Speciation of Mercury in Crude Oil Using Speciated Isotope Dilution Mass Spectrometry

Mercury, in a variety of chemical and physical forms, is contributed to the environment from many sources, both natural and anthropogenic. It exists naturally in mineral form, usually associated with ores and other geological materials. Mercury enters and is recycled in the environment from a variety of sources, including both natural and anthropogenic (1). The anthropogenic activities are of the greatest importance in the mobilization of this metal and its compounds. Among the contributors of mercury to the environment, the burning of fossil fuel is a significant concern. While many scientific studies have focused upon coal and coal-fired power plants as industrial sources of mercury in the environment, other sources containing different species and concentrations also have generated interest. A deeper understanding of the origins of these species can be obtained by measuring speciated components of this reactive heavy metal. Other contributing sources include natural gas, gas condensate, crude oil, and petroleum, which also can contain significant amounts of mercury (2). Depending upon the region, crude oil produced around the world can contain varying levels of mercury and can range from 0.1 to 20,000 μg/kg (3).

The nature of mercury compounds in crude oil is not well-specified and quantified. Based upon previous studies on its chemical reactivity, the mercury in crude oil has been identified mostly as metallic with trace amounts of inorganic content (for example, mercuric chloride) and organometallics, including, monomethylmercury, monoethylmercury, dimethylmercury and diethylmercury, as well as nonsoluble mercury, such as mercury sulfide and mercury selenide (4). The content and concentration of different species of mercury in crude oil cause processing and environmental concerns. Accurate measurement of major species of mercury is therefore highly desirable for all stakeholders in the fossil fuel continuum.

Figure 1
Determination of mercury species in crude oil is difficult due to the volatile nature of many mercury species and complexity of the matrix. The traditional hot acid digestion procedures could cause lower recovery or transform mercury species during sample preparation. Therefore, an improved sample preparation protocol necessitates different strategies, such as sample preparation in a closed vessel microwave system followed by element-selective determination in the resulting aqueous medium (2).

From the literature, it was found that popular techniques for the determination of total mercury use cold vapor atomic absorption spectrometry (AAS) (2) or atomic fluorescence spectroscopy (AFS) (5) and isotope dilution cold vapor inductively coupled plasma–mass spectrometry (ID-CV-ICP-MS) (6). Mercury speciation analysis in crude oil matrices mainly consists of combinations of hyphenated chromatography for separation such as gas chromatography (GC) (7,8) and high performance liquid chromatography (HPLC) (9) with different detectors and detection methods. The detection methods used in GC for the determination of mercury species are ICP-MS (7), AFS (5), atomic absorption spectrometry (AAS) (9), electron-capture detection (ECD) (10), atomic emission spectrometry (AES) (11). The main problem of using GC-based separation in conventional ways is that the analyte must be volatile and thermally stable. To produce a volatile and stable mercury compound, the analyte must be treated with a suitable derivatizing reagent to make sure all the mercury compounds present in the extracted solvent are derivatized completely and back-extracted into the organic solvent, which are then injected into the GC system or extracted on a solid-phase microextraction sorbent in the headspace. Although HPLC separation can be used without derivatization, this method suffers from high detection limits. Many crude oil samples contain low concentrations of mercury species of interest.

The method reported in this article is an improvement of the sample preparation and detection inadequacies and represents a significant progress in the determination of mercury species in crude oil: the closed vessel microwave digestion system completely dissolves the difficult matrix without loss of analyte, and traditional corrections for analyte recovery are not necessary because the isotopically enriched analogues of the species are spiked before extraction–digestion and are equilibrated with the corresponding mercury species present in the sample during extraction to yield a fixed isotope ratio. This ratio is unaffected by any subsequent postequilibration loss of mercury species. Speciated isotope dilution mass spectrometry (SIDMS), the underlying metrology in EPA Method 6800, was used as a diagnostic tool and analytical determinative technique in this study. The unique SIDMS capability performed a dual role of quantifying and correcting for species transformation of up to three or four species simultaneously. In this case, SIDMS was used as a protocol step-evaluation tool to trap and evaluate errors from specific procedural steps. Thermally or chemically caused species interconversions occurring postspiking are traceable using the isotope enrichments in the species and can be corrected quantitatively by monitoring the values of the enriched species-isotopes equilibrated with each sample species. The SIDMS protocol enabled accurate correction for species transformations and determination of species concentrations at the point of spiking. To optimize the protocol, the sample was spiked both before and after its extraction to assess transformation during critical procedural steps. SIDMS was used to identify procedure-altered species distributions in a multistep protocol, and as importantly, to minimize and correct for these species introversions that would have otherwise invalidated our results.

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