A method using high performance liquid chromatography (HPLC) coupled with vapor generation atomic fluorescence spectrometry
(VGAFS) has been developed for the separation and determination of mercury species in sediment samples. The method is based
on the extraction of samples using 0.1% (v/v) 2-mercaptoethanol as the only extractant. The separation and determination of
mercury species was achieved by direct injection of the extractant to the HPLC–VGAFS system. Extraction conditions were optimized,
and extraction efficiency was validated against an established method, with RSD below 8%. Three mercury species including
inorganic mercury (Hg
), methylmercury (MeHg
), and ethylmercury (EtHg
) were baseline separated using reversed-phase C18 columns with a mobile phase of 3% (v/v) acetonitrile containing 60 mM ammonium
acetate-acetic acid (pH 4.5) and 0.1% (v/v) 2-mercaptoethanol pumped at 2.0 mL/min
. At optimized conditions, the limit of detection was 0.58 ng/g for MeHg
, 1.1 ng/g for EtHg
and 0.48 ng/g for Hg
. The average recoveries were 96.2% (MeHg
), 88.7% (EtHg
), and 95.8% (Hg
) by spiking three concentration levels of mercury species into samples. The developed method was validated by the determination
of certified reference materials and was further applied in analyses of sediment samples from the Sichuan area in China.
It is well known that the toxicity, biogeochemical behavior, and transportation of mercury in the environment are heavily
dependent on its chemical form (1,2). Methylmercury (MeHg+), the most toxic among mercury species, has been found in sediments from areas polluted by mercury because many micro-organisms
are involved with sulfate-reducing bacteria and have the ability to methylate the inorganic mercury. The determination of
mercury species in sediments is among the most required analysis in environmental studies, not only because of the toxicity
of mercury, but also because it is a good indicator of anthropogenic pollution sources (3) and enables scientists to identify
locations where MeHg is formed. Our research is focused on the development of simple, fast, and robust methods for the analysis
of mercury species in sediment samples.
The most common methods of mercury speciation are gas chromatography (GC) (2,4), high performance liquid chromatography (HPLC)
(2,4–6), ion chromatography (IC) (7–9), and capillary electrophoresis (CE) (10,11) coupled with a mercury-specific detector,
such as atomic absorption spectrometry (AAS) (2,4,5,12,13), atomic fluorescence spectrometry (AFS) (2,4,5,14–17), inductively
coupled plasma–mass spectrometry (ICP-MS) (7,16,18,19), inductively coupled plasma–atomic emission spectrometry (ICP-AES)
(20,21), and microwave-induced plasma atomic emission spectrometry (MIP-AES) (2,4,22). Only a few mercury speciation techniques
have been reported that do not incorporate chromatographic separation procedures into these atomic spectrometry techniques
Even though GC has been used to separate inorganic and organic mercury, this technique requires chemical derivatization of
mercurials into nonpolar, volatile, and thermally stable derivatives before GC separation, which is laborious and time-consuming.
In addition, interconversion of mercury species resulting from the high column temperature also may provide erroneous results
(26). CE combines rapid separation with high efficiency and very small sample volumes. However, sample requirements and inferior
detection limits render it unsuitable for analyzing most real-world samples (27). HPLC using reversed-phase chromatography
(26,28–30), which is based on the introduction of complexing agents, such as sodium, ammonium pyrrolidine dithiocarbamate,
or sulfhydryl-containing modifiers to the mobile phase (31–35), has been developed in both isocratic and gradient modes for
the separation of inorganic and organic mercury. Therefore, HPLC is considered the method that is least prone to species conversion
and that allows the most expedient separation of these mercurial species.
The determination of ultratrace levels of mercury using atomic optical or mass spectrometry methods requires the assistance
of vapor generation (VG) techniques to separate the analyte from the complex matrix, reduce interferences, and improve detection
sensitivity and selectivity (36). Among the reported chemical VG methods, SnCl2 and NaBH4 are the most frequently used reductants for the determination of mercury. However, according to Bramanti and colleagues (37,38),
the vaporization efficiency of Hg2+ could be severely suppressed because of the formation of thiol complexes and a molar excess of NaBH4. To minimize the adverse effects resulting from a thiol-containing effluent, Bramanti (39) suggested using an oxidation system
to convert various mercurial thiol complexes to Hg2+ before NaBH4 reduction. Presently, chemical oxidation and UV irradiation techniques for on-line postcolumn oxidation of mercury compounds
have been reported using various oxidants, including KBr/KBrO3 (40), K2S2O8 (41,42), and K2Cr2O7 (43–45).
Despite the excellent sensitivity and selectivity provided by most of the analytical techniques, sample preparation has been,
and continues to be, the primary concern in mercury speciation analysis. A reliable, simple, and rapid extraction method for
the determination of mercury species in complex matrices is of considerable interest. Although complexes of mercury species
with 2-mercaptoethanol are widely used in reversed-phase separation because of its chemical structure, until now the reagent
was not solely used for extraction of mercury species in any solid sample.
The aim of this study was to develop a simple, sensitive, and robust analytical method that would enable the speciation and
analysis of inorganic mercury (Hg2+), methylmercury (MeHg+), and ethylmercury (EtHg+) in sediments. The extraction procedure was performed in a closed-vessel microwave-assisted extraction (MAE) system using
2-mercaptoethanol as the only extractant. The separation and determination of mercury species was achieved by direct injection
of the extractant to a HPLC–VGAFS system. The application of 2-mercaptoethanol for mercury species extraction from sediments
has been found to be a very advantageous idea. This method was much more simple and rapid compared with others. The accuracy
of the method was tested against a set of certified reference materials, and its applicability was validated against a wide
number of natural sediments collected from Sichuan, China.