Determination of methyl-and ethylmercury compounds using gas chromatography atomic fluorescence spectrometry following aqueous derivatization with sodium tetraphenylborate

Occurrence of monoethylmercury in the Florida Everglades: identification and verification

A few studies have reported the occurrence of monoethylmercury (CH(3)CH(2)Hg(+)) in the natural environment, but further verification is needed due to the lack of direct evidence and/or uncertainty in analytical procedures. Various analytical techniques were employed to verify the occurrence of CH(3)CH(2)Hg(+) in soil of the Florida Everglades. The identity of CH(3)CH(2)Hg(+) in Everglades soil was clarified, for the first time, by GC/MS. The employment of the recently developed aqueous phenylation-purge-and-trap-GC coupled with ICPMS confirmed that the detected CH(3)CH(2)Hg(+) was not a misidentification of CH(3)SHg(+). Stable isotope-tracer experiments further indicated that the detected CH(3)CH(2)Hg(+) indeed originated from Everglades soil and was not an analytical artifact. All these evidence clearly confirmed the occurrence of CH(3)CH(2)Hg(+) in Everglades soil, presumably as a consequence of ethylation occurring in this wetland. The prevalence of CH(3)CH(2)Hg(+) in Everglades soil suggests that ethylation could play an important role in the biogeochemical cycling of Hg.

Simultaneous speciation of monomethylmercury and monoethylmercury by aqueous phenylation and purge-and-trap preconcentration followed by atomic spectrometry detection

A new method for the detection of trace levels of organomercury species has been developed by combining the high enrichment capacity of purge and trap with aqueous phenylation derivatization. Phenylation products of monomethylmercury (MeHg) and monoethylmercury (EtHg) were first separated by capillary gas chromatography and then detected by atomic fluorescence spectrometry (AFS) or inductively coupled plasma mass spectrometry (ICPMS). This combination made it possible to simultaneously quantify trace or ultratrace level of MeHg and EtHg in environmental samples. Method detection limits were 0.03 ng/L for both MeHg and EtHg when AFS was used as the detector and 0.02 and 0.01 ng/L for MeHg and EtHg with ICPMS, respectively. Certified reference materials, IAEA-405 and DORM-2, were analyzed and the results were in accordance with certified values. Both MeHg and EtHg were detected in sediment samples collected from the Florida Everglades and a Canadian wetland. This new method has been validated for the direct detection of trace organomercury species in freshwater samples and has the additional benefits of being free from interference by Cl (-) and dissolved organic matter.

Production of artifact methylmercury during the analysis of certified reference sediments: Use of ionic exchange in the sample treatment step to minimise the problem

Production of artifact methylmercury (MeHg(+)) during the analysis of two certified reference sediments, CRM-580 and IAEA-405, was investigated. Leaching of the analyte from the solid sample was achieved by ultrasound assisted acidic extraction. The aqueous leachate was either ethylated (NaBEt(4)) or phenylated (NaBPh(4)) using acetic/acetate or citric/citrate to buffer the solution. Preconcentration of the volatile compounds was carried out by extraction with an organic solvent (n-hexane) or solid phase microextraction (SPME). MeHg(+) was finally separated and detected by gas chromatography with atomic emission or mass spectrometry detection (GC-MIP-AED or GC-MS). In all the cases the concentrations obtained for MeHg(+) in the CRM-580 were significantly higher than the certified value. For the IAEA-405, however, the MeHg(+) concentration found was always statistically indistinguishable from the certified value. Experiments were also conducted with synthetic samples, such as aqueous mixtures of MeHg(+) and inorganic mercury (Hg(2+)) or silica-gel spiked with both compounds. The methylation rates found (defined as the percentage of Hg(2+) present in the sample which methylates to give artifact MeHg(+)) ranged from not observable (in certain synthetic aqueous mixtures) to 0.57% (analysis of CRM-580 under certain conditions). As the amount of Hg(2+) available in the sample seems to be the main factor controlling the magnitude of the artifact, several experiments were conducted using an ionic exchange resin (Dowex M-41) in order to minimise the concentration of this chemical in the reaction medium. First, a hydrochloric leachate of the sample was passed through a microcolumn packed with the exchanger. Second, the resin was mixed with the sample prior to extraction with HCl. In both cases, the predominant Hg(2+) species, HgCl(4)(2-), was adsorbed on the resin, whereas MeHg(+), mainly as MeHgCl, remained in solution. Following the second option, a new method to analyse MeHg(+) in conflictive matrices like certain sediments was proposed. This approach produced better results for the CRM-580, but a MeHg(+) concentration slightly, but statistically significant, higher than the reference value was still obtained.

Determination of mercury species in fish reference materials by gas chromatography-atomic fluorescence detection after closed-vessel microwave-assisted extraction

A simple and rapid method has been developed for speciation analysis of inorganic mercury and monomethylmercury (MMHg) in biological tissues. The procedure is based on the quantitative closed-vessel microwave-assisted leaching of mercury from biological samples with an alkaline extractant. The extracted mercury species are ethylated and analysed by capillary gas chromatography coupled to an atomic fluorescence detector via pyrolysis (CGC-pyro-AFS). The coupling between capillary gas chromatography and atomic fluorescence detector was optimized with the aim of minimizing the detection limits and time necessary for the species-selective determination of mercury compounds. The use of closed-vessel microwave-assisted extraction along with no clean-up steps before the ethylation leads to a significant total analysis time decrease with respect to similar procedures. The detection limit was 2 pg for MMHg (as Hg) and 1 pg for inorganic mercury. The method was validated by the analysis of DORM-2 (dogfish muscle) and DOLT-3 (dogfish liver) certified reference materials. The inorganic mercury and methylmercury concentrations found were in good agreement with the certified values. Recovery studies of fish muscle tissue spiked with inorganic mercury and MMHg were done to check the reliability of the method. In all cases satisfactory recoveries (92-105%) were obtained.

Improvements in the methylmercury extraction from human hair by headspace solid-phase microextraction followed by gas-chromatography cold-vapour atomic fluorescence spectrometry

Improvements in the methylmercury extraction from human hair by solid-phase microextraction followed by gas chromatography coupled to cold-vapour atomic fluorescence spectrometry (GC-CVAFS) have been carried out. They consisted in the optimisation of the digestion step prior to the aqueous-phase ethylation and in the GC-CVAFS interface set-up. The main digestion parameters such as acid type, concentration, temperature and time have been optimised for hair sample analysis, thereby avoiding methylmercury degradation. Moreover, the stability of the digested samples was evaluated to improve the sample throughput.

Application of isotope dilution to the determination of methylmercury in fish tissue by solid-phase microextraction gas chromatography–mass spectrometry

Species-specific isotope dilution (ID) calibration using solid-phase microextraction (SPME) in combination with gas chromatography-mass spectrometry (GC-MS) for separation and detection of methylmercury (MeHg) in fish tissue is described. Samples were digested with methanolic potassium hydroxide. Analytes were propylated and headspace sampled with a polydimethylsiloxane-coated SPME fused-silica fiber. ID analysis was performed using a laboratory-synthesized 198Hg-enriched methylmercury (Me 198Hg) spike. Using selective ion monitoring (SIM) mode, the intensities of Me 202HgPr+ at m/z 260 and Me 198HgPr+ at m/z 256 were used to calculate the m/z ratio at 260/256, which was used to quantify MeHg in NRCC CRM DORM-2 fish tissue. A MeHg concentration of 4.336 +/- 0.091 microg g(-1) (one standard deviation, n = 4) as Hg was obtained in DORM-2, in good agreement with the certified value of 4.47 +/- 0.32 microg g(-1) (95% confidence interval). A concentration of 4.58 +/- 0.31 microg g(-1) was determined by standard additions calibration using ethylmercury (EtHg) as an internal standard. The three-fold improvement in the precision of measured MeHg concentrations using ID highlights its superiority in providing more precise results compared to the method of standard additions. A method detection limit (3 S.D.) of 0.037 microg g(-1) was estimated based on a 0.25 g subsample of DORM-2.

Methylmercury determination in biological samples by derivatization, solid-phase microextraction and gas chromatography with microwave-induced plasma atomic emission spectrometry

A method for the extraction and gas chromatographic determination of methylmercury in biological matrices is presented. By combining the advantages of two extraction techniques-microwave-assisted extraction (MAE) and solid-phase microextraction (SPME)--the separation of methylmercury from biological samples is possible. Specifically, the procedure involves microwave extraction with 3 M hydrochloric acid, followed by aqueous-phase derivatization with sodium tetraphenylborate and headspace SPME with a silica fibre coated with polydimethylsiloxane (PDMS). For optimization of the derivatization-SPME procedure, a central composite experimental design with alpha = 1.682 and two central points was used to model gas-chromatographic peak areas as functions of pH, extraction temperature and sorption time. A desirability function was then used for the simultaneous optimization for methylmercury and Hg(II). The optimal derivatization-SPME conditions identified were close to pH 5, temperature 100 degrees C, and sorption time 15 min. The identification and quantification of the extracted methylmercury is carried out by gas chromatography with microwave-induced plasma atomic emission spectrometry detection. The validity of the new procedure is shown by the results of analyses of certified reference materials.