Thiourea catalysis of MeHg ligand exchange between natural dissolved organic matter and a thiol-functionalized resin: a novel method of matrix removal and MeHg preconcentration for ultratrace Hg speciation analysis in freshwaters

A high-throughput, straightforward procedure for biomonitoring organomercury species in human hair

Mercury is a pervasive and concerning pollutant due to its toxicity, mobility, and tendency to biomagnify in aquatic and terrestrial ecosystems. Speciation analysis is crucial to assess exposure and risks associated with mercury, as different mercury species exhibit varying properties and toxicities. This study aimed at developing a selective detection method for organic mercury species in a non-invasive biomonitoring matrix like human hair. The method is based on frontal chromatography (FC) in combination with inductively coupled plasma mass spectrometry (ICP-MS), using a low pressure, homemade, anion exchange column inserted in a standard ICP-MS introduction system, without requiring high-performance liquid chromatography (HPLC) hyphenation. In addition to the extreme simplification and cost reduction of the chromatographic equipment, the proposed protocol involves a fast, streamlined and fully integrated sample preparation process (in contrast to existing methods): the optimized procedure features a 15-min ultrasonic assisted extraction procedure and 5 min analysis time. Consequently, up to 100 samples could be analyzed daily, making the method highly productive and suitable for large-scale screening programs in public and environmental health. Moreover, the optimized procedure enables a limit of detection (LOD) of 5.5 μg/kg for a 10 mg hair microsample. All these features undeniably demonstrate a significant advancement in routine biomonitoring practices. To provide additional evidence, the method was applied to forty-nine human hair samples from individuals with varying dietary habits successfully finding a clear correlation between methylmercury levels (ranging from 0.02 to 3.2 mg/kg) in hair and fish consumption, in line with previous literature data.

The present and potential future of aqueous mercury preservation: a review

Mercury is considered to be one of the most toxic elements to humans. Due to pollution from industry and artisanal gold mining, mercury species are present globally in waters used for agriculture, aquaculture, and drinking water. This review summarises methods reported for preserving mercury species in water samples and highlights the associated hazards and issues with each. This includes the handling of acids in an uncontrolled environment, breakage of sample containers, and the collection and transport of sample volumes in excess of 1 L, all of which pose difficulties for both in situ collection and transportation. Literature related to aqueous mercury preservation from 2000-2021 was reviewed, as well as any commonly cited and relevant references. Amongst others, solid-phase extraction techniques were explored for preservation and preconcentration of total and speciated mercury in water samples. Additionally, the potential as a safe, in situ preservation and storage method for mercury species were summarised. The review highlighted that the stability of mercury is increased when adsorbed on a solid-phase and therefore the metal and its species can be preserved without the need for hazardous reagents or materials in the field. The mercury species can then be eluted upon return to a laboratory, where sensitive analytical detection and speciation methods can be better applied. Developments in solid phase extraction as a preservation method for unstable metals such as mercury will improve the quality of representative environmental data, and further improve toxicology and environmental monitoring studies.

Ultratrace mercury speciation analysis in rice by in-line solid phase extraction - liquid chromatography - atomic fluorescence spectrometry

For the first time, Hg²⁺ and methylmercury speciation analysis was accomplished by in-line SPE-LC-AFS. After modification with 0.1 mL of 0.001% (m:v) sodium diethyldithiocarbamate, a C₁₈ microcolumn retained Hg²⁺ and MetHg in rice extract within 3 min; the captured Hg species were separated within 12 min in 0.25% (v:v) 2-mercaptoethanol + 60 mmol L^-1 (m:v) ammonium acetate + 4% (v:v) acetonitrile. Under optimized conditions, the detection limits were 0.3 ng L^-1 for Hg²⁺ and 0.2 ng L^-1 for MetHg, respectively, with 10 mL injection vs. 0.1 mL eluent; in-line SPE achieved ∼ 100x enrichment. Method precision and accuracy were satisfactory at < 2% relative standard deviations (RSDs) for 20 ng L^-1 of Hg²⁺ and MetHg and 95-102% recoveries for real rice samples. In-line SPE obviated human involvement and avoided invalid transportation between interfaces, rendering this SPE-LC-AFS method easy, compact, robust, yet sensitive in mercury speciation analysis to uphold food safety.

Preconcentration and speciation analysis of mercury: 3D printed metal scavenger-based solid-phase extraction followed by analysis with inductively coupled plasma mass spectrometry

A selective method for preconcentration and determination of methylmercury (MeHg) and inorganic mercury (iHg) in natural water samples at the ng L^-1 level has been developed. The method involves adsorption of Hg species into a 3D printed metal scavenger and sequential elution with acidic thiourea solutions before ICP-MS determination. Experimental parameters affecting the preconcentration of MeHg and iHg such as the sample matrix, effect of the flow rate on adsorption, eluent composition, and elution mode have been studied in detail. The obtained method detection limits, considering the preconcentration factors of 42 and 93, were found to be 0.05 ng L^-1 and 0.08 ng L^-1 for MeHg and iHg, respectively. The accuracy of the method was assessed with a certified groundwater reference material ERM-CA615 (certified total iHg concentration 37 ± 4 ng L^-1). The determined MeHg concentration was below MDL while iHg concentration was determined to be 41.2 ± 0.5 ng L^-1. Both MeHg and iHg were also spiked to natural water samples at 5 ng L^-1 concentration and favorable spiking recoveries of 88-97% were obtained. The speciation procedure was successfully applied to two lake water samples where MeHg and iHg concentrations ranged from 0.18 to 0.24 ng L^-1 and 0.50-0.62 ng L^-1, respectively. The results obtained demonstrate that the developed 3D printed metal scavenger-based method for preconcentration and speciation of Hg is simple and sensitive for the determination of Hg species at an ultra-trace level in water samples.

Addition of thiourea and hydrochloric acid: Accurate nanogram level analysis of mercury in humic-rich natural waters by inductively coupled plasma mass spectrometry

An analytical method was developed for the direct determination of total mercury in natural waters at low ng L^-1 level by inductively coupled plasma mass spectrometry (ICP-MS). The presented method overcomes previously observed problems relating to poor spike recoveries by adding 0.12% thiourea in addition to 3% HCl to all samples and standards. The sample preparation is fast and easy to perform by the developed method since it requires only the addition of HCl and thiourea to the water samples. A very low instrument detection limit (0.4 ng L^-1) was obtained without time-consuming preconcentration procedures. The accuracy and precision of the developed method were found excellent by the analysis of a certified groundwater reference material (ERM-CA615). The determined Hg concentration of 38.6 ± 0.5 ng L^-1 was within the 95% confidence interval of the certified concentration of 37 ± 4 ng L^-1. The analysis of natural water samples showed that total mercury levels ranged from concentrations lower than the method detection limit (2.0 ng L^-1) to 10.9 ng L^-1. Excellent recoveries of 96-108% for inorganic mercury (iHg) and 102-110% for methylmercury (MeHg) were obtained for spiked humic-rich natural water samples. To our knowledge, the developed method is the first ICP-MS method for the analysis of humic-rich natural water samples at ng L^-1 concentrations without the need for hyphenated techniques or preconcentration procedures.

Thiol-functionalized silica microspheres for online preconcentration and determination of mercury species in seawater by high performance liquid chromatography and inductively coupled plasma mass spectrometry

Thiol-functionalized silica microspheres were synthesized from aminosilica for online mercury preconcentration, followed by high performance liquid chromatographic separation and inductively coupled plasma mass spectrometry detection.

Aeration prevents methyl mercury production in dental wastewater

Although research has demonstrated that Hg is methylated in the reducing conditions of the dental clinic wastewater collection system, studies are inconclusive as to whether further methylation occurs in the aeration basin of activated sludge wastewater treatment plant (WWTP) which typically treats this waste. Given the high levels of methyl Hg reported in dental wastewater (DWW), it is important to determine whether additional methylation occurs once it enters the WWTP. To achieve this objective, we incubated DWW under conditions designed to mimic the oxidized conditions of the activated sludge aeration basin in a WWTP. Duplicate bioreactors were charged with raw DWW collected from a 12-chair dental clinic and incubated both with and without aeration. Aeration was continued for 15 days, consistent with the typical mean cell residence time (MCRT) necessary for both heterotrophic carbon oxidation (typically 5-6 days) and nitrification (typically 12-15 days), thus ensuring that incubation time exceeded those for most conceivable MCRTs used in the activated sludge process. Results demonstrate a rapid increase in pH concomitant with an increase in dissolved oxygen (DO) to near saturation in the aerated reactor. The non-aerated reactor remained low or at zero DO due to low surface reaeration coupled with the high levels of organic matter. The rate of mercury methylation increased in the unaearated reactors rapidly upon incubation, reaching highest levels when DO was at the lowest levels during the experiment. In great contrast, methyl mercury levels were much lower and net mercury methylation does not appear to occur at any significant rate under aeration. These results imply that although some mercury methylation may occur in the sewer collection system (or anaerobic digesters), net methylation is unlikely to occur in the aeration basin in activated sludge WWTPs, and thus methyl Hg influent levels from DWW represent an upper bound on effluent levels.

Automated speciation of mercury in the hair of breastfed infants exposed to ethylmercury from thimerosal-containing vaccines

A simplified thiourea-based chromatography method, originally developed for methyl and inorganic mercury, was adapted to separate methylmercury (MeHg), ethylmercury (EtHg), and inorganic mercury (Hg(II)) in infants' hair. Samples were weighed and leached with an acidic thiourea solution. Leachates were concentrated on a polymeric resin prior to analysis by Hg-thiourea liquid chromatography/cold vapor atomic fluorescence spectrometry. All but one sample showed small amounts of EtHg, and four of the six analyzed samples had proportionally higher Hg(II) as a percent of total Hg. Breastfed infants from riverine Amazonian communities are exposed to mercury in breast milk (from high levels of maternal sources that include both fish consumption and dental amalgam) and to EtHg in vaccines (from thimerosal). The method proved sensitive enough to detect and quantify acute EtHg exposure after shots of thimerosal-containing vaccines. Based on work with MeHg and Hg(II), estimated detection limits for this method are 0.050, 0.10, and 0.10 ng g⁻¹ for MeHg, Hg(II), and EtHg, respectively, for a 20-mg sample. Specific limits depend on the amount of sample extracted and the amount of extract injected.

Methods for the determination and speciation of mercury in natural waters--a review

This review summarises current knowledge on Hg species and their distribution in the hydrosphere and gives typical concentration ranges in open ocean, coastal and estuarine waters, as well as in rivers, lakes, rain and ground waters. The importance of reliable methods for the determination of Hg species in natural waters and the analytical challenges associated with them are discussed. Approaches for sample collection and storage, pre-concentration, separation, and detection are critically compared. The review covers well established methods for total mercury determination and identifies new approaches that offer advantages such as ease of use and reduced risk of contamination. Pre-concentration and separation techniques for Hg speciation are divided into chromatographic and non-chromatographic methods. Derivatisation methods and the coupling of pre-concentration and/or separation methods to suitable detection techniques are also discussed. Techniques for sample pre-treatment, pre-concentration, separation, and quantification of Hg species, together with examples of total Hg determination and Hg speciation analysis in different natural (non-spiked) waters are summarised in tables, with a focus on applications from the last decade.

Automated simultaneous analysis of monomethyl and mercuric Hg in biotic samples by Hg-thiourea complex liquid chromatography following acidic thiourea leaching

A simple leaching procedure has been validated for quantitative isolation of both monomethyl (CH3Hg+) and inorganic (Hg(II)) mercuryfrom fresh or dried biotic tissue for simultaneous analysis via separation and quantification with Hg-thiourea complex liquid chromatography cold vapor atomic fluorescence spectrometry (HgTU/LC-CVAFS). The leaching solution comprises thiourea, hydrochloric acid, and glacial acetic acid and works by protonating thiol binding sites and forming water-soluble cationic CH3HgS=C(NH2)2+ and Hg[S=C(NH2)2]2(2+) complexes, which are easily separated from the solid matrix. The isolated complexes are preconcentrated online by either thiol resin trapping or a new iodide-complex polydivinylbenzene resin trapping (I-PDVB). The I-PDVB trapping involves only one reagent addition, requires no pH adjustments, and is quantitative over a large range of volumes and flow rates. The chromatography system can use either ion chromatography or a new ion-pairing reversed phase separation coupled to cold vapor generation and atomic fluorescence detection. The system allows quantitative sample introduction and yields absolute detection limits of 0.4 pg and 0.7 pg, for CH3Hg+ and Hg(II) respectively, enabling relative detection limits as low as 4 and 7 pg g(-1) with 100 mg samples, and yields % CV routinely less than 5% with well homogenized samples. Accuracy for both forms of mercury has been validated with multiple biotic reference materials and by comparison of the sum of CH3Hg+ and Hg(II) with total Hg on a variety of different biotic sample types (n = 49). The system can be calibrated with either aqueous standards or leached reference materials.

Characterization of methyl mercury in dental wastewater and correlation with sulfate-reducing bacterial DNA

Dental wastewater (DWW) was collected over two months from a 12-chair clinic and a single-chair office to identify conditions that may affect Hg methylation. DWW was settled for 24 h and samples were collected from the top and bottom of the supernatant to simulate a range of particles that may escape in-line traps. Total Hg spanned 5 orders of magnitude (0.02-5000 microM), following a log-normal distribution with p10, p50, and p90 concentration values of 0.24, 31 and 4000 microM, respectively; typically well in excess of free aqueous Hg solubility. Methyl Hg was present in high levels (2-270 nM), also following a log-normal distribution with p10, p50, and p90 concentration values of 2.8, 17, and 100 nM, respectively. There were no statistically significant differences (90% CI) in p50 methyl Hg or total Hg between the clinic and office. Methyl Hg was predicted from total Hg data by (+/- 95% CI): Log (Me-Hg) = 0.33 (+/- 0.06) x Log (T-Hg) - 2.27 (+/- 0.13). Total methyl Hg from DWW to U.S. wastewater collection systems is estimated to be 2-5 kg yr(-1). Equilibrium speciation modeling predicted that DWW Hg was primarily in sulfide-Hg complexes, except at high total Hg levels where organo-Hg complexes become significant. DNA extracts amplified by quantitative polymerase chain reaction with primers for total eubacteria and sulfate-reducing bacteria (SRB) indicated that the total eubacterial DNA was composed primarily of SRB, and highly significant correlations were found between methyl Hg and both amplified Desulfobacteraceae (p < 0.0001) and Desulfovibrionacaea DNA (p < 0.00001). Both are known Hg methylators. In marked contrast, there was no significant correlation between methyl Hg and amplified Desulfobulbus DNA, a genus generally not known to methylate Hg at high rates. These results strongly suggest that SRB are implicated in DWW Hg methylation.