Growth of Carbonaceous Nanoparticles on Steel Fiber from Candle Flame for the Long-Term Preservation of Ultratrace Mercury by Solid-Phase Microextraction

Advances in atmospheric pressure plasma-based optical emission spectrometry for the analysis of heavy metals

Over the past decade, miniaturized optical emission spectrometry (OES) systems utilizing atmospheric pressure plasmas (APPs) as radiation sources have exhibited impressive capabilities in trace heavy metal analysis. As the core of the analytical system, APPs sources possess unique properties such as compact size, light weight, low energy requirement, ease of fabrication, and relatively low manufacturing cost. This critical review focuses on recent progress of APP-based OES systems employed for the determination of heavy metals. Influences of technical details including the sample introduction manner, the sampling volume, the sample flow rate, the pH of the solutions on the plasma stability and the intensity of analytical signals are comprehensively discussed. Furthermore, the review emphasizes the analytical challenges faced by these techniques and highlights the opportunities for further development in the field of heavy metal detection.

Solid Phase Photothermo-Induced Chemical Vapor Generation: A New Desorption Method for Mercury Analysis by High-Throughput 20-Fiber Direct Immersion Solid Phase Microextraction

A simple solid phase photothermo-induced chemical vapor generation (SP-PT-CVG) is described and used as an environmentally friendly desorption method for the sensitive determination of mercury in water by direct immersion solid phase microextraction (DI-SPME) atomic fluorescence spectrometry (AFS). A DI-SPME array equipped with 20 nano-TiO2-coated tungsten fibers was employed to simultaneously preconcentrate mercury from 20 samples, enabling an extraction throughput of 40 samples per hour. Subsequently, the fibers were drawn from the sample solutions and inserted into an inner tube sealed in a specially designed UV lamp in turn for SP-PT-CVG to generate Hg0, which was swept to an AFS detector for its detection. It is worth noting that the tube served as both a vapor generator and a desorption chamber. This proof-of-concept study confirms the feasibility of solid phase CVG. Compared to conventional CVG carried out in the liquid phase, solid phase CVG not only retains the advantages of conventional CVG but also alleviates the matrix interference on vapor generation and preconcentrates analyte prior to vapor generation, improving analytical performance for liquid state samples. DI-SPME-SP-PT-CVG-AFS provides a limit of detection of 2.3 ng L-1 for mercury determination by AFS. In the proposed method, the combination of DI-SPME and SP-PT-CVG eliminates the tedious derivatization steps required in conventional headspace SPME, thus minimizing toxic reagent consumption and improving extraction throughput. The practicality of DI-SPME-SP-PT-CVG-AFS was evaluated by analyzing two different certified reference materials and river water samples with good spike recoveries (98-107%).

Battery-Operated and Self-Heating Solid-Phase Microextraction Device for Field Detection and Long-Term Preservation of Mercury in Soil

The application of headspace solid-phase microextraction (HS-SPME) for mercury preservation and detection still has several shortcomings, including the use of high-temperature desorption chamber, the consumption of expensive reagent (NaBEt4 or NaBPr4), and analyte loss during sample storage. Herein, a self-heating HS-SPME device using a gold-coated tungsten (Au@W) fiber was developed for the field detection of mercury in soil by miniature point discharge optical emission spectrometry (μPD-OES). Hg2+ was reduced to Hg0 with NaBH4 solution and then preconcentrated with the Au@W fiber. The adsorbed Hg0 could be rapidly desorbed by directly heating the fiber with a mini lithium battery and subsequently detected by μPD-OES. A limit of detection (LOD) of 0.008 mg kg-1 was obtained with a relative standard deviation (RSD) of 2.4%. The accuracy of the self-heating HS-SPME was evaluated by analyzing a soil certified reference material (CRM) and nine soil samples with satisfactory recoveries (86-111%). Compared to the conventional external heating method, the proposed method reduces desorption time and power consumption from 80 s and 60 W to 20 s and 2.5 W, respectively. Moreover, the self-heating device enables the μPD-OES system to remove the high-temperature desorption chamber, making it more compact and suitable for field analytical chemistry. Interestingly, the Au@W SPME fiber can be also used for the long-term preservation of mercury with a sample loss rate <5% after 30 days of storage at room temperature.

One-Pot Pretreatment Coupled to Microplasma Optical Emission Spectrometry for Field and Sensitive Determination of Inorganic Mercury and Methylmercury in Fish

Field and sensitive analysis of mercury species in seafood is helpful to assess the risk of human exposure to mercury, but the cumbersome pretreatment process is time-consuming and laborious. Herein, a simple one-pot pretreatment system is designed for extraction, separation, and enrichment of inorganic mercury (Hg(II)) and methylmercury (MeHg) in fish, and coupled to dielectric barrier discharge (DBD) microplasma optical emission spectrometry (OES). Both Hg(II) and MeHg species in fish can be effectively extracted by tetramethylammonium hydroxide under ultrasound, then separated from the fish matrix by vapor generation and photochemical vapor generation, and finally enriched on the activated carbon electrode tips. Mercury trapped on the activated carbon electrode tips can be rapidly released to produce OES under the DBD microplasma excitation for quantitative analysis. The pretreatment and analysis of a batch of 12 samples are completed within 50 min, and the extraction efficiency of total mercury is up to 90% for 100 mg of freeze-dried fish or 86% for 1 g of fresh fish. Under the optimized conditions, the detection limits are 2 μg kg^-1 for Hg(II) and 1.2 μg kg^-1 for MeHg in freeze-dried fish, and precisions are 3.2% for Hg(II) and 3.9% for MeHg. The present method is applied to the analysis of the certified reference material and real marine fishes, giving rise to spiked recoveries of 95-103%. The present system hardly leads to MeHg and Hg(II) transforming into each other during extraction, providing a simple, convenient, and low-cost analytical tool to evaluate the risk of mercury species in fish.

Recent applications and novel strategies for mercury determination in environmental samples using microextraction-based approaches: A review

The growing need to monitor Hg levels in the environment to control its emissions and evaluate the effectiveness of reduction policies is driving the scientific community to focus efforts on creating analytical methods that are simpler, lower cost, more performing, and environmentally sustainable. In this context, an important contribution is provided by microextraction techniques, which have long proven to be simple, reliable, and to ensure an environmentally responsible sample preparation. This manuscript reviews the recent progress in the determination of environmental Hg using microextraction techniques. The considered studies involve all environmental compartments (i.e., air, water, soil, and biota) and have been discussed by grouping them according to the employed technique while pointing out the main advances achieved and the most important limitations. The ultimate goal is to provide an up-to-date overview of the analytical potential of microextraction techniques that can be exploited in various investigation fields and to highlight the most important knowledge gaps that should be addressed in the coming years, such as in-situ sampling, the use of natural materials, and the value of metrological support to obtain data SI-traceable and comparable.

Gas-cycle-assisted headspace solid-phase microextraction coupled with gas chromatography for rapid analysis of organic pollutants

Herein, a new gas-cycle-assisted (GCA) headspace solid-phase microextraction (HS-SPME) device was designed to rapidly extract organic pollutants with high K_ow and boiling points, which have difficulty in volatilization from matrix to headspace. Organic pollutants, including three polycyclic aromatic hydrocarbons (PAHs), four polychlorinated biphenyls (PCBs), and five phthalate esters (PAEs), were selected to evaluate the performance of GCA HS-SPME. Compared with conventional HS-SPME, the equilibrium times of GCA HS-SPME for extraction of PAHs, PCBs, and PAEs were greatly shortened from 70-90 to 5-11 min. Moreover, the limits of detection for analysis of PAHs were achieved at pg mL^-1 level by GCA HS-SPME coupled with gas chromatography-flame ionization detection.

Methanol-Enhanced Liquid Electrode Discharge Microplasma-Induced Vapor Generation of Hg, Cd, and Zn: The Possible Mechanism and Its Application

Despite increased interest in microplasma-induced vapor generation (μPIVG) over the past several years, applications in real sample analyses remain limited due to their relatively low vapor generation efficiency and ambiguous mechanism. In this work, a novel method using methanol for significantly enhancing the liquid electrode discharge μPIVG efficiency was developed for the simultaneous and sensitive determination of Hg, Cd, and Zn by atomic fluorescence spectrometry (AFS). It is worth noting that the possible enhancement mechanism was investigated via the characterizations of volatile products by AFS, microplasma optical emission spectrometry, online gas chromatography, and gas chromatography-mass spectrometry, which involved the reductive species such as electrons, hydrogen radicals (·H), methyl radicals (·CH₃), and other intermediates in the argon plasma adding methanol. Under the optimized conditions, the limits of detection of 0.007, 0.05, and 0.5 μg L^-1 were obtained for Hg, Cd, and Zn, respectively, with relative standard deviations of 3.1, 3.7, and 5.2% for these elements, respectively. Vapor generation efficiencies of 90, 83, and 55% were achieved for Hg, Cd, and Zn, respectively, and improved 2.7-, 4.8-, and 7.9-fold, respectively, compared to those obtained in the absence of methanol. The accuracy and practicability of the proposed method were validated by the determination of Hg, Cd, and Zn in a certified reference material (CRM, Lobster hepatopancreas, TORT-3) and crayfish samples collected from three different provinces of China.

MnFe2O4 micromotors enhanced field digestion and solid phase extraction for on-site determination of arsenic in rice and water

Despite the increased interest and great progress obtained on arsenic test, it is still a challenge to accomplish the on-site determination of arsenic in rice due to the expensive instrumentation and harsh digestion process. In this work, MnFe₂O₄ micromotors were found to retain high catalytic activity to simultaneously produce large amounts of hydroxyl radicals and O₂ bubbles in the presence of H₂O₂. Interestingly, the generated bubbles autonomously propel the micromotors and prevent them from depositing, thus keeping their high catalytic activity. As a result, a MnFe₂O₄ micromotors enhanced digestion method was developed for the field digestion of rice samples within 100 min only using H₂O₂, which was further utilized to realize the on-site detection of arsenic in rice by coupling with the Gutzeit method followed headspace solid phase extraction. A quantification limit of 40 μg kg^-1 was obtained for the determination of arsenic in rice. Owing to their capabilities of the efficient and rapid adsorption of arsenic and continuous movement, a MnFe₂O₄ micromotors enhanced solid phase extraction was also established for the sensitive determination of arsenic in water with a 1 μg L^-1 of quantification limit. The accuracy of the developed method was validated via analysis of a Certified Reference Material of rice (GBW10043) and a series of rice and water samples with satisfactory results, showing promising potential in the sensitive on-site detection of arsenic in rice and water samples.

In-site and solvent-free exfoliation of porous graphene oxide from pencil lead fiber for solid-phase microextraction of cadmium ion before GF-AAS determination

Graphene oxide (GO)-functionalized pencil lead fiber was prepared for the first time by in situ oxidation and exfoliation of graphite contained in pencil lead fiber to porous graphene oxide structure via a one-step solvent-free dielectric barrier discharge (DBD) microplasma treatment. This new fiber was demonstrated as a highly efficient and low-cost solid-phase microextraction (SPME) fiber for the determination of toxic metal ions. The fiber extraction performance was evaluated by using cadmium as a model analyte in a direct immersing SPME mode. Unlike most commercially available and other lab-built fibers, the preparation of the graphene oxidized pencil lead fiber is environmentally friendly, low cost, and non-toxic without using any organic solvents. The fiber is robust due to its coating-free configuration. Furthermore, high extraction efficiency and high sensitivity for cadmium can be obtained due to the abundant oxygen-containing functional groups on the surface of the novel fiber. After extraction, the cadmium adsorbed on the fiber was desorbed to 150-μL solution. Graphite furnace atomic absorption spectrometry (GF-AAS) with low sample consumption was used to determine cadmium. The calibration curve for cadmium ions was linear in a range 0.04-0.26 μg L^-1 with a detection limit of 0.005 μg L^-1. A relative standard deviation (RSD, n = 5) of 2.1% was obtained at 0.1 μg L^-1 of cadmium. The sensitivity enhancement factor (EF) value of the proposed SPME method was 25. The SPME fiber was successfully applied to determine cadmium in tap water, river water, and pond water with spike recoveries ranging from 94 to 105%. Pipe network water samples were also analyzed to evaluate the cadmium release to drinking water due to the corrosion of tubes.

"Insert-and-Go" Activated Carbon Electrode Tip for Heavy Metal Capture and In Situ Analysis by Microplasma Optical Emission Spectrometry

The miniaturized optical emission spectrometry (OES) devices based on various microplasma excitation sources provide reliable tools for on-site analysis of heavy metal pollution, while the development of convenient and efficient sample introduction approaches is essential to improve their performances for field analysis. Herein, a small activated carbon electrode tip is employed as solid support to preconcentrate heavy metals in water and subsequently served as an inner electrode of the coaxial dielectric barrier discharge (DBD) to generate microplasma. In this case, heavy metal analytes in water are first adsorbed on the surface of the activated carbon electrode tip via a simple liquid-solid phase transformation during the sample loading process, and then, fast released to produce OES during the DBD microplasma excitation process. The corresponding OES signals are synchronously recorded by a charge-coupled device (CCD) spectrometer for quantitative analysis. This activated carbon electrode tip provides a new tool for sample introduction into the DBD microplasma and facilitates "insert-and-go" in subsequent DBD-OES analysis. With a multiplexed activated carbon electrode tip array, a batch of water samples (50 mL) can be loaded in parallel within 5 min. After drying the activated carbon electrode tips for 5 min, the DBD-OES analysis is maintained at a rate of 6 s per sample. Under the optimized conditions, the detection limits of 0.03 and 0.6 μg L^-1 are obtained for Cd and Pb, respectively. The accuracy and practicability of the present DBD-OES system have been verified by measuring several certified reference materials and real water samples. This analytical strategy not only simplifies the sample pretreatment steps but also significantly improves the sensitivity of the DBD-OES system for heavy metal detection. By virtue of the advantages of high sensitivity, fast analysis speed, simple operation, low cost, and favorable portability, the upgraded DBD-OES system provides a more powerful tool for on-site analysis of heavy metal pollution.