Reduction of mercury(II) by electrons contained in carbon dots: An environmentally friendly cold vapor generation for mercury analysis

In this work, the reduction of mercury ions (Hg²⁺) to elemental mercury (Hg⁰) was easily achieved using highly reductive carbon dots (r-CDs), which synthesized from sucrose by a simple and cost-effective method. After a careful mechanistic study, the reduction was probably accomplished with the large numbers of electrons contained in r-CDs rather than the oxidation of its functional groups. Additionally, a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay showed that the r-CDs were nontoxic to wildlife and human beings. Consequently, the r-CDs were used as an alternative to toxic reductants (SnCl₂ or NaBH₄) for the sensitive and in situ determination of mercury by cold vapor generation (CVG) coupled to a miniature point discharge optical emission spectrometer (μPD-OES). Limit of detection of 0.05 μg/L was obtained for Hg²⁺, with relative standard deviation (RSD) less than 5.4% at a concentration of 5 μg/L. The accuracy of r-CDs induced CVG-μPD-OES was validated by the determination of mercury in a certified reference material (DOLT-5, dogfish liver) and five natural water samples collected from different rivers and lakes in Chengdu City. Since r-CDs are nontoxic and prepared from abundant and inexpensive sucrose, the r-CDs induced CVG-μPD-OES retains the great potential for the inexpensive and environmentally friendly field analysis of mercury in natural water. The accuracy of the proposed method was validated by the analysis of a certified reference material and several water samples with satisfactory results.     

Green method for ultrasensitive determination of Hg in natural waters by electrothermal-atomic absorption spectrometry following sono-induced cold vapor generation and ‘in-atomizer trapping’

Sono-induced cold vapor generation (SI-CVG) has been used for the first time in combination with a graphite furnace atomizer for determination of Hg in natural waters by electrothermal-atomic absorption spectrometry after in situ trapping onto a noble metal-pretreated platform (Pd, Pt or Rh) inserted into a graphite tube. The system allows ‘in-atomizer trapping’ of Hg without the use of conventional reduction reactions based on sodium borohydride or tin chloride in acid medium for cold vapor generation. The sono-induced reaction is accomplished by applying ultrasound irradiation to the sample solution containing Hg(II) in the presence of an organic compound such as formic acid. As this organic acid is partly degraded upon ultrasound irradiation to yield CO, CO₂, H₂ and H₂O, the amount of lab wastes is minimized and a green methodology is achieved.     

Photo-induced cold vapor generation with low molecular weight alcohol, aldehyde, or carboxylic acid for atomic fluorescence spectrometric determination of mercury

With UV irradiation, Hg²⁺ in aqueous solution can be converted into Hg⁰ cold vapor by low molecular weight alcohols, aldehydes, or carboxylic acids, e.g., methanol, formaldehyde, acetaldehyde, glycol, 1,2-propanediol, glycerol, acetic acid, oxalic acid, or malonic acid. It was found that the presence of nano-TiO₂ more or less improved the efficiency of the photo-induced chemical/cold vapor generation (photo-CVG) with most of the organic reductants. The nano-TiO₂-enhanced photo-CVG systems can be coupled to various analytical atomic spectrometric techniques for the determination of ultratrace mercury. In this work, we evaluated the application of this method to the atomic fluorescence spectrometric (AFS) determination of mercury in cold vapor mode. Under the optimized experimental conditions, the instrumental limits of detection (based on three times the standard deviation of 11 measurements of a blank solution) were around 0.02–0.04 μg L⁻¹, with linear dynamic ranges up to 15 μg L⁻¹. The interference of transition metals and the mechanism of the photo-CVG are briefly discussed. Real sample analysis using the photo-CVG-AFS method revealed that it was promising for water and geological analysis of ultralow levels of mercury. Image of the photo-CVG instrumentation showing the photoreactor inside the water cooling unit     

Determination of total mercury and methylmercury in biological samples by photochemical vapor generation

Cold vapor atomic absorption spectrometry (CV-AAS) based on photochemical reduction by exposure to UV radiation is described for the determination of methylmercury and total mercury in biological samples. Two approaches were investigated: (a) tissues were digested in either formic acid or tetramethylammonium hydroxide (TMAH), and total mercury was determined following reduction of both species by exposure of the solution to UV irradiation; (b) tissues were solubilized in TMAH, diluted to a final concentration of 0.125% m/v TMAH by addition of 10% v/v acetic acid and CH₃Hg⁺ was selectively quantitated, or the initial digests were diluted to 0.125% m/v TMAH by addition of deionized water, adjusted to pH 0.3 by addition of HCl and CH₃Hg⁺ was selectively quantitated. For each case, the optimum conditions for photochemical vapor generation (photo-CVG) were investigated. The photochemical reduction efficiency was estimated to be ∼95% by comparing the response with traditional SnCl₂ chemical reduction. The method was validated by analysis of several biological Certified Reference Materials, DORM-1, DORM-2, DOLT-2 and DOLT-3, using calibration against aqueous solutions of Hg²⁺; results showed good agreement with the certified values for total and methylmercury in all cases. Limits of detection of 6 ng/g for total mercury using formic acid, 8 ng/g for total mercury and 10 ng/g for methylmercury using TMAH were obtained. The proposed methodology is sensitive, simple and inexpensive, and promotes “green” chemistry. The potential for application to other sample types and analytes is evident.     

电化学冷蒸气发生-原子荧光光谱法测定人发中的痕量汞

采用自制的电化学流通池作为汞蒸气发生器,以玻碳为阴极材料,结合原子荧光光谱法,在断续流动条件下,建立了电化学冷蒸气发生法-原子荧光光谱联用技术(ECVG-AFS)对汞的分析方法.在优化的实验条件下,汞在0～5.0μg/L范围内荧光强度与浓度呈良好的线性关系,汞的检出限为1.2 ng/L.对1μg/L Hg测定的相对标准偏差为1.8%(n=11).可用于人发标准样品中汞的测定.     

A sensitive and compact mercury analyzer by integrating dielectric barrier discharge induced cold vapor generation and optical emission spectrometry

An environmentally friendly,low power consuming,sensitive and compact mercury analyzer was developed for the determination of mercury in water samples by integrating a thin film dielectric barrier discharge induced cold vapor reactor and a dielectric barrier discharge optical emission spectrometer into a small polymethyl methacrylate plate(10.5 cm length×8.0 cm width×1.2 cm height).Mercury cold vapor was generated when standard or sample solutions with or without formic acid were introduced to the reactor to form thin film liquid and exposed to microplasma irradiation and subsequently separated from the liquid phase for transport to the microplasma and detection of its atomic emission.Limits of detection of 0.20 μg L~1 and 2.6 μg L~1 were obtained for the proposed system using or not using formic acid,respectively.Compared to the conventional microplasma optical emission spectrometry used for mercury analysis,this system not only retains the good limit of detection amenable to the determination of mercury in real samples,but also reduces power consumption,eliminates the generation of hydrogen and avoids the use of toxic or unstable reductant.Method validation was demonstrated by analysis of a certified reference material of water sample and three real water samples with good spike recoveries(88-102%).     

Determination of mercury by intermittent flow electrochemical cold vapor generation coupled to atomic fluorescence spectrometry

A novel method for determination of mercury was developed using an intermittent flow electrochemical cold vapor generation coupled to atomic fluorescence spectrometry (IF-ECVG-AFS). The mercury vapor was generated on the surface of glassy carbon cathode in the flow cell. The operating conditions for the electrochemical generation of mercury vapor were investigated in detail, and the interferences from various ions were evaluated. Under the optimized conditions, no evident memory effects of mercury were observed. The calibration curve was linear up to 5 μg L⁻¹ Hg at 0.54 A cm⁻². A detection limit of 1.2 ng L⁻¹ Hg and a relative standard deviation of 1.8% for 1 μg L⁻¹ Hg were obtained. The accuracy of method was verified by the determination of mercury in the certified reference human hair. The ECVG avoided the use of reductants, thereby greatly reducing the contamination sources. In addition, the manifold of IF-ECVG-AFS was simple and amenable to automation.     

Determination of methylmercury and inorganic mercury by coupling short-column ion chromatographic separation,on-line photocatalyst-assisted vapor generation,and inductively coupled plasma mass spectrometry

We have combined short-column ion chromatographic separation and on-line photocatalyst-assisted vapor generation (VG) techniques with inductively coupled plasma mass spectrometry to develop a simple and sensitive hyphenated method for the determination of aqueous Hg²⁺ and MeHg⁺ species. The separation of Hg²⁺ and MeHg⁺ was accomplished on a cation-exchange guard column using a glutathione (GSH)-containing eluent. To achieve optimal chromatographic separation and signal intensities, we investigated the influence of several of the operating parameters of the chromatographic and photocatalyst-assisted VG systems. Under the optimized conditions of VG process, the shortcomings of conventional SnCl₂-based VG techniques for the vaporization of MeHg⁺ was overcome; comparing to the concentric nebulizer-ICP-MS system, the analytical sensitivity of ICP-MS toward the detection of Hg²⁺ and MeHg⁺ were also improved to 25- and 7-fold, respectively. With the use of our established HPLC–UV/nano-TiO₂–ICP-MS system, the precision for each analyte, based on three replicate injections of 2 ng/mL samples of each species, was better than 15% RSD. This hyphenated method also provided excellent detection limits—0.1 and 0.03 ng/mL for Hg²⁺ and MeHg⁺, respectively. A series of validation experiments—analysis of the NIST 2672a Standard Urine Reference Material and other urine samples—confirmed further that our proposed method could be applied satisfactorily to the determination of inorganic Hg²⁺ and MeHg⁺ species in real samples.     

Evaluation of a new electrolytic cold vapor generation system for mercury determination by AFS

In this study we firstly report a new electrolytic cold vapor generation system for mercury determination on Pt/Ti cathode in the presence of organic acid catholyte. Comparing with the traditional inorganic acid, formic acid increased the signal intensity of Hg vapor from electrolytic generation on Pt cathode and reduced the impact of cathode erosion on the stability of signal intensity. Moreover, formic acid has better interference tolerance. The introduction location for carrier gas is probably the most important factor that influences the signal intensity of Hg from electrolytic vapor generation. The effects of the electrolytic conditions and interference ions on the ECVG have been studied. Under the optimized conditions, the detection limit (3σ) of Hg (II) in aqueous solutions is 1.4 ng L⁻¹; a relative standard deviation of 2.3% for 1 μg L⁻¹ Hg was obtained. The accuracy of this method was verified by the determination of mercury in the certified reference materials. This system has been applied satisfactorily to the determination of Hg in Traditional Chinese Medicines samples.     

Determination of mercury by electrochemical cold vapor generation atomic fluorescence spectrometry using polyaniline modified graphite electrode as cathode

An electrochemical cold vapor generation system with polyaniline modified graphite electrode as cathode material was developed for Hg (II) determination by coupling with atomic fluorescence spectrometry. This electrochemical cold vapor generation system with polyaniline/graphite electrode exhibited higher sensitivity; excellent stability and lower memory effect compared with graphite electrode electrochemical cold vapor generation system. The relative standard deviation was 2.7% for eleven consecutive measurements of 2 ng mL^− 1 Hg (II) standard solution and the mercury limit of detection for the sample blank solution was 1.3 рg mL^− 1 (3σ). The accuracy of the method was evaluated through analysis of the reference materials GBW09101 (Human hair) and GBW 08517 (Laminaria Japonica Aresch) and the proposed method was successfully applied to the analysis of human hairs.     

Electrochemical generation of mercury cold vapor and its in-situ trapping in gold-covered graphite tube atomizers

The combination of more efficient flow-through electrochemical mercury cold vapor generation with its in-situ trapping in a graphite tube atomizer is described. This coupled technique has been optimized to attain the maximum sensitivity for Hg determination and to minimize the limits of detection and determination. A laboratory constructed thin-layer flow-through cell with a platinum cathode served as the cold vapor generator. Various cathode arrangements with different active surface areas were tested. Automated sampling equipment for the graphite atomizer with an untreated fused silica capillary was used for the introduction of the mercury vapor. The inner surface of the graphite tube was covered with a gold foil placed against the sampling hole.     

Evaluation and application of argon and helium microstrip plasma for the determination of mercury by the cold vapor technique and optical emission spectrometry

The suitability of a 2.45-GHz atmospheric pressure, low-power microwave microstrip plasma (MSP) operated with Ar and He for the determination of Hg by continuous-flow cold vapor (CV) generation, using SnCl₂/HCl as the reducing agent, and optical emission spectrometry (OES) using a small CCD spectrometer was studied. The areas of stability for a discharge in the Ar and in the He MSP enclosed in a cylindrical channel in a quartz wafer were investigated. The excitation temperatures as measured for discharge gas atoms (Ar I, He I), and the electron number densities at 35–40 W and 15–400 mL min⁻¹ were found to be at the order of 3,200–5,500 K and 0.8 × 10¹⁴–1.6 × 10¹⁴ cm⁻³, respectively. The relative intensity of the Hg I 253.6-nm line and the signal-to-background ratio as a function of the forward power (35–40 W) as well as of the flow rate of the working gas (15–400 mL min⁻¹) were evaluated and discussed. For the selected measurement conditions, the Ar MSP was established to have the lower detection limit for Hg (0.6 ng mL⁻¹) compared with the He MSP. The linearity range is up to 300 ng mL⁻¹ and the precision is on the order of 1–3%. With the optimized CV Ar MSP-OES method a determination of Hg in spiked domestic and natural waters at concentration levels of 20–100 μg L⁻¹ and an accuracy of 1–4% could be performed. In an NIST domestic sludge standard reference material, Hg (3.64 μg g⁻¹) could be determined with a relative standard deviation of 4% and an agreement better than 4%.     

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.     

Direct determination of mercury in white vinegar by matrix assisted photochemical vapor generation atomic fluorescence spectrometry detection

This paper proposes the use of photochemical vapor generation with acetic acid as sample introduction for the direct determination of ultra-trace mercury in white vinegars by atomic fluorescence spectrometry. Under ultraviolet irradiation, the sample matrix (acetic acid) can reduce mercury ion to atomic mercury Hg⁰, which is swept by argon gas into an atomic fluorescence spectrometer for subsequent analytical measurements. The effects of several factors such as the concentration of acetic acid, irradiation time, the flow rate of the carrier gas and matrix effects were discussed and optimized to give detection limits of 0.08 ng mL⁻¹ for mercury. Using the experimental conditions established during the optimization (3% v/v acetic acid, 30 s irradiation time and 20 W mercury lamp), the precision levels, expressed as relative standard deviation, were 4.6% (one day) and 7.8% (inter-day) for mercury (n = 9). Addition/recovery tests for evaluation of the accuracy were in the range of 92–98% for mercury. The method was also validated by analysis of vinegar samples without detectable amount of Hg spiked with aqueous standard reference materials (GBW(E) 080392 and GBW(E) 080393). The results were also compared with those obtained by acid digestion procedure and determination of mercury by ICP-MS. There was no significant difference between the results obtained by the two methods based on a t-test (at 95% confidence level).     

Investigation of ultraviolet photolysis vapor generation with in-atomizer trapping graphite furnace atomic absorption spectrometry for the determination of mercury

Generation of mercury vapor by ultraviolet irradiation of mercury solutions in low molecular weight organic acid solutions prior to measurement by Atomic Absorption Spectrometry is a cheap, simple and green method for determination of trace concentrations of mercury. In this work mercury vapor generated by ultraviolet photolysis was trapped onto a palladium coated graphite furnace significantly improving the detection limit of the method. The system was optimized and a detection limit of 0.12 µg L^− 1 (compared to 2.1 µg L^− 1 for a previously reported system in the absence of trapping) with a precision of 11% for a 10 µg L^− 1 mercury standard (RSD, N = 5).     

Speciation analysis of mercury in water samples by cold vapor atomic absorption spectrometry after preconcentration with dithizone immobilized on microcrystalline naphthalene

Trace amounts of inorganic mercury (Hg²⁺) and methylmercury cations (MeHg²⁺) were adsorbed quantitatively from acidic aqueous solution onto a column packed with immobilized dithizone on microcrystalline naphthalene. The trapped mercury was eluted with 10 ml of 7 mol L^–1 hydrochloric acid solution. The Hg²⁺ was then directly reduced with tin (II) chloride, and volatilized mercury was determined by cold vapor atomic absorption spectrometry (CVAAS). Total mercury (Hgt) was determined after decomposition of MeHg⁺ into Hg²⁺. Hg²⁺ and MeHg⁺ cations were completely recovered from the water with a preconcentration factor of 200. The relative standard deviation obtained for eight replicate determinations at a concentration of 0.3 g L^–1 was 1.8%. The procedure was applied to analysis of water samples, and the accuracy was assessed via recovery experiment.     

Solution cathode glow discharge induced vapor generation of mercury and its application to mercury speciation by high performance liquid chromatography-atomic fluorescence spectrometry

A novel solution cathode glow discharge (SCGD) induced vapor generation was developed as interface to on-line couple high-performance liquid chromatography (HPLC) with atomic fluorescence spectrometry (AFS) for the speciation of inorganic mercury (Hg(2+)), methyl-mercury (MeHg) and ethyl-mercury (EtHg). The decomposition of organic mercury species and the reduction of Hg(2+) could be completed in one step with this proposed SCGD induced vapor generation system. The vapor generation is extremely rapid and therefore is easy to couple with flow injection (FI) and HPLC. Compared with the conventional HPLC-CV-AFS hyphenated systems, the proposed HPLC-SCGD-AFS system is very simple in operation and eliminates auxiliary redox reagents. Parameters influencing mercury determination were optimized, such as concentration of formic acid, discharge current and argon flow rate. The method detection limits for HPLC-SCGD-AFS system were 0.67 μg L(-1) for Hg(2+), 0.55 μg L(-1) for MeHg and 1.19 μg L(-1) for EtHg, respectively. The developed method was validated by determination of certified reference material (GBW 10029, tuna fish) and was further applied for the determination of mercury in biological samples.     

Organic,inorganic and total mercury determination in fish by chemical vapor generation with collection on a gold gauze and electrothermal atomic absorption spectrometry

A method for organic, inorganic and total mercury determination in fish tissue has been developed using chemical vapor generation and collection of mercury vapor on a gold gauze inside a graphite tube and further atomization by electrothermal atomic absorption spectrometry. After drying and cryogenic grinding, potassium bromide and hydrochloric acid solution (1 mol L^− 1 KBr in 6 mol L^− 1 HCl) was added to the samples. After centrifugation, total mercury was determined in the supernatant. Organomercury compounds were selectively extracted from KBr solution using chloroform and the resultant solution was back extracted with 1% m/v L-cysteine. This solution was used for organic Hg determination. Inorganic Hg remaining in KBr solution was directly determined by chemical vapor generation electrothermal atomic absorption spectrometry. Mercury vapor generation from extracts was performed using 1 mol L^− 1 HCl and 2.5% m/v NaBH₄ solutions and a batch chemical vapor generation system. Mercury vapor was collected on the gold gauze heated resistively at 80 °C and the atomization temperature was set at 650 °C. The selectivity of extraction was evaluated using liquid chromatography coupled to chemical vapor generation and determination by inductively coupled plasma mass spectrometry. The proposed method was applied for mercury analysis in shark, croaker and tuna fish tissues. Certified reference materials were used to check accuracy and the agreement was better than 95%. The characteristic mass was 60 pg and method limits of detection were 5, 1 and 1 ng g^− 1 for organic, inorganic and total mercury, respectively. With the proposed method it was possible to analyze up to 2, 2 and 6 samples per hour for organic, inorganic and total Hg determination, respectively.     

Continuous flow electrolytic cold vapor generation atomic fluorescence spectrometric determination of Hg in water samples

The determination of trace concentrations of Hg in water samples by the use of electrolytic cold vapor generation (ECVG) system and AFS was studied. Several buffer solutions were used and the detection limits with these systems were found to be by a factor of 1–2 lower than in the conventional electrolytic cold vapor generation system. Comparing with the traditional inorganic acid, phosphate buffer solution (PBS) increased the signal intensity of Hg vapor from electrolytic generation on Pt cathode and reduced the impact of cathode erosion on the stability of signal intensity. Moreover, buffer solution has better interference tolerance. The effects of the electrolytic conditions and interference ions on the ECVG have been studied. Under optimized conditions and with PBS as catholyte the detection limit for Hg was found to be 0.27 ng L⁻¹. The relative standard deviation was 2.8% for 11 consecutive measurements of 1 μg L⁻¹ Hg. This method has been applied in the determination of inorganic Hg in Yangtze River water.     

A new vapor generation system for mercury species based on the UV irradiation of mercaptoethanol used in the determination of total and methyl mercury in environmental and biological samples by atomic fluorescence spectrometry

A new vapor generation system for mercury (Hg) species based on the irradiation of mercaptoethanol (ME) with UV was developed to provide an effective sample introduction unit for atomic fluorescence spectrometry (AFS). Preliminary investigations of the mechanism of this novel vapor generation system were based on GC–MS and FT–IR studies. Under optimum conditions, the limits of determination for inorganic divalence mercury and methyl mercury were 60 and 50 pg mL⁻¹, respectively. Certified reference materials (BCR 463 tuna fish and BCR 580 estuarine sediment) were used to validate this new method, and the results agreed well with certified values. This new system provides an attractive alternative method of chemical vapor generation (CVG) of mercury species compared to other developed CVG systems (for example, the traditional KBH₄/NaOH–acid system). To our knowledge, this is the first systematic report on UV/ME-based Hg species vapor generation and the determination of total and methyl Hg in environmental and biological samples using UV/ME–AFS. Figure A new vapor generation system for mercury species using mercaptoethanol under UV irradiation was developed as an effective sample introduction unit for atomic fluorescence spectrometry