Mercury-cycling in surface waters and in the atmosphere — Species analysis for the investigation of transformation and transport properties of mercury

The river Elbe has been one of the most contaminated rivers with regard to mercury for many years. In 1991 a length-profile has been measured for mercury and methylmercury (CH₃Hg⁺) from Obristvi, Czech Republic, to the German bight. Total mercury has been measured by cold vapor atomic absorption spectrometry (CVAAS). The organo mercury compounds have been separated by high performance liquid chromatography (HPLC) connected on-line to an atomic fluorescence spectrometer (AFS) by a continuous flow-system. Total mercury up to 120 mg Hg⁺/kg and CH₃Hg⁺ concentrations up to 130 μg CH₃ Hg⁺/kg could be detected in special sites. The formation of CH₃Hg⁺ in sediments can be caused besides the methylation of mercury, by sulphate reducing or methanogenic bacteria and transmethylation reactions with organometals. Atmospheric mercury concentrations have been measured at three different European sites. Samples have been collected on goldcoated glass balls or on quartz wool, respectively. After thermal desorption mercury has been determined using the two step amalgamation technique with AFS detection. Compared to natural background concentrations of total gaseous mercury (TGM), slightly increased levels could be detected at a rural site in Germany. This increase can probably be explained by long-range transport processes. Within the vicinity of a inactivated mercury production plant high concentrations of up to 13.5 ng/m³ particle associated mercury (Hg_part) have been detected. Consequently, dry deposition of mercury in the particulate form can intensify the total deposition flux close to Hg-emitting sources.     

Mercury speciation in thawed out and refrozen fish samples by gas chromatography coupled to inductively coupled plasma mass spectrometry and atomic fluorescence spectroscopy

Different sub-sampling procedures were applied for the determination of mercury species (as total mercury Hg, methylmercury MeHg⁺ and inorganic mercury Hg²⁺) in frozen fish meat. Analyses were carried out by two different techniques. After the sample material was pre-treated by microwave digestion, atomic fluorescence spectroscopy (AFS) was used for the determination of total Hg. Speciation analysis was performed according to the following procedure: dissolution of sample material in tetramethylammonium hydroxide (TMAH), derivatisation with sodium tetraethylborate (NaBEt₄), extraction into isooctane and measurement with gas chromatography inductively coupled plasma mass spectrometry (GC-ICPMS) for the identification and quantification of methylmercury (MeHg⁺) and inorganic mercury (Hg²⁺). The concentration range of total Hg measured in the shark fillets is between 0.9 and 3.6 g g^–1 thawed out shark fillet. Speciation analysis leads to 94% Hg present as MeHg⁺. Homogeneity, storage conditions and stability of analytical species and sample materials have great influence on analytical results. Sub-sampling of half-frozen/partly thawed out fish and analysis lead to significantly different concentrations, which are on average a factor of two lower.     

Application of stable isotopes in environmental tracer studies – Measurement of monomethylmercury (CH3Hg+) by isotope dilution ICP-MS and detection of species transformation

The monovalent cation monomethylmercury (CH₃Hg⁺) was determined in certified reference materials by isotope dilution GC/ICP-MS and good agreement between measured and certified values has been found. The use of enriched stable isotopes with subsequent detection by ICP-MS is a powerful tracer technique to study dynamic environmental processes. For the first time, it was possible to monitor opposite processes like Hg²⁺ methylation and CH₃Hg⁺ demethylation at ambient tracer levels simultaneously in the same sample. A scheme for calculating the formation of new species from stable tracers used in environmental studies is presented. The sensitivity of stable tracer methods is superior to traditional tracer or radiotracer techniques. In case of mercury methylation, where the generation of a new compound is monitored, the limit of detection depends only on the precision of the isotope ratio measurements and the concentration of the ambient CH₃Hg⁺ already present in the sample, not on the absolute detection limit of the GC/ICP-MS technique used for analyzing CH₃Hg⁺. A 0.25% change in concentration of CH₃Hg⁺ is detectable. In the case of CH₃Hg⁺ demethylation, where the decrease of the added tracer is monitored, the detection limit again depends on the precision of the isotope ratio measurement and the ambient CH₃Hg⁺ concentration but additionally on the amount of the added tracer as well. A decrease in the CH₃Hg⁺ level of 2% of the added tracer is detectable. The validity of the calculation scheme was tested and no difference was found between individually measured isotope concentrations and calculated concentrations from solutions with multiple stable tracers.     

Preconcentration and speciation of inorganic and methyl mercury in waters using polyaniline and gold trap-CVAAS

Applicability of polyaniline (PANI) has been investigated for the preconcentration and speciation of inorganic mercury (Hg²⁺) and methyl mercury (CH₃Hg⁺) in various waters (ground, lake and sea waters). Preliminary experiments (batch) with powdered PANI for the quantitative removal of both Hg²⁺ and CH₃Hg⁺ showed that the retention of Hg²⁺ was almost independent of pH while a pH dependent trend from pH 1 to 12 was seen for CH₃Hg⁺ with maximum retention at pH > 5. Time dependence batch studies showed that a contact time of 10 min was sufficient to reach equilibrium. The K_d values were found to be ∼8 × 10⁴ and ∼7 × 10³ for Hg²⁺ and CH₃Hg⁺, respectively.Subsequently column experiments were carried out with PANI and the separation of the species was carried out by selective and sequential elution with 0.3% HCl for CH₃Hg⁺ and 0.3% HCl-0.02% thiourea for Hg²⁺. This was then followed by further pre-concentration of mercury on a gold trap and its determination by CVAAS. The uptake efficiency studies showed that the PANI column was able to accumulate up to 100 mg Hg²⁺/g and 2.5 mg CH₃Hg⁺/g. This method allows both preconcentration and speciation of mercury with preconcentration factors around 120 and 60 for Hg²⁺ and CH₃Hg⁺, respectively. The interfering effects of various foreign substances on the retention of mercury were investigated.     

Determination of mercury and methylmercury in seafood by ion chromatography using photo-induced chemical vapor generation atomic fluorescence spectrometric detection

A novel method based on photo-induced chemical vapor generation (CVG) as interface to on-line coupled Hg-cysteine ion chromatograpy (IC) with atomic fluorescence spectrometry (AFS) was developed for rapid determination of methylmercury (MHg) in seafood. Separation of inorganic mercury (Hg²⁺) and methylmercury(CH₃Hg⁺) was accomplished on a Hamilton PRP X-200 polymer-based exchange column with a mobile of 3% acetonitrile, 1% (w/w) L-cysteine and 20 mmol L^{− 1} pyridine and 160 mmol L^{− 1} formic acid, at pH 2.4 within 7 min. Once separated, both species are reduced by formic acid in mobile phase under UV radiation to convert Hg⁰ on-line, which is subsequently swept (by argon carrier gas) into an atomic fluorescence spectrometry (AFS) for measurement. Under the optimized experiment conditions, the detection limits (as Hg), based on three times the standard deviation of a standard solution, were found to be 0.1 ng mL^{− 1} for mercury and 0.08 ng mL^{− 1} for methylmercury, with an injection volume of 100 μL. The developed method was validated by determination of certified reference material DORM-2 and was further applied in determination of seafood samples.     

Robust microwave-assisted extraction protocol for determination of total mercury and methylmercury in fish tissues

A rapid and efficient closed vessel microwave-assisted extraction (MAE) method based on acidic leaching was developed and optimized for the extraction of total mercury (Hg), inorganic mercury (Hg²⁺) and methylmercury (CH₃Hg⁺) from fish tissues. The quantitative extraction of total Hg and mercury species from biological samples was achieved by using 5 mol L⁻¹ HCl and 0.25 mol L⁻¹ NaCl during 10 min at 60 °C. Total Hg content was determined using inductively coupled plasma mass spectrometry (ICP-MS). Mercury species were measured by liquid chromatography hyphenated with inductively coupled plasma mass spectrometry (LC-ICP-MS). The method was validated using biological certified reference materials ERM-CE464, DOLT-3, and NIST SRM-1946. The analytical results were in good agreement with the certified reference values of total Hg and CH₃Hg⁺ at a 95% confidence level. Further, accuracy validation using speciated isotope-dilution mass spectrometry (SIDMS, as described in the EPA Method 6800) was carried out. SIDMS was also applied to study and correct for unwanted species transformation reactions during and/or after sample preparation steps. For the studied reference materials, no statistically significant transformation between mercury species was observed during the extraction and determination procedures. The proposed method was successfully applied to fish tissues with good agreement between SIDMS results and external calibration (EC) results. Interspecies transformations in fish tissues were slightly higher than certified reference materials due to differences in matrix composition. Depending on the type of fish tissue, up to 10.24% of Hg²⁺ was methylated and up to 1.75% of CH₃Hg⁺ was demethylated to Hg²⁺.     

Validation of a simplified field-adapted procedure for routine determinations of methyl mercury at trace levels in natural water samples using species-specific isotope dilution mass spectrometry

A field-adapted procedure based on species-specific isotope dilution (SSID) methodology for trace-level determinations of methyl mercury (CH₃Hg⁺) in mire, fresh and sea water samples was developed, validated and applied in a field study. In the field study, mire water samples were filtered, standardised volumetrically with isotopically enriched CH₃²⁰⁰Hg⁺, and frozen on dry ice. The samples were derivatised in the laboratory without further pre-treatment using sodium tetraethyl borate (NaB(C₂H₅)₄) and the ethylated methyl mercury was purge-trapped on Tenax columns. The analyte was thermo-desorbed onto a GC-ICP-MS system for analysis. Investigations preceding field application of the method showed that when using SSID, for all tested matrices, identical results were obtained between samples that were freeze-preserved or analysed unpreserved. For DOC-rich samples (mire water) additional experiments showed no difference in CH₃Hg⁺ concentration between samples that were derivatised without pre-treatment or after liquid extraction. Extractions of samples for matrix–analyte separation prior to derivatisation are therefore not necessary. No formation of CH₃Hg⁺ was observed during sample storage and treatment when spiking samples with ¹⁹⁸Hg²⁺. Total uncertainty budgets for the field application of the method showed that for analyte concentrations higher than 1.5 pg g^–1 (as Hg) the relative expanded uncertainty (REU) was approximately 5% and dominated by the uncertainty in the isotope standard concentration. Below 0.5 pg g^–1 (as Hg), the REU was >10% and dominated by variations in the field blank. The uncertainty of the method is sufficiently low to accurately determine CH₃Hg⁺ concentrations at trace levels. The detection limit was determined to be 4 fg g^–1 (as Hg) based on replicate analyses of laboratory blanks. The described procedure is reliable, considerably faster and simplified compared to non-SSID methods and thereby very suitable for routine applications of CH₃Hg⁺ speciation analysis in a wide range of water samples.     

Speciation of volatile mercury species present in digester and deposit gases

Volatile mercury compounds have been speciated in gases evolved from fermentation of sewage sludge as well as municipal waste. The species were trapped by sequential sampling, using a noble‐metal trap in series with an activated‐carbon trap. Thermally desorbed Hg⁰ and (CH₃)₂Hg were separated by GC at 70 °C and detected by cold vapour atomic fluorescence spectroscopy after thermal reduction. The amounts of mercury detected in the sewage gas correspond to concentrations in the range 50–110 ng m⁻³ for both species whereas the deposit gases were found to contain only elemental mercury. Monomethylmercury species could not be positively identified in any of the gas samples. Copyright © 1999 John Wiley & Sons, Ltd.     

Selective detection of mercury(II) and methylmercury(II) via coordination-induced emission of a small-molecule probe

Mercury is one of the major toxic pollutants and has many adverse effects on human health. The main mercury species in the environment or in living organisms are inorganic mercuric ion (Hg²⁺) and organic methylmercury (CH3Hg+). Detection of the two mercury ions is a particularly active topic in the molecular sensing field during the past decade. However, efficient sensors that can sensitively detect and discriminate the two species are rare. In this work, we adopt the concept of restriction of intramolecular rotations which is the basis of aggregation induced emission, and design a molecular probe with pyridyl group as the chelating unit and 1,8-naphthalimide as the fluorescent unit for the detection of both Hg²⁺ and CH₃Hg⁺. When the probe is free in solution, it exhibits weak fluorescence because free intramolecular rotations of the 1,8-naphthalimide moieties non-radiatively annihilate its excited state. However, upon coordination with Hg²⁺ or CH₃Hg⁺, the rotation of 1,8-naphthalimide moieties would be restricted due to the chelation between 1,8-naphthalimide and Hg²⁺ or CH₃Hg⁺, leading to significantly enhanced fluorescent emission. The response induced by Hg²⁺ is much stronger than CH₃Hg⁺; but for specific detection of CH₃Hg⁺, we introduced a T-rich DNA fragment which could completely mask Hg²⁺ in solution. Furthermore, we have employed the sensor for confocal imaging of Hg²⁺ and CH₃Hg⁺in immobilized cells. We expect the probe design tactics can be generally useful for sensing many other analytes.     

Routine analysis of mercury species using commercially available instrumentation: chemometric optimisation of the instrumental variables

Mercury speciation analysis (inorganic mercury, Hg²⁺, methylmercury, CH₃Hg⁺ and dimethylmercury, (CH₃)₂Hg) by gas chromatography (GC) coupled to atomic emission spectroscopy with microwave induced plasma as excitation source (MIP-AES), after ethylation of the sample and extraction of the derivatised species into an organic phase, has been optimised using factorial design, analysis of variance and MultiSimplex techniques. Standard conditions were used in the derivatisation step with sodium tetraethylborate (NaB(C₂H₅)₄) and in the extraction step into hexane. Good separation of the species investigated and maximum sensitivity was achieved using an OV-1701 capillary column. The sensitivity was found to be maximum with an helium flow rate (make-up flow) of 100 ml min⁻¹. Procedures for a correct cleaning of glass and plastic ware, as well as for the purification of reagents used throughout the analytical process, are also suggested in order to avoid unacceptably high blank signals. The effect that ageing of stock solutions used in calibrations has on the artefact formation of CH₃Hg⁺ has been also investigated. Using the optimum conditions found, good quality calibration curves (R²>0.995) for the three mercury species were obtained. Absolute detection limits of 0.5, 3 and 15 pg of (CH₃)₂Hg, CH₃Hg⁺ and Hg²⁺, respectively, were estimated. The repeatability of the analysis was found to be better than 5% (n=5) in relative standard deviation (R.S.D.) units. The optimised procedure for the speciation of mercury in standard samples is the first step in the development of a method for routine analysis of mercury species in aquatic environmental samples.