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Development of Fast Sampling and High Recovery Extraction Method for Stable Isotope Measurement of Gaseous Mercury

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Development of Fast Sampling and High Recovery Extraction Method for Stable Isotope Measurement of Gaseous Mercury applied sciences Article Development of Fast Sampling and High Recovery Extraction Method for Stable Isotope Measurement of Gaseous Mercury Satoshi Irei Department of Environment and Public Health, National Institute for Minamata Disease, 4058-18 Hama, Minamata, Kumamoto 867-0008, Japan; irei@nimd.go.jp; Tel.: +81-966-63-3111 (ext. 760) Received: 24 August 2020; Accepted: 22 September 2020; Published: 24 September 2020 Abstract: A method involving fast large-volume sampling and bag extraction of total gaseous mercury (TGM) using a 5 mL acid solution was developed for stable mercury isotope ratio measurements. A big gold-coated sand trap (BAuT)—a 45 (i.d.) 300 mm (length) quartz tube with 131 times more × trapping material than a conventional gold trap—was used for the collection of a large amount of TGM. The collected TGM was extracted using 5 mL inversed aqua regia in a 2 L Tedlar bag followed by isotope measurements using a cold vapor generator coupled with a multicollector inductively coupled plasma mass spectrometer. Sampling tests demonstrated that the collection efficiency of the 1 BAuT was 99.9% or higher during the 1–24 h sampling period under the flow rate of 20–100 L min− . Recovery tests of 24 h bag extraction using 100 ng NIST SRM 8610 exhibited nearly 100% recovery yields. The five measured stable mercury isotope ratios agreed with reference values within 2σ intervals. The overall methodology tested during the pilot field and laboratory studies demonstrated its successful application in analysis, promising highly precise stable mercury isotopic data with a time resolution of less than 24 h. Keywords: total gaseous mercury (TGM); stable mercury isotopes; multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS); sampling technique; acid extraction for gaseous elemental mercury; Minamata Convention on Mercury 1. Introduction Mercury is a notorious metal pollutant that can trigger serious adverse health effects [1]. It has a unique nature that it evaporates under room temperature and pressure, resulting in global spread through the atmosphere and ubiquity in the natural environment. The international regulation on the use and trade of manmade mercury, the Minamata Convention on Mercury, was enforced by the United Nations to lower the level of global mercury pollution; however, its effectiveness is questionable [2] because of the possibly significant natural sources such as volcanoes [3], the ocean surface [4], forests [5], and permafrost [6]. Recent stable mercury isotope analysis at the natural abundance has the potential to untangle the complex source apportionment that concentration measurements alone cannot solve. There are seven stable isotopes of mercury, namely 196Hg, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg, and 204Hg with typical abundances of 0.16%, 10.0%, 16.9%, 23.1%, 13.2%, 29.7%, and 6.8%, respectively [7]. These isotopic compositions can be simultaneously measured by injecting a solution containing Hg2+ and a tin chloride solution into a cold vapor generator (CV), followed by a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS). To date, several methods to determine the stable isotopic compositions of atmospheric total gaseous mercury (TGM), which is a combination of gaseous elemental mercury (GEM) and gaseous oxidized mercury (GOM), have been reported [5,8–12]; however, these methods require long sampling durations, from a few days to weeks, thus making Appl. Sci. 2020, 10, 6691; doi:10.3390/app10196691 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x 2 of 11 Appl. Sci. 2020, 10, 6691 2 of 11 [5,8–12]; however, these methods require long sampling durations, from a few days to weeks, thus making source apportionment difficult. Additionally, the efficient conversion of 100 ng or more of source apportionment difficult. Additionally, the efficient conversion of 100 ng or more of GEM into GEM into Hg2+ in a small amount of solution is also technically challenging due to the slow oxidation Hg2+ in a small amount of solution is also technically challenging due to the slow oxidation reaction. reaction. A dynamic oxidation method (i.e., slowly bubbling GEM in an acid solution) has been used A dynamic oxidation method (i.e., slowly bubbling GEM in an acid solution) has been used for this for this conversion [5,8,13–15], but the reproduction of this method is difficult and strongly depends conversion [5,8,13–15], but the reproduction of this method is difficult and strongly depends on the on the technical skills and knowledge of analysts. Herein, we report novel and simple offline technical skills and knowledge of analysts. Herein, we report novel and simple offline techniques of techniques of fast large-volume sampling and high recovery extraction for stable mercury isotope fast large-volume sampling and high recovery extraction for stable mercury isotope measurements. measurements. 2. Materials and Methods 2. Materials and Methods 2.1. Sampling and Preconcentration of TGM 2.1. Sampling and Preconcentration of TGM TGM was collected using a big gold-coated sand trap (BAuT) (Figure1). A BAuT is a custom-made 50 (o.d.)TGM 45was (i.d.) collected300 using mm (length) a large gold-coated quartz tube withsand atrap 45 (o.d.)(BAuT) 5(Figure mm (thickness) 1). A BAuT fritted is a custom- quartz × × × platemade (COSMOS 50 (o.d.) × VID, 45 (i.d.) Fukuoka, × 300 Japan).mm (length) Approximately quartz tube 10.5 with g gold-coated a 45 (o.d.) sand,× 5 mm which (thickness) traps TGM fritted at roomquartz temperature plate (COSMOS and releases VID, Fukuoka, TGM as GEM Japan). at high Appr temperaturesoximately 10.5 and g is gold-coated reused more sand, than 50,000which timestraps (NipponTGM at room Instruments temperature Corporation, and releases Osaka, TGM Japan) as GEM was layered at high ontemperatures the fritted plate, and is and reused approximately more than 0.150,000 g quartz times wool(Nippon was Instruments then stuffed Corporation, on the top of Osaka, the sand. Japan) The was BAuT layered was on used the verticallyfritted plate, during and samplingapproximately because 0.1 the g trappingquartz wool material was settled,then stuffe but wasd on not the secured top of ontothe thesand. fritted The plate.BAuT Compared was used tovertically a commercially during sampling available because conventional the trapping gold-coated mate sandrial settled, trap (a but 4 (i.d.) was not160 secured mm quartz onto tubethe fritted with × 0.08plate. g gold-coatedCompared to silica a commercially sand, capable available of capturing conven 70tionalµg of gold-coated TGM; Nippon sand Instruments trap (a 4 (i.d.) Corporation), × 160 mm thequartz mouth tube of with the BAuT0.08 g is gold-coated approximately silica 11 timessand, wider,capable and of thecapturing amount 70 of μ trappingg of TGM; material Nippon is approximatelyInstruments Corporation), 131 times greater, the mouth allowing of the a BAuT larger is air approximately flow with the 11 same times collection wider, and effi theciency amount and capturingof trapping capacity material of 9.2is approxim mg of TGM.ately To 131 meet times the samegreater, linear allowing velocities a larger as those air flow of the with conventional the same collection efficiency and capturing capacity of 9.2 mg of1 TGM. To meet the same linear velocities as trap under the typical flow rates of 0.5 and 1.0 L min− , the sampling flow rates used for the BAuT those of the conventional trap1 under the typical flow rates of 0.5 and 1.0 L min−1, the sampling flow were 63.3 and 126.6 L min− , respectively. Techniques reported in the literature use sampling flow rates used for the 1BAuT were 63.3 and 126.6 L min−1, respectively. Techniques reported in the rates of 2–20 L min− with a sampling duration of 12 h–20 days (using a carbon trap impregnated with literature use sampling flow rates of 2–201 L min−1 with a sampling1 duration of 12 h–20 days (using a iodine or chlorine [10,16]), 14.4 L min− with 2–3 days (1.8 L min− 8 conventional gold traps [5,8]), carbon trap impregnated1 with iodine1 or chlorine [10,16]), 14.4 L min× −1 with 2–3 days (1.8 L min−1 × 8 and 16 L min− with 3 days (1 L min− 16 conventional gold traps [9]). Assuming identical collection × −1 −1 econventionalfficiencies, the gold BAuT traps sampling [5,8]), and is 16 3–9-fold L min fasterwith 3 than days that (1 L ofmin the × other 16 conventional methods. Angold additional traps [9]). advantageAssuming ofidentical the BAuT collection is the ease effici ofencies, preconditioning, the BAuT sampling its long lifetime, is 3–9-fold and faster the use than of a that single of the trap other that makesmethods. the An postsampling additional procedureadvantage simple.of the BAuT is the ease of preconditioning, its long lifetime, and the use of a single trap that makes the postsampling procedure simple. Figure 1. A schematic of the large gold-coated sand trap (BAuT). Figure 1. A schematic of the large gold-coated sand trap (BAuT). TGMTGM waswas sampledsampled by by the the BAuT BAuT using using a a diaphragm diaphragm pump pump (N860FTE, (N860FTE, KNF, KNF, Freiburg, Freiburg, Germany Germany or DOA-P501-DB,or DOA-P501-DB, GAST GAST MFG MFG Corp., Corp., Benton Benton Harbor, Harb MI,or, USA) MI, through USA) through a flow meter a flow (RK230, meter KOFLOC, (RK230, Kyoto,KOFLOC, Japan). Kyoto, PTFE Japan). connectors PTFE for connectors the BAuT werefor the custom-made BAuT were (COSMOS custom-made VID, Fukuoka,(COSMOS Japan), VID, andFukuoka, half an Japan), inch or and 3/8 ofhalf an an inch inch (o.d.) or 3/8 PFA of tubingan inch (Tombo (o.d.) PFA 9003-PFA, tubing NICHIAS (Tombo 9003-PFA, Corporation, NICHIAS Tokyo, Japan),Corporation, PFA connectors Tokyo, Japan), (Swagelok, PFA Solon,connectors OH, USA),(Swagelok, and/or Solon, PTFE connectorsOH, USA), (Flowell and/or 30PTFE series, connectors Flowell Corporation,(Flowell 30 series, Yokohama, Flowell Japan) Corporatio were alson, Yokohama, used.
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