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Unexpected occurrence of volatile dimethylsiloxanes in Antarctic soils, vegetation, phytoplankton and krill Josep Sanchís, Ana Cabrerizo, Cristobal Galban, Damia Barcelo, Marinella Farré, and Jordi Dachs Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es503697t • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Environmental Science & Technology
WET DEPOSITION
Ice melting
Antarctic krill
Snow melting
Phytoplankton
ACS Paragon Plus Environment
Soil
Vegetation
Environmental Science & Technology
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Submitted to Environmental Science & Technology
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Unexpected occurrence of volatile dimethylsiloxanes in Antarctic soils,
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vegetation, phytoplankton and krill
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Josep Sanchís1, Ana Cabrerizo1,2, Cristóbal Galbán-Malagón1,3,
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Damià Barceló1,4, Marinella Farré1*, Jordi Dachs1
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1
Barcelona, Catalonia, Spain.
8 9
2
3
Present Address: Department of Ecology and Biodiversity, Universidad Andrés Bello, Chile
12 13
Present Address: Institute for Environment and Sustainability, European Commission, Via Enrico Fermi 2749, 21027 Ispra, Italy.
10 11
Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-26, 08034,
4
Catalan Institute for Water Research (ICRA), H2O Building, Scientific and
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Technological Park of the University of Girona, Emili Grahit 101, 17003 Girona,
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Catalonia, Spain.
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*Corresponding author: Marinella Farré.
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Address: C/Jordi Girona, 18-26, 08034 Barcelona
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Phone : 0034 934 006 100
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Fax: 0034 932 045 904
20
E-mail :
[email protected] 21 22 23 1 ACS Paragon Plus Environment
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ABSTRACT
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Volatile methyl siloxanes (VMS) are high-production synthetic compounds,
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ubiquitously found in the environment of source regions. Here, we show for the first
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time, the occurrence of VMS in soils, vegetation, phytoplankton and krill samples
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from the Antarctic Peninsula region, which questions previous claims that these
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compounds are “flyers” and do not significantly reach remote ecosystems. Cyclic
30
VMS are the predominant compounds, with concentrations ranging from the limits of
31
detection to 110 ng/g in soils. Concentrations of cyclic VMS in phytoplankton are
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negatively correlated with sea surface salinity, indicating a source from ice and snow
33
melting, and consistent with snow depositional inputs. After the summer snow
34
melting, VMS accumulate in the Southern Ocean and Antarctic biota. Therefore, once
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introduced into the marine environment, VMS are eventually trapped by the
36
biological pump and, thus, behave as “single hoppers”. Conversely, VMS in soils and
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vegetation behave as “multiple hoppers”, due to their high volatility.
38 39 40
KEYWORDS volatile dimethylsiloxanes, soils, vegetation, krill,
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phytoplankton, Antarctica, Southern Ocean
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Introduction
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Cyclic and linear volatile methyl siloxanes (cVMS and lVMS, respectively) are
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synthetic chemicals, the chemical structure of which is characterised by the reiterative
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alternation between oxygen and silicon atoms with methyl groups. These compounds
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are used in the manufacture of many industrial and consumer products, such as
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silicones, in the formulation of many personal care products (PCPs), paints, cleaning
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products and coatings, among others [1, 2]. Their main emission route into the
49
environment is the direct release into the atmosphere during the manufacturing
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process and product formulation, as well as through the normal use of siloxane-
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containing consumer products [3]. However, other important sources of emission are
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landfills disposal, solid waste incineration plants [4], and wastewater treatment plants
53
[5-7].
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The unusual combination of physic-chemical properties of VMS (cVMS and lVMS),
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makes difficult the prediction of their environmental fate. High octanol-water partition
56
coefficients (KOW), extremely low solubility in water, [8] and high values of Henry’s
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law constant (H) (Supplementary Table S1) are exhibited by VMS. However, due to
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the combination of their high KOW and H, the octanol-air partition coefficients (KOA)
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are relatively low. Furthermore, some VMS are resistant to oxidation, reduction, and
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photo-degradation, favouring their persistence in the environment [9-11], even though
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little is known about biotic mechanisms of degradation. In the atmosphere, VMS can
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be degraded, due to reaction with OH radicals [12]. With these physico-chemical
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properties, VMS reservoirs will be associated with the organic carbon pools in
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terrestrial and aquatic environments, while in the atmosphere they can be ultimately
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degraded [13]. VMS bio-accumulate in living organisms [14, 15], although the bio-
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accumulation and bio-magnification factors do not follow the same trends as for other
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POPs and are difficult to determine [15]. Due to the high tendency to escape to the
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atmosphere (low KOA and high H values), cVMS have been identified as flyers [16,
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17], favouring their long-range atmospheric transport (LRAT) [9, 18-20]. Flyers do
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not reach remote terrestrial and marine ecosystems, because of their low atmospheric
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depositional fluxes. Nevertheless, VMS and, in particular, cVMS, are ubiquitously
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distributed in the environment close to source regions [15, 21, 22], and during the last
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decade, the occurrence of VMS has been reported in water [5, 23], soils [24, 25],
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sediments [26], air [27-29], and biota [15, 22, 30]. Several monitoring programmes 3 ACS Paragon Plus Environment
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have been conducted to assess the occurrence of VMS in Canada, [31] Sweden [32],
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the UK [33, 34], and the Nordic countries [35], confirming the ubiquitous presence of
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these compounds in the environment, including remote lakes withoug direct sources
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[36]. However, most of the information has been focused on the study of a limited
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number of cVMS. Even though the LRAT potential of VMS, and the results reporting
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their occurrence in remote atmospheres, their potential to affect surface environments
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(either terrestrial or marine), has not been properly evaluated. It has been assumed that
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the atmospheric deposition of these compounds is negligible, due to their high
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volatility, but there have been scarce field studies for assessing this assumption.
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VMS have been related to different toxicological effects [11, 37-39] and they have
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raised concern by regulating agencies. Recently, octamethylcyclotetrasiloxane (D4)
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was included by the US-EPA in a list of 23 chemicals to be reviewed in 2013, for
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their further assessment under the Toxic Substance Control Acta [40]. The European
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Commission classifies D4 as an endocrine disruptor, based on evidence that it
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interferes with human hormone function [41] and that it may impair human fertility
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[42].
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decamethylcyclopentasiloxane (D5), concluded that these two compounds are toxic,
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persistent, and have the potential to be bio-accumulated in the aquatic organisms [43,
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44]. In contrast, dodecamethylcyclohexasiloxane (D6) has a similar structure to D4
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and D5, and its occurrence in the environment has been confirmed in the literature [5,
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45]. These three cVMS (D4, D5 and D6) are those with larger emissions into the
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environment [15, 34, 46].
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As committed in The Protocol on Environmental Protection to the Antarctic Treaty,
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the Antarctic environment and the dependent/associated ecosystems are objects of
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comprehensive protection. The study of the transport, distribution and bio-
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accumulation of persistent organic pollutants (POPs) in the Antarctic environment has
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been recognised as a research priority [47]. However, there is still a gap of knowledge
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about the distribution of emerging organic contaminants, such as VMS, in the
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Antarctic Region. Snow scavenging, is a very effective process sequestering
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atmospheric pollutants, and could drive the occurrence of VMS in Antarctic soils and
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biota.
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In this work, we present the occurrence of eight VMS, four cVMS (D3, D4, D5 and
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D6) and four lVMS (Octamethyltrisiloxane (MDM or L3), Decamethyltetrasiloxane
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(MD2M
Environment
or
L4),
Canada,
after
the
Dodecamethylpentasiloxane
assessment
(MD3M
of
or
D4
L5)
and
and 4
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Tetradecamethylhexasiloxane (MD4M or L6)), in terrestrial Antarctica (soils, lichens,
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mosses and grass), and we assess for the first time, the occurrence of VMS in
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phytoplankton and krill from the Southern Ocean. In addition, we aim at assessing the
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fate, accumulation and sources of VMS in remote cold ecosystems.
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Experimental Section
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Sampling. The soil and vegetation samples were mostly collected at Livingston and
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Deception Islands (Southern Shetlands, Antarctica) during period January-February
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2009, while the phytoplankton and krill samples were collected in the Southern Ocean
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during the ATOS II sampling cruise, on board RV Hespérides, covering the Drake
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Passage, Bransfield Strait, and the South Scotia, Bellingshausen, and Weddell Seas
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(February 2009). Sample locations and ancillary information are given in Figure 1,
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Supplementary Figure S1, and Supplementary Tables S2 (a, b, c, d). All sampling
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operations were performed using silicone-free materials, in order to avoid any
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possible contamination source during these operations. The occurrence and cycling of
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other POPs during the ATOS II campaign have been reported in companion papers
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[34, 35, 48-50].
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Surface soil (top cm) and vegetation samples (lichens, grass, mosses) were collected
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at eleven and seventeen sites, respectively, as described elsewhere [47]. Except for a
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few samples, located proximate to the Juan Carlos I research station, most sample
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sites were located in isolated areas in Deception and Livingston Islands (including the
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Byers Peninsula, an Antarctica Specially Protected Area), but some of the soils
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correspond to sites with Penguin colonies and soils covered with vegetation.
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Samples of krill individuals (Euphausia superba) were collected at eleven stations.
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Krill swarms were detected using a SimradTM EK60 (Norway) multi-frequency echo
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sounder. Specimens were collected using an Isaacs-Kidd mid water trawl (IKMT)
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with nets of 1 cm mesh size. Trawls were taken during 20 minutes at the depth where
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the swarms were detected (ranging from 20 to 30 m [51]). After retrieval, krill
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individuals were collected and wrapped in aluminium foil envelopes in glass flasks.
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Then, samples were frozen and stored at -20 ºC until analysis.
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Phytoplankton samples were collected at eleven sampling sites, using a trawl net
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(50 µm mesh) in the photic zone, from 10 m below the deep chlorophyll maximum
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depth (average: 54 m depth) to surface. Copepods and E. superba individuals were 5 ACS Paragon Plus Environment
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actively separated, when present in the sample, with a clean Pasteur pipette and
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tweezers, respectively. The samples were then filtrated with pre-combusted (4 hours
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at 450 ºC) GF/D filter (47 mm diameter, 2.7 µm mesh size, Whatman GE, UK),
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wrapped in aluminium foil, sealed in polyethylene bags, and frozen at -20 ºC until
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analysis.
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Filter blanks consisted of GFD filters, following the same precombustion, storage and
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analyses conditions as samples.
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Chemicals. cVMS and lVMS standards hexamethylcyclotrisiloxane (D3, 98 %),
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octamethylcyclotetrasiloxane (D4, 98 %), decamethylcyclopentasiloxane (D5, 97 %),
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octamethyltrisiloxane (MDM, 98 %), decamethyltetrasiloxane (MD2M, 97 %),
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dodecamethylpentasiloxane (MD3M, 97 %) and tetrakis(trimethylsilyloxy)-silane
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(Si(OTMS)4, 97 %) were purchased from Sigma Aldrich (Madrid, Spain).
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Dodecamethylcyclohexasiloxane (D6) and tetradecamethylhexasiloxane (MD4M) of
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the highest available purity were purchased from Fluorochem (Hadfield, UK).
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Supplementary Table S1 summarises the physico-chemical properties of the selected
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analytes. Standards m-xylene-d10 (98 %) and naphthalene-d8 (99 %) were purchased
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from Sigma Aldrich (Madrid, Spain). m-xylene-d10 and naphthalene-d8 were used as
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surrogate, while Si(OTMS)4 was used as an internal standard.
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Individual stock standard solutions of each compound and internal standards
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(1000 mg/L) were prepared in hexane from their respective pure standards in amber
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glass bottles. Secondary individual standard solutions were prepared by successive
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dilution of the stock standard solutions in hexane to give a concentration of 100 mg/L.
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A standard mixture solution of 10 mg/L in hexane containing all compounds was
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prepared. This solution was stored at -20 °C and monthly prepared. Standard mixtures
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solutions at concentrations ranging from 1 to 5,000 µg/L were prepared in hexane by
167
dilution from the mixed stock standard solution.
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Hexane Suprasolv® (purity ≥98%), was purchased from Merck (Darmstadt,
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Germany). Liquid nitrogen was obtained from Abello Linde (Barcelona, Spain).
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Samples pre-treatment and extraction. Extraction of the samples was performed by
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an ultrasound-assisted extraction procedure with hexane [5], with some minor
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modifications depending on the type of matrix. Very briefly, three/four krill
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individuals (1.00–1.73 g) were homogenized using an agate mortar, previously rinsed
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with hexane. Then, the mixture was transferred to a glass vial of 5.0 mL and it was
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spiked with the surrogates (15 µL of m-xylene-d10 and naphthalene-d8 10 ng/µL) and 6 ACS Paragon Plus Environment
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let reach the equilibrium for 3 hours at 4 ºC. The extraction was carried out inside the
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vial with 3.0 mL of hexane during 25 minutes. After this process, the vials were
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centrifuged at 1700 g.
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Filters with phytoplankton (~1 g of phytoplankton) were lyophilized in 15 mL
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polyethylene terephthalate (PET) tubes and spiked with surrogates (30 µL of m-
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xylene-d10 and naphthalene-d8 10 ng/µL). After 3 hours at 4 ºC, 6.0 mL of hexane
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were added and the extraction was performed using the ultrasonic bath during 25
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minutes. Finally, the tubes were centrifuged 10 minutes at 1700 g.
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Soil samples were grinded with an agate mortar and 3 grams of soil were weighted
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and placed in 5 mL glass vials. 15 µL of m-xylene-d10 and naphthalene-d8 10 ng/µL
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were used as surrogates. Extraction was carried out with 3.0 mL of hexane in the
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ultrasonic bath during 25 minutes.
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Vegetation samples were frozen with liquid N2 in a Teflon vase and grinded with a
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blender. About 3 grams of pulverized sample were placed in 10 mL vial and they
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were spiked with 15 µL of m-xylene-d10 and naphthalene-d8 10 ng/µL. Samples were
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let for equilibration during 3 hours at 4 ºC, and were extracted as in the case of soils.
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For all the matrices, 1.0 mL of extract was collected and transferred to a vial. 5 µL of
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Si(OTMS)4 10 ng/µL were spiked as internal standard prior to their analysis.
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Instrumental analysis. The analysis of VMS was performed using gas
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chromatography coupled to tandem mass spectrometry (GC-MS/MS) adapting the
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method described elsewhere [5], using a Trace GC-Ultra coupled to a triple
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quadrupole (QqQ) mass spectrometer TSQ Quantum (Thermo Fischer Scientific). The
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inlet temperature was maintained at 200 °C and the injection (injection volume
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2.0 µL) was carried out in split-less mode. The chromatographic separation was
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achieved with a DB-5 MS (5 % phenyl, 95 % methylpolysiloxane) fused silica
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capillary column (Agilent Technologies, CA, USA), 30 m×0.25 mm I.D., and
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0.25 µm film thickness, using helium as carrier gas. The oven temperature was
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programmed as follows: 50 ºC during 1 min, increased to 100 ºC at 6.0 ºC/min, then
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linearly increased to 250 ºC at 12 °C/min, then linearly increased at 25 °C/min to
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310 °C and finally held at 310 ºC during 1 min.
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Transfer line and ion source temperatures were set at 250 °C and 280 °C, respectively.
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Electron ionization (EI) was set at 70 eV and solvent delay was maintained during 3.5
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minutes. Acquisition was performed during 3.5 to 20 min by selected reaction
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monitoring (SRM) mode (see Supplementary Table S3). 7 ACS Paragon Plus Environment
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Quality assurance/Quality Control. In order to ensure the quality of the data a series
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of precautions were included to minimize and assess the contamination during sample
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manipulation, extraction and analysis: As in previous works [5], glass materials were
213
heated at 400 ºC overnight. After the heating process, glass materials were covered
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with aluminium fold during their storage and were washed with hexane prior to their
215
use. In addition, all the silicon-based materials were avoided during the sampling and
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the analytical process and, during the GC-MS/MS analysis, PTFE-folded silicone-
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based septa were replaced by aluminium-foil. In order to assess the potential
218
contamination during the whole analytical procedure, instrumental, procedural and
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field blanks were analysed together with the samples.
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To assess the potential matrix interferences and the adsorption of cVMS during the
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extraction procedure, blank matrixes with a close composition to the real samples
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were used as procedural blanks and analysed together with the real samples. One
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procedural blank was analysed each three real samples. Krill and prawns are both
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crustaceans belonging to the class Malacostraca with similar contents in lipid and
225
proteins. Therefore, for krill samples, a pool of blank prawns was used as procedural
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blank. For both phytoplankton and vegetation, a pool of blank tea-leaves was
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employed as procedural blank. Finally, for the soils analysis, a prepared blank soil
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matrix was used. Prior to their extraction and analysis, blank matrices were dried
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during 48 hours at 60 ºC in order to facilitate the evaporation of residual cVMS.
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The contamination levels of procedural blanks were subtracted from real samples.
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Several examples of extracted ion chromatograms of real samples vs. blank signals
232
are presented in the Supplementary section (Supplementary Figure S2). The typical
233
intensities of blank and samples signals are also represented there. The procedural
234
contamination was maintained constant, with extremely low standard deviation
235
(Supplementary Figure S3). The mean values of the signal of the compounds
236
quantified in the samples were at least four times the mean value of the standard
237
deviation of the contamination. The KOA values of VMS are low, and thus there is a
238
low potential to contaminate the sample from the ambient air, and the blank levels
239
seem to be of instrumental origin.
240
To assess the potential contamination during sampling, the storage and the shipment,
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field blank samples were carried out and were analysed together with the real
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samples. Field blanks consisted of GFF and GFD filters that were transported during
243
the whole sampling campaign. The filters were transported in closed containers as 8 ACS Paragon Plus Environment
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those used as for sampling. During sampling, the blanks were exposed to the ambient
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air conditions. After this time, the field blanks were stored using the same recipients
246
as the samples and were transported back to the laboratory frozen under the same
247
condition used for the samples. The concentrations of cVMS in field blanks did not
248
presented significant differences with the levels of contamination of procedural
249
blanks. Consequently, no extra contamination was introduced during sampling (See
250
Figure S3).
251
Instrumental quality parameters (instrumental limits of detection (ILOD) and
252
quantification (ILOQ), linearity and accuracy) were determined with hexane standard
253
solutions. For lVMS, ILOD and ILOQ were defined as the amount of analyte that
254
provides a response in the field blank equal to the mean noise plus three and ten times
255
the standard deviation, respectively and they were experimentally assessed by
256
consecutive dilution of the same standards. In the case of cVMS, which were present
257
in the blanks at concentrations higher than the theoretical limits of detection and
258
quantification, ILOQ were calculated by analysing the blank background in 10
259
instrumental blanks using a statistical approach based on Warner et al. 2012 [52].
260
ILOQ was defined as t0.95,n×sins, where t0.95 represents the one-tailed t-value at 95%
261
confidence, n represents the number of individual procedural blanks and sins refers to
262
the standard deviation of the background concentration in the hexane blanks.
263
Instrumental inter-day and intraday repeatability were calculated as the relative
264
standard deviation obtained when standards solutions were analysed at three
265
concentration levels (15, 50 and 100 pg/µL).
266
In order to validate the analytical method, linearity, method intra-assay precision
267
(n=10), method interday accuracy (n=10), and the method limits of detection (MLOD)
268
and quantification (MLOQ) were determined using spiking experiments in blank
269
matrices.
270
For lVMS, MLOD and MLOQ were defined as the amount of analyte that provides a
271
response equal to three and ten times the procedural blank noise. For cVMS, MLODs
272
and MLOQs were determined with blank matrix applying t=0.95 and n=10.
273
Recoveries, method inter-day and intraday repeatability were calculated as the relative
274
standard deviation obtained when spiked matrices solutions were analysed at three
275
concentration levels (16.7, 33.3 and 66.7 ng/g). The method parameters are
276
summarized in the Table S4 of the Supporting Information.
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Statistical Analyses. The statistical analysis was performed using SPSS version 22.
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The normality and log-normality distribution were confirmed by means of the Q-Q
279
normal test. The regression between concentrations, or between concentrations and
280
other environmental variables were performed by linear (simple or multiple) least
281
squares fitting of the data set. The significance of each one of the fitted parameters
282
was confirmed by a t-student test, and the significance of the regression line by a F-
283
Snedecor test. Only those regressions with all fitted parameters significantly different
284
than zero are shown.
285
Results
286
cVMS were found in almost all the samples (soils, vegetation, phytoplankton and
287
krill), while lVMS were only detected in soil and phytoplankton. Table 1 summarizes
288
the concentrations of each VMS in soils, vegetation, phytoplankton and krill, which
289
are shown in Figures 1 and 2, and the concentration of each compound for each
290
sample is given in Supplementary Table S5.
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Occurrence of VMS in Antarctic soils. Concentration of cVMS were between 2 and
292
3 orders of magnitude higher than those of lVMS, consistent with the relative
293
magnitude reported in other studies [5, 35, 53]. D5 was the predominant compound,
294
ranging from
