detect total mercury and mercury species, respectively, for all extraction procedures.

Results

To prevent relative biases from being interpreted as accurate species measurement, all the methods tested could be corrected by applying isotopic correction using SIDMS protocol (Figure 3). This shows that, providing isotopic equilibration is achieved, the sample preparation and extraction errors can be corrected to ensure precise and legally justifiable and accurate results.

Figure 3. Speciation of inorganic mercury and monomethyl mercury. Column: Nova-Pak C18 RP column, 60 Å, 4 μm, 3.9 mm × 150 mm; eluent: 0.4% L-Cysteine (pH 5); flow rate: 1 mL/min; m/z 199, 200, 202

Microwave-assisted extraction protocol for analysis of total mercury and methylmercury in fish tissues

To extract methylmercury (CH₃Hg⁺), inorganic mercury (Hg²⁺), and total mercury (Hg) from fish tissues, a fast and efficient closed vessel microwave-assisted extraction (MAE) method based on acid leaching was developed.

Humans are mainly exposed to methylmercury (CH₃Hg⁺) through the intake of fish (85–90% of total Hg is present in aquatic organisms). Microbial action on Hg²⁺ in aquatic ecosystems leads to the formation of CH₃Hg⁺, one of the most toxic forms of Hg. Readily bioavailable and able to bioaccumulate in the food chain, CH₃Hg⁺ reaches relatively high concentrations in large predator fish such as swordfish, marlin, shark and certain tuna species. In the past, the intake of contaminated fish has led to severe neurological damage in growing fetuses and children (Aschner, T., 2005; Gochfeld, M., 2005; Burger, J., 2005).

Results

A quick and efficient MAE method was developed for extracting total Hg, Hg(II) and CH₃Hg⁺ in fish tissues. Optimum conditions for the method were 0.5 g of sample, 10 mL of solvent containing 0.25 mol/L NaCl and 5 mol/L HCl, and an irradiation time of 10 minutes at 60 °C, as illustrated in Figure 4. Analyzing both total Hg and Hg species in different biological certified reference materials proved the accuracy of the method. SIDMS was successful as a diagnostic tool for validating the MAE method.

Figure 4. Dog fish liver DOLT-3 (top) and marlin fish tissue (bottom) samples spiked with ¹⁹⁹Hg(II) and CH₃ ²⁰⁰Hg⁺ before MAE. Column: PS-DVB C18 RP (150mm × 4.6mm, 2 μm); eluent: 50 mmol/L pyridine, 0.5% (w/v) L-cysteine, and 5% (v/v) methanol (pH 3); flow rate: 1mL/min.

Mercury – further applications with IC-ICP/MS

Comparison of methods with respect to efficiencies, recoveries, and quantitation of mercury species interconversions in food demonstrated using tuna fish. Reyes, L. H.; Mizanur Rahman, G. M.; Fahrenholz, T.; 'Skip' Kingston, H. M. (2008) Anal. Bioanal. Chem. 390(8), 2123–2132

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