Mercury and Arsenic Speciation Analysis by IC-ICP-MS

Ion chromatography (IC) with conductivity detection has been used successfully to determine anionic and cationic substances as well as polar compounds such as amines and organic acids. However, in environmental samples, higher sensitivity and selectivity are required to test for potentially toxic substances with low maximum contaminant levels (MCLs).

The coupling of IC with multidimensional detectors such as electrospray ionization-mass spectrometers (ESI-MS) or inductively coupled plasma-mass spectrometers (ICP-MS) solves even complex separation problems, simultaneously achieving high sensitivity and selectivity. Additionally, these hyphenated techniques allow unambiguous peak identification and are less prone to matrix interferences than conductivity detection.

Especially in light of the zero tolerance policy concerning chromium, arsenic, and selenium compounds in drinking water and mercury in food samples, ICP-MS detectors have gained increasing importance. IC-ICP-MS can distinguish between different oxidation states and chemical forms of a given element. This approach is called speciation analysis. From a toxicological point of view, individual concentrations of element-containing species are far more significant than total element concentrations because different valence states of an element often have completely different properties. For example, while chromium(III) is an essential trace element for mammals since it is involved in glucose metabolization, all forms of hexavalent chromium are regarded as highly toxic and carcinogenic.

Arsenic

Arsenic is found ubiquitously in a high number of minerals, and its use as a weed killer and rat poison illustrates its high toxicity. Inorganic arsenical derivatives are considered to be carcinogenic and possibly teratogenic. Therefore, the U.S. EPA proposes a maximum allowable drinking water concentration of 10 ppb. In environmental and biological samples, more than 20 arsenic species have been identified. Depending on their binding characteristics, they have different toxicities and chemical properties. Based on structural data, IC-ICP-MS allows separation and unambiguous identification of different arsenic species in inorganic and organic forms.

Mercury

Mercury is found in several forms, particularly as elemental (Hg), inorganic (Hg2+), and alkylated mercury (CH3Hg+). Of the most common mercury species found in the environment, methylmercury is considered the most toxic species. It is classified as a neurotoxin that rapidly bioaccumulates and can cause major health problems or death, even in small quantities. According to the U.S. FDA, the major exposure pathway to methylmercury in humans and wildlife is through consumption of contaminated fish. The U.S. EPA stipulates a reference dose for methylmercury (Rf) of 0.1 µg/kg of body weight per day, while the World Health Organization (WHO) has set a tolerable dose of 1.6 µg/kg of body weight per week.1

However, mercury species are prone to interconversion. Mercury shows a pronounced transformation from inorganic mercury (Hg2+) to the biologically active and highly toxic methylmercury (methylation) and vice versa (demethylation). Similarly, the extraction techniques used for separation and preconcentration tend to alter the original distribution of mercury species, which affects the legal defensibility of the data.

Speciated isotope dilution mass spectrometry (SIDMS) has been developed to correct for species conversions. According to U.S. EPA Method 6800,2 each species is labeled with a different isotope-enriched spike in the corresponding form. By measuring the isotope ratio of both the unspiked and spiked sample and knowing the isotopic ratio of the addition, interconversions between the species become traceable and can be corrected.

This article discusses the determination of organic and inorganic arsenic and mercury compounds by means of IC-ICP-MS. Arsenic species (monoisotopic) are not prone to interconversion and are thus determined by traditional speciation analysis. Several established extraction techniques used for mercury speciation in biological samples are evaluated by applying both internal SIDMS and external calibration.

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