Vol. 2 (2013), S0019
Mass SPectrometrY DOI: 10.5702/massspectrometry.S0019
Chiral Chemicals as Tracers of Atmospheric Sources and Fate Processes in a World of Changing Climate Terry F. Bidleman,*,1,2 Liisa M. Jantunen,2 Perihan Binnur Kurt-Karakus,3 Fiona Wong,4 Hayley Hung,5 Jianmin Ma,5 Gary Stern,6,7 and Bruno Rosenberg6 1 Chemistry Department, Umeå University, Umeå, SE-901 87, Sweden Centre for Atmospheric Research Experiments, Environment Canada, 6248 Eighth Line, Egbert, ON, L0L 1N0, Canada 3 Faculty of Natural Sciences, Architecture and Engineering, Dept. of Env. Eng., Bursa Technical University, Gaziakdemir Mah. Mudanya Cad. No: 4/10 Osmangazi/Bursa, Turkey 4 Department of Applied Environmental Science (ITM), Stockholm University, Stockholm, SE-106 91, Sweden 5 Science & Technology Branch, Environment Canada, 4905 Dufferin St., Toronto, ON, M3H 5T4, Canada 6 Freshwater Institute, Department of Fisheries & Oceans, 501 University Crescent, Winnipeg, MB R3T 2N6, Canada 7 Centre for Earth Observation Science, University of Manitoba, 474 Wallace Building, 125 Dysard Road, Winnipeg, MB, R3T 2N2, Canada 2
Elimination of persistent organic pollutants (POPs) under national and international regulations reduces “primary” emissions, but “secondary” emissions continue from residues deposited in soil, water, ice and vegetation during former years of usage. In a future, secondary source controlled world, POPs will follow the carbon cycle and biogeochemical processes will determine their transport, accumulation and fate. Climate change is likely to aﬀect mobilisation of POPs through e.g., increased temperature, altered precipitation and wind patterns, flooding, loss of ice cover in polar regions, melting glaciers, and changes in soil and water microbiology which aﬀect degradation and transformation. Chiral compounds oﬀer advantages for following transport and fate pathways because of their ability to distinguish racemic (newly released or protected from microbial attack) and nonracemic (microbially degraded) sources. This paper discusses the rationale for this approach and suggests applications where chiral POPs could aid investigation of climate-mediated exchange and degradation processes. Multiyear measurements of two chiral POPs, trans-chlordane and α-HCH, at a Canadian Arctic air monitoring station show enantiomer compositions which cycle seasonally, suggesting varying source contributions which may be under climatic control. Largescale shifts in the enantioselective metabolism of chiral POPs in soil and water might influence the enantiomer composition of atmospheric residues, and it would be advantageous to include enantiospecific analysis in POPs monitoring programs. Keywords:
persistent organic pollutants, climate change, chiral, atmospheric transport, air-surface exchange (Received October 30, 2012; Accepted January 12, 2013)
INTRODUCTION Over the last four decades persistent organic pollutants (POPs) have come under increasingly stringent control through national and regional regulations.1,2) In 2001, twelve POPs were targeted for worldwide elimination or control under the Stockholm Convention and another ten substances were added since 2009.1) These eﬀorts have reduced “primary” emissions of POPs into the environment and levels have declined in the atmosphere3,4) and arctic biota.5) While encouraging, monitoring data for organochlorine pesticides in arctic air suggest that downward trends have slowed and in some cases reversed since ∼2000, 3,4,6) buﬀered by “secondary” emissions from residues deposited in soil, water, ice and vegetation during former years of usage.6–12) Secondary sources are expected to dominate in the future, when POPs transport and accumulation will be controlled by air-surface exchange and the biogeochemical cycle of carbon.7,13) Several publications in recent years discuss predicted impacts of climate change on sources, transport and fate of POPs.2,13–17) POPs are expected to experience increased mobility from primary and secondary sources due to higher temperatures, * Correspondence to: Terry F. Bidleman, Chemistry Department, Umeå University, Umeå, SE-901 87, Sweden, e-mail: [email protected]
© 2013 The Mass Spectrometry Society of Japan
loss of ice cover in polar regions, melting glaciers and ice caps, and increased frequency of extreme events such as flooding. Shifts in wind patterns, ocean currents, and precipitation amount and distribution influence POPs transport pathways. Fate processes are impacted through changes in temperature, UV radiation, albedo, and soil and water microbial communities which aﬀect degradation rates. Changes in these physical and chemical factors will be felt throughout the biological realm, resulting in altered bioaccumulation and biomagnification of POPs.13,15,18–20) One consequence is to confound interpretation of environmental monitoring records, which would otherwise be expected to show declines following POPs control measures.15,16) Suggestions have been made to include the influence of climate change on the environmental behaviour of chemicals in regulatory decision-making.2) This article and our recent review21) present the rationale for employing chiral POPs in the investigation of climatemediated air-surface exchange and degradation processes. Enantiomers of chiral POPs have identical vapour pressures, water solubilities and partition coeﬃcients among air, water and octanol. Transport (advection, deposition, volatilisation, diﬀusion) and reactions (photolysis, hydrolysis, OH radical attack) will not change enantiomer proportions provided they take place in achiral environments. However, enzymes are chiral and enantioselective metabolism is the Page 1 of 7 (page number not for citation purpose)
CHIraL CHemIcaLs as AtmosPHerIc Tracers oF CLImate CHange
Vol. 2 (2013), S0019
“rule rather than the exception.”22) A list of chiral POPs in the Stockholm Convention and some of their chiral metabolites is given in Table 1. Other chemical classes not in the Stockholm Convention which have chiral members are currently used pesticides, brominated flame retardants (other than brominated diphenyl ethers), polycyclic musks and pharmaceuticals. At least one industrial organophosphorus compound is chiral, tris(2-chloro-1-methylethyl) phosphate (TCPP).23) Most chiral compounds are produced as racemates (equal proportion of enantiomers), and nonracemic residues in the environment indicate enantioselective microbial degradation in soil and water or processes in higher organisms (e.g., absorption, translocation, metabolism, excretion, preferential membrane transport). Separation of individual enantiomers by chromatography on chiral stationary phases, with detection by mass spectrometry, provides the ability to distinguish emission of chemicals from two source types: racemic (newly released or not subjected to microbial attack) and nonracemic (enantioselectively degraded by microbial action in soil and water).
MATERIALS AND METHODS Most information in this paper is taken from our review21) and other published papers. Our analytical methods for the chiral organochlorine pesticides (OCPs) mentioned here are based on capillary gas chromatography using columns with chiral stationary phases and detection by low-resolution mass spectrometry in the electron capture negative ion mode (GC-ECNI-LRMS) with selected monitoring of two ions for each compound. 24–26) Columns typically employed are BGB-172 (20% tert-butyldimethylsilylβ-cyclodextrin in OV-1701, 15 m×0.25 mm i.d., 0.25 µm film, BGB Analytik AG, Switzerland), Betadex-120 (20% permethylated β-cyclodextrin in SPB-25, 30 m×0.25 mm i.d., 0.25 µm film, Supelco, U.S.A.) and Rtx β-DEXcst (proprietary phase, 30 m×0.25 mm i.d., 0.25 µm film thickness, Restek, U.S.A.), BGB-172 is the primary column for enantiomer separations of α-hexachlorocyclohexane (α-HCH), Table 1.
cis- and trans-chlordane (CC, TC), heptachlor exo-epoxide (HEPX), oxychlordane (OXY) and o,p′-DDT. Due to diﬀerences in enantiomer elution order, the Betadex-120 column is used for confirmatory analysis of the chlordanes24–26) and either Rtx β-DEXcst or Betadex-120 for α-HCH.24,26) Monitored ions are given in these references. Analytical methods for PCB atropisomers by other research groups27–32) involve chromatography on chiral-phase columns with detection by electron impact MS. The most popular column is ChirasilDex (10% permethylated 2,3,6-tri-O-methyl β-cyclodextrin as chiral selector in polysiloxane, 25 or 30 m×0.25 mm i.d., 0.25 µm film, Chrompack or Varian), and in one case29) Cyclosil-B (30% heptakis (2,3-di-O-methyl-6-Otert-butyldimethylsilyl)-β-cyclodextrin in 14% cyanopropylphenyl/86% methylpolysiloxane, 30 m×0.25 mm i.d., 0.5 µm film, Agilent, U.S.A.) was also used. Two-dimensional GC×GC (heartcut)27,28,30) and MS/MS methods have also been used.27–30) Enantiomer proportions are expressed as enantiomer fraction, EF=(+)/[(+)+(−)] (optical signs), or E1/ (E1+E2) (chromatographic elution order if optical signs are not known).
RESULTS AND DISCUSSION Enantioselective degradation in soil and water
Chiral POPs undergo enantioselective degradation in soil, resulting in accumulation of nonracemic residues. Frequencies of enantiomer depletions for organochlorine pesticides (OCPs) in agricultural and background soils worldwide are summarised in Fig. 1.21) Dominant depletions are (−) α-HCH), (−)CC and (+)TC, although opposite preferences and racemic residues are also common. Residues of o,p′DDT are almost equally divided among (+) depletion, (−) depletion or racemic. Strong preference for enrichment of the (+) enantiomer has been reported for metabolites HEPX (97%) and OXY (81%), which may reflect their preferential formation from parent compounds rather than degradation of the (−) enantiomer. 21,26) Average EFs and pooled standard
Chiral POPs in the Stockholm Convention and some of their chiral metabolites. Typea
Atropisomeric PCBs Atropisomeric PCB methyl sulfones and OH-PCBs Perfluorooctane sulfonate (PFOS) and precursors α-Hexachlorocyclohexane (α-HCH) β-Pentachlorocyclohexene γ-Pentachlorocyclohexene Chlordane Oxychlordane Heptachlor Heptachlor exo-epoxide Toxaphene o,p′-DDT o,p′-DDD a)
I M I P, Bb Mc Md Pe M P M Pe P, Bf M
I=industrial chemical, P=pesticide, B=unintentional by-product M=metabolite. b) By-product from production of lindane (γ-HCH). c) Chiral metabolite of α-HCH. d) Chiral metabolite of achiral γ-HCH. e) Complex mixture, some components are chiral. f) By-product from production of the pesticide dicofol. © 2013 The Mass Spectrometry Society of Japan
Percent of soils showing depletion of the (+) enantiomer (blue, EF 0.5) and containing racemic residues (green, EF=0.5) for α-HCH, cis-chlordane (CC), trans-chlordane (TC) and o,p′-DDT (number of soils in parentheses), based on reports from the 1990s to the present where such information is given or can be deduced. Data sources are given in ref. 21. Figure reproduced with modifications from ref. 21. Page 2 of 7
CHIraL CHemIcaLs as AtmosPHerIc Tracers oF CLImate CHange
deviations from merging individual data sets of research groups21) are: α-HCH 0.530±0.097, CC 0.531±0.073, TC 0.480±0.067, HEPX 0.668±0.032, OXY 0.554±0.033, and o,p′DDT 0.511±0.064. Enantioselective degradation in soils has been associated with higher humus and organic nitrogen content, clay vs. sand, and microbial biomass/activity.28) Our review of the literature21) found that soil organic matter and pH were significant factors in some studies, but not in others. Soil pH aﬀects carbon and nutrient availability, solubility of metals, and microbial and fungal communities.33) Climate change may impact soil microbial diversity and respiration by altering CO2, soil temperature, precipitation patterns, soil moisture, vegetation communities and productivity, and the rate of organic matter decomposition.34,35) The relative abundance of bacteria and fungi in soil was changed by manipulating CO2, temperature and precipitation,36) pH33) and soil frost.37) Eﬀects of such changes on the diagenesis of chiral POPs are poorly known, but evidence suggests that enantioselectivity will be aﬀected. Changes in the enantiomer degradation preference of the organophosphate pesticide cruformate and the phenoxy herbicide methyl dichlorprop were found when these compounds were incubated in soils which had received simulated climate change manipulations (warming the soil by 5°C), nutrient amendments or changes in land use (conversion of forest to pasture). 38) Enantiomer fractions (EFs) of atropisomeric PCBs in 101 soils from Switzerland varied with land use categories which included deciduous and coniferous forest, permanent and pasture grassland, various agricultural operations, marshland and urban parks.27) Wide variations in enantioselective degradation were found, with depletions of either enantiomer. Median EFs showed the following trends: PCB-95 0.5 elsewhere; PCB-174 racemic in most cases, slightly >0.5 in coniferous forests and slightly