Environ. Sci. Technol. 2002, 36, 12-15
Trifluoroacetate in Ocean Waters H A R T M U T F R A N K , * ,† EUGEN H. CHRISTOPH,† OSMUND HOLM-HANSEN,‡ AND JOHN L. BULLISTER§ Department of Environmental Chemistry and Ecotoxicology, University of Bayreuth, D-95440 Bayreuth, Germany, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093, and NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington 98115
Trifluoroacetate (TFA) is a ubiquitous xenochemical presently increasing in concentration in some environmental compartments, especially in the plant biomass of industrialized countries. Direct anthropogenic emissions of TFA are probably low, and the major anthropogenic sources are most likely various TFA precursors. As TFA has been found in ocean waters from remote locations, the question arose whether it is also a naturally occurring environmental chemical. Determination of the depth dependence of TFA in the ocean water column should shed some light on this question. However, in environmental analytical studies, the risk of systematic errors can be high and may lead to wrong conclusions. Therefore, special attention has been paid to the fact that TFA is a common atmospheric pollutant in the urban environment and that contributions from sampling, storage, and transport potentially lead to artificially high TFA values. The results of the ocean water sampling campaigns indicate that TFA is a naturally occurring chemical, homogeneously distributed in ocean waters of all ages with a concentration of about 200 ng/L.
Introduction The sources of environmental trifluoroacetate (TFA) (1) are only partly known. TFA is an atmospheric breakdown product of the CFC-substitute compounds 1,1,1,2-tetrafluoroethane (HFC 134a), 1,1-dichloro-2,2,2-trifluoroethane (HCFC 123), and 1-chloro-1,2,2,2-tetrafluoroethane (HCFC 124) and of the inhalation anaesthetics halothane and isofluran (2). These are oxidized to trifluoroacetyl halides (3-5) and hydrolyzed to TFA (6). Atmospheric degradation of pesticides containing trifluoromethyl-substituted aromatic rings (7) are believed to give rise to TFA. The direct use of TFA as laboratory chemical and of its derivatives and precursors such as trifluoroacetyl anhydride or trifluoroethanol may also contribute to environmental TFA as well as to waste incineration of fluorinated organopolymers, e.g., polytetrafluoroethene (8-11). Determination of TFA in ocean waters has been undertaken in order to estimate the global abundance of this strongly hydrophilic, persistent environmental chemical and to assess whether today’s levels are mainly of anthropogenic origin or whether biological and/or geological sources must be considered. * Corresponding author telephone: +49-921-552252; fax: +49921-552334; e-mail:
[email protected]. † University of Bayreuth. ‡ University of California at San Diego. § NOAA/Pacific Marine Environmental Laboratory. 12
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Materials and Methods Chemicals. Sodium chloride, potassium chloride, calcium sulfate dihydrate, sodium sulfate, potassium bicarbonate, sodium fluoride (all analytical grade, p.a.), sodium bromide, and boric acid (both extra pure) were obtained from Merck (Darmstadt, Germany). Magnesium sulfate (p.a.), sulfuric acid (98%), tert-butyl methyl ether (98%) (MTBE), and heptafluorobutyric acid (g99%) were from Fluka (Buchs, Switzerland). Pentafluoroacetophenone (97%) and sodium trifluoroacetate (g98%) were from Sigma-Aldrich (Steinheim, Germany). Ocean Water Sampling Locations. Ocean water samples were collected during a cruise of the Russian RV Yuzhmorgeologiya in the Southern Ocean near Elephant Island on January 19 and 25, 1998, at 60.6° S, 56.5° W; on January 23, 1999, at 60.5° S, 57.5° W; on January 26, 1999, at 60.25° S, 54.5° W; and during a journey of the NOAA RV Ronald H. Brown in the Mid-Atlantic Ocean from Las Palmas de Gran Canaria (Canary Islands, Spain) to Miami (Florida) on January 29 and 30, 1998, at 24° N, 28° W. Sampling and Sample Storage. The water was sampled with Niskin samplers. During the sampling campaigns of 1998, the water samples were filled into new 500-mL brown soda-lime glass bottles (rectangular 7 × 7 cm, h ) 18.6 cm; Laborcenter, Nu ¨ rnberg, Germany) previously washed with 1 N NaOH, deionized water, 1 N H2SO4, and deionized water. The bottles were rinsed once with ocean water of the same sample, filled to leave an air volume of about 10 mL, and closed with gastight polypropylene (PP)-lined screw caps. Three samples were taken for each depth. During the sampling campaign of 1999, new (prewashed as above) cylindrical 500-mL PP bottles (L ) 7.3 cm, h ) 16.8 cm; Laborcenter, Nu ¨ rnberg, Germany) were used. Upon arrival at Bayreuth, all samples were stored at 4 °C. Sample Preparation. The samples were allowed to warm to room temperature (22 °C). Three aliquots of 10 mL were transferred by means of an adjustable pipet (EppendorfVario, Ko¨ln, Germany) into 30-mL PP screw-cap vials (25 × 95 mm; Nalgene, Rochester, NY). Each sample aliquot was spiked with a solution of heptafluorobutyric acid in deionized water leading to an in-sample concentration of 134 ng/L heptafluorobutyrate (CF3CF2CF2COO-). After addition of 2 g of sodium chloride and acidification to pH 1 with 350 µL of concentrated (98%) sulfuric acid, each sample was extracted with 1 mL of MTBE under agitation, and the ethereal phases were transferred with glass Pasteur pipets (John Poulten Ltd., Barking, Essex, U.K.) into silanized 1-mL crimp-cap vials (Chromacol Ltd., Herts, U.K.). The acids in the ethereal extracts were derivatized to their pentafluorophenylethyl esters (12) with 5 µL of a solution of 1-pentafluorophenyldiazoethane, 8 vol % in MTBE, prepared from pentafluoroacetophenone (13). Gas Chromatography/Mass Spectrometry. One microliter of the derivatized ethereal extract was injected oncolumn by means of an automated sampler (A 200 S, Fisons Instruments, Thermo Finnigan, San Jose, CA) onto a fusedsilica capillary (15 m × 0.25 mm) coated with 0.25 µm of methyl(95%)-phenyl(5%)-silicone (CP-Sil 8 CB-MS Chrompack, Middelburg, Netherlands) installed in a gas chromatograph (Hewlett-Packard HP 5890 series II) coupled to a mass spectrometer (HP 5989A) operated in the negativechemical ionization mode (NCI-MS) with methane as reactant gas at a pressure at the source gauge between 1.2E-2 and 2.1E-2 Pa. Carrier gas was helium at an inlet pressure of 80 kPa; injection temperature was 60 °C. The oven temperature was maintained at 60 °C for 1.5 min, then programmed at a rate of 25 K/min to 240 °C, and kept isothermally for 1.3 10.1021/es0101532 CCC: $22.00
2002 American Chemical Society Published on Web 11/30/2001
FIGURE 1. Capillary GC/negative-chemical ionization selected-ion monitoring trace (m/z 113) for TFA and the internal standard heptafluorobutyrate (m/z 213): (a) Southern Ocean water sample from January 26, 1999, at 750-m depth (205 ( 18 ng/L plus blank of 25 ( 8 ng/L); (b) ASW spiked with 200 ng/L TFA plus blank of 8 ( 4 ng/L; (c) nonspiked ASW plus blank of 8 ( 4 ng/L. min. Total run time was 10 min. The GC/MS interface temperature was 240 °C, the ion-source temperature was 200 °C, and the temperature of the quadrupole was 100 °C. The mass spectrometer was tuned daily using perfluorotributylamine, optimizing on the fragment ions m/z 127, 195, 245, and 302. The ion selected for quantification of trifluoroacetate was m/z 113, relative to the ion m/z 213 of the internal standard heptafluorobutyrate. Identification of TFA was done by verification of retention times (Figure 1a-
c) and occasional recording of the mass spectrum of the respective GC peak. Calibration. Artificial seawater (ASW) was prepared by dissolving pure salts in deionized water (14). Calibration was performed by spiking ASW with a solution of sodium trifluoroacetate in deionized water, leading to calibration concentrations ranging from 28 to 339 ng/L TFA, followed by sample preparation and GC/MS analysis as described before. Blanks and Controls. Blanks obtained by analyzing deionized water were different between different sample preparation periods in 1998 and 1999. During the last sample preparation period following the ocean water sampling in 1999, the blanks were between 2 and 13 ng/L. The mean value was 8 ng/L with a standard deviation of 4 ng/L (n ) 6). The limit of detection is calculated as the sum of the mean blank and 3-fold of its standard deviation, yielding 20 ng/L; the limit of quantification is the sum of the mean blank and 6-fold of its standard deviation, yielding 32 ng/L. Sampling Period 1998. For the sampling period of 1998, about 400-year-old mineral water (Kondrauer Mineralwasser, Kondrau, Germany) was used as the control, forwarded together with the empty glass bottles to the sampling sites. At the time and location of sampling, the control water was poured into the Niskin sampling device, the sampler was slowly rotated for 15-20 min, and the water was then poured back into the original shipping bottle. In this fashion, any contamination during sample transfer, transport of the empty or filled bottles to the sampling site, and back to laboratory would have been detected. The TFA values obtained for forwarded mineral water were 35 ( 4 ng/L (n ) 3), for mineral water stored at the University of Bayreuth were 35 ( 5 ng/L (n ) 3), for deionized water were 33 ( 3 ng/L (n ) 3), and for ASW were 34 ( 1 ng/L (n ) 3). Thus, the differences between the various control samples were smaller than their standard deviations, indicating that there is no contamination during sampling, transport, and storage. Sampling Period 1999. For the sampling period of 1999, eight samples of 500 mL of ASW were forwarded (four in PP, four in glass) to the Antarctic in addition to the empty PP bottles. Controls were performed in parallel using PP and glass bottles in order to determine whether different materials give rise to different TFA values. At each of the two sampling stations, water from 10, 50, 100, 200, and 750 m was drained directly from the Niskin sampler into the rinsed sample bottles. For testing with ASW samples, the Niskin bottles used for the 200- and 750-m samples were removed from the sampling device and rinsed with 250 mL of the ASW for 5-10 min, which was then discarded. Two other portions of ASW (500 mL each) were then poured into the two Niskin bottles, which were rotated slowly in horizontal position, so that the entire inside surface was rinsed for 15-20 min. These control samples were then drained out of the Niskin bottle directly back into the same bottle (either glass or PP), the cap was tightly screwed on, and the bottles were returned with all other sample bottles. Two bottles were also stored and kept sealed in the laboratory. All these controls showed an average TFA level of 25 ( 8 ng/L (n ) 24) (Table 1). Statistical analyses (F-test: error probability R ) 5%) verified that for opened (forwarded) control samples there were only slight nonsignificant differences between those stored in glass or those stored in PP. Applying the student t-test, one control sample series (Table 1, stored and kept sealed at Bayreuth) showed a significant difference (R ) 5%) between samples stored in PP and those stored in glass bottles. The slightly smaller control values in glass bottles may have been caused by adsorption of TFA on alkali glass surfaces as it has been found before with radioactively labeled TFA. However, the differences were not consistent and were VOL. 36, NO. 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Results and Discussion
TABLE 1. TFA Control Levels in ASWa container, concn ( SD (N; n) (ng/L) control sample history forwarded; kept sealed forwarded; opened; refilled stored; kept sealed all samples all forwarded samples all stored samples all sealed samples all opened samples a
glass
PP
25 ( 5 (1; 3) 24 ( 5 (1; 3) 19 ( 7 (2; 6) 26 ( 7 (2; 6) 21 ( 9 (1; 3) 37 ( 4 (1; 3) 21 ( 7 (4; 12) 28 ( 8 (4; 12) 23 ( 6 (6; 18) 29 ( 11 (2; 6) 24 ( 4 (2; 6) 22 ( 7 (4; 12)
N, number of bottles; n, number of replicates.
TABLE 2. TFA Concentrations and CFC-12 Age of Mid-Atlantic Seawater Samplesa depth (m)
TFA concn ( SD (n ) 6) (ng/L)
CFC-12 apparent age (yr)
0 SFC 2 40 120 380 1000 4000 4150
190 ( 10 200 ( 8 210 ( 12 205 ( 16 210 ( 6 205 ( 16 195 ( 16 200 ( 16
(
