Environ. Sci. Technol. 2000, 34, 239-245
Chloro- and Bromoacetates in Natural Archives of Firn from Antarctica LENA M. VON SYDOW,* ANNIKA T. NIELSEN, ANDERS B. GRIMVALL, AND H A N S B . B O R EÄ N Department of Water and Environmental Studies, Linko¨ping University, SE-581 83 Linko¨ping, Sweden
A firn core was drilled in Dronning Maud Land, Antarctica, to investigate the presence of haloacetates in snow that had accumulated over the past 200 years. By employing GC-MS detection of methyl esters of haloacetic acids, we were able to measure haloacetate concentrations down to one or a few nanograms per liter. Trichloroacetate (TCA) and dibromoacetate (DBA) were found in firn at concentrations that clearly exceeded the blank level of the applied analytical procedure, with mean concentrations estimated to 12 and 6 ng/L, respectively. There were also indications that mono- and dichloroacetate (MCA and DCA) were present in firn, whereas monobromoacetate (MBA) was found only in samples of surficial snow. We concluded that there is a significant natural background level of TCA and DBA in precipitation based on the following: (i) several of samples represented snow accumulated in the 19th century; (ii) haloacetates can be expected to be immobile in Antarctic firn; (iii) extensive measures were taken to prevent sample contamination; and (iv) blank levels of the analytical procedure used were low and stable. In addition, our results suggested that MCA and DCA also occur naturally in precipitation.
Introduction Chloroacetates are widely distributed in rain and snow (e.g., refs 1-10) and are often observed at higher concentrations than any other chloroorganics (1, 11). Some of the cited authors, in particular Frank and co-workers (e.g., refs 1214), have claimed that this widespread occurrence of chloroacetates is due to atmospheric degradation of chlorinated solvents, such as tri- and tetrachloroethene and 1,1,1trichloroethane. Other scientists have questioned the proposed pathways in the atmosphere (15), and investigators analyzing samples of glacier ice and firn (10, 16) have suggested that natural sources may play a significant role. In principle, the presence of chloroacetates in ice or firn of preindustrial origin should provide evidence that the compounds are of natural origin. However, the firn samples we analyzed in a previous investigation (10) were collected and archived for purposes other than analysis of trace organics, and in such cases contamination of the samples cannot be completely ruled out. Furthermore, the ice samples in the studies cited above were taken from temperate glaciers where recent precipitation may have percolated to older ice * Corresponding author phone: +46-13-28 10 00; fax: +46-13-13 36 30; e-mail:
[email protected]. 10.1021/es9812150 CCC: $19.00 Published on Web 12/04/1999
2000 American Chemical Society
FIGURE 1. Map of Antarctica showing the site designated M (74°59′59′′ S, 15°00′06′′ E) where the firn core was drilled.
TABLE 1. Location of Sites for Sampling of Surficial Snow sample
location
vol (L w.e.)
D G I K L M
72°30′04′′ S, 03°00′00′′ E 73°02′44′′ S, 05°02′70′′ E 73°43′29′′ S, 07°56′13′′ E 74°21′18′′ S, 11°06′13′′ E 74°38′53′′ S, 12°47′23′′ E 74°59′59′′ S, 15°00′06′′ E
0.286 0.688 0.499 0.712 0.700 0.635
layers during the summer season (10, 16). Therefore, a new study was undertaken to obtain more conclusive results regarding the natural occurrence of chloro- and bromoacetates in precipitation. A firn core representing the past 200 years of snow accumulation was drilled in Dronning Maud Land, East Antarctica, and extensive measures were taken to prevent and monitor contamination during sampling, transport, storage, and chemical analysis. The analyzed haloacetates were mono-, di-, and trichloroacetate (MCA, DCA, and TCA) and mono- and dibromoacetate (MBA and DBA).
Materials and Methods Sampling of Firn and Surficial Snow. The field work was conducted during the 1996-1997 Norwegian Antarctic Research Expedition (NARE) organized by the Norwegian Polar Institute, within the framework of the European Project for Ice Coring in Antarctica (EPICA). A 20-m firn core was drilled in Dronning Maud Land (site M in Figure 1), East Antarctica. The sampling site was located 3453 m above sea level, and the average temperature was -51.3 °C at a depth of 10 m (17). The drill system was of type PICO (Polar Ice Coring Office; 18) and consisted of a 75-mm polyester cylinder connected through 1 m long extensions to an electric drill (19). The core was cut into 10-40 cm long segments, which were immediately packed in polyethylene bags (0.15 mm; Labora, Stockholm) that were heat-sealed. Five of the segments (M3, M8, M21, M24, and M 51) were further protected from contamination during transport and storage by putting the plastic bags in stainless steel cylinders made airtight with a copper gasket (Leybold Inc., Gothenburg, Sweden). Surficial snow was collected at seven different sites (see Table 1) by pressing 2-L polyethylene bottles horizontally through the snow just below the surface of the sampled VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Physical Characteristics of Firn Sampled in Dronning Maud Land, East Antarctica (74°59′59′′ S, 15°00′06′′ E) sample
mean depth (m)
density (kg/L)
vol (L w.e.)
M1 M3 M5 M8 M 10 M 13 M 14 M 18-19 M 21 M 23 M 24 M 25 M 26 M 27 M 28 M 29 M 30 M 35 M 37 M 38 M 41 M 42-43 M 44 M 46-48 M 51
1.6 3.1 4.3 5.4 6.6 7.3 8.0 9.6 10.2 11.0 11.4 11.8 12.1 12.5 12.8 13.0 13.4 15.1 16.3 16.5 17.5 17.9 18.4 19.1 20.2
0.329 0.360 0.390 0.421 0.428 0.464 0.497 0.493 0.483 0.500 0.491 0.504 0.502 0.512 0.505 0.519 0.520 0.558 0.543 0.547 0.518 0.562 0.523 0.524 0.587
0.315 0.193 0.285 0.379 0.220 0.335 0.253 0.632 0.332 0.348 0.342 0.281 0.218 0.403 0.148 0.385 0.266 0.217 0.431 0.302 0.297 0.279 0.235 0.533 0.357
FIGURE 2. Electrical conductivity records for a firn core drilled next to the core analyzed in the present study. Peaks corresponding to major volcanic eruptions are indicated by location and year of the eruption. Source: Isaksson et al. (17). snowpacks. The air-tight steel cylinders were washed with Milli-Q water (Millipore) and ethanol prior to the sampling, and the purity of the polyethylene bags and bottles was checked by analysis of haloacetates in Milli-Q water that had been in contact with such materials (see also the section about control experiments). Sample Characteristics. The locations of the sampling sites for surficial snow and the physical characteristics of the firn samples are listed in Tables 1 and 2. The age of the sampled firn was calculated from measurements made on a core drilled next to the core analyzed in the present study. Dating of the core was performed using β-activity measurements, which revealed the 1954-1955 and 1964-1965 snow layers, and electrical conductivity measurements (ECM) identifying major volcanic eruptions (Figure 2; 17). The deepest part of the firn core (20.2 m) was estimated to represent snow that accumulated about 190 years ago. Sample Handling. All samples were transported and stored frozen (-20 °C) pending analysis. Contaminants that might have adsorbed onto the surface of the firn core were 240
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FIGURE 3. Device used for drilling a 50-mm inner core in the original 75-mm firn core.
TABLE 3. Characteristic Mass Numbers Used in GC-MS-SIM Analysis of Methyl Esters of Haloacetic Acids and the Internal Standard (2,2-Dichloropropionic Acid)
a
compound
tR (min)
m / za
MCA MBA DCA DCPA TCA DBA
6.9 9.6 10.1 10.8 12.9 16.1
49, 51, 108 93, 95, 152 83, 85, 87 97, 99, 101, 121 117, 119, 121 171, 173, 175
Ions used for quantification are shown in italics.
removed prior to the analysis by using a coring device made of Teflon and stainless steel (Figure 3) to drill a 50-mm inner core in the original 75-mm firn core. The coring was performed in a cold room to avoid melting, and the inner core was then thawed in polyethylene bottles. The samples of surficial snow were thawed in the polyethylene bottles in which the snow had been sampled and stored. Analytical Procedure. The procedure used to determine the presence of haloacetates in the analyzed samples comprised the following steps: (i) preconcentration of haloacetates by solid-phase extraction; (ii) diethyl ether extraction of haloacetic acids at low pH; (iii) methylation of carboxylic acids; (iv) gas chromatographic analysis of methyl esters of haloacetic acids. Preconcentration of haloacetates was accomplished by passing the original sample through a strongly basic anion exchanger (Isolute SAX, 500 mg; IST, Sorbent AB). The ion exchanger had been previously conditioned with methanol and Milli-Q. The haloacetates were then desorbed with strong acid and salt. Two different combinations of salt and acid were used (one including 5 mL of 1.2 M hydrochloric acid and 1.7 M sodium chloride and one with 5 mL of 1 M sulfuric acid and 0.7 M sodium sulfate) in various parts of the study. The water passed through the ion exchanger was extracted with 5 mL of diethyl ether, and the water was removed from the ether phase obtained by freezing at -80 °C and then shaking the ether with water-free sodium sulfate. After evaporation to a volume of approximately 1 mL, carboxylic acids were derivatized using an excess of diazomethane prepared from N-methyl-N-nitroso-p-toluenesulfonamide according to Vogel (20). The ether extract was then evaporated to a volume of 100 µL using a gentle stream of nitrogen gas, and the methyl esters of haloacetic acids were analyzed by gas chromatography-mass spectrometry in the selected ion monitoring mode (GC-MSSIM). The mass numbers used are summarized in Table 3. 2,2-Dichloropropionic acid (DCPA; 500 ng) was added to the original water sample as internal standard. GC Parameters and Mass Spectrometric Conditions. GC-MS-SIM analyses were performed on a Hewlett-Packard 6890 GC equipped with an HP 5973 mass spectrometer. Gas
FIGURE 4. GC-MS analysis of the methyl ester of trichloroacetic acid in extracts derived from (A) firn collected in Dronning Maud Land (site M, sample M 46-48) and (B) Milli-Q water used as a blank sample. The chromatograms show the response in the SIM mode to m/z 117, 119, and 121. The age of the firn sample was approximately 180 years, and the concentration of TCA was estimated to 39 ng/L.
TABLE 4. Blank Levels (ng) Recorded Prior to the Analysis of Haloacetates in Five 100-mL Aliquots of Milli-Q Watera
TABLE 5. Blank Levels (ng) Recorded Prior to the Analysis of Haloacetates in Five 100-mL Aliquots of Milli-Q Watera
blank level expressed in ng
blank level expressed in ng
aliquot
MCA
DCA
TCA
MBA
DBA
aliquot
MCA
DCA
TCA
MBA
DBA
1 2 3 4 5
35 33 31 32 33
14 13 12 13 13
1 1 1 1 1
nd nd nd nd nd
1 1 nd 1 1
1 2 3 4 5
4 5 5 5 4
5 5 7 7 7
nd nd nd nd nd
nd nd nd nd nd
nd nd nd nd nd
a Haloacetates were desorbed with 5 mL of 1.2 M hydrochloric acid and 1.7 M sodium chloride.
chromatographic conditions: HP-5 column (30 m × 0.25 mm, phase thickness 0.25 µm); carrier gas helium, flow rate 39 cm/s; splitless injection (1 min), 2 µL injected; pulsed splitless auto-injection, injector temperature 250 °C; temperature program, 30 °C (5 min), 5 °C/min, 105 °C; transfer line temperature, 280 °C. Blank Levels. Prior to the analysis of any samples, it was confirmed that the blank concentrations of all analyzed compounds were stable and low. The results in Tables 4 and 5 illustrate the response expressed in nanograms when 100mL aliquots of Milli-Q water were analyzed with each of the two desorption techniques. In addition, 15 aliquots of Milli-Q water were analyzed as blank samples throughout the study. Quantification. Quantification of the concentrations of haloacetates in the analyzed samples was based on the peak areas of the ions characteristic of haloacetates and 2,2dichloropropionic acid, as indicated in Table 3. Calibration functions were established by analyzing a total of 12 100-mL samples of Milli-Q water spiked with 0, 25, 50, 100, 200, or 300 ng of each of the analyzed compounds. The response was found to be linear, and the R 2 values ranged from 0.991 to 0.999. Due to the different blank levels, different calibration functions were used for quantification when desorption was accomplished with chloride and sulfate, respectively. Spiked
a Haloacetates were desorbed with 5 mL of 1 M sulfuric acid and 0.7 M sodium sulfate.
samples were also analyzed throughout the study to confirm the initially determined calibration functions. Control Experiments. The risk of different types of contamination was examined by analyzing the following: (i) Milli-Q water stored for 48 h in the polyethylene bottles used for snow sampling; (ii) Milli-Q water with submerged pieces of the polyethylene bags used to protect the firn samples; (iii) Milli-Q water stored in an open beaker exposed to laboratory air for 24 h; (iv) firn from the outer parts of the original core that were removed when an inner core was drilled; (v) firn from the inner core thawed in a glass beaker filled with purified nitrogen gas.
Results The GC-MS analyses of derivatives of five haloacetates (MCA, DCA, TCA, MBA, and DBA) demonstrated that it is possible to detect such compounds in extracts derived from firn and snow. The SIM chromatograms Figure 4A illustrate the distinct response to the ions characteristic of the methyl ester of trichloroacetic acid observed when analyzing one of the firn extracts; the SIM chromatograms in Figure 4B show the VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Firn samples and corresponding blank samples desorbed with Na2SO4. The blank samples (to the left of the corresponding firn samples) were analyzed along with the real firn samples. The area ratios between the methyl esters of haloacetic acids and the internal standard were plotted for all samples.
FIGURE 6. Firn samples and corresponding blank samples desorbed with NaCl. The blank samples (to the left of the corresponding firn samples) were analyzed along with the real firn samples. The area ratios between the methyl esters of haloacetic acids and the internal standard were plotted for all samples. much weaker response to the same ions exhibited by an extract derived from Milli-Q water. The response to the ions characteristic of the other methyl esters of haloacetates was also very distinct. The blank levels recorded at the onset of the study have already been shown in Tables 4 and 5. Further analyses of blanks and samples showed that stable blank levels were 242
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maintained throughout the study. Moreover, it was obvious that there was a significant difference between real samples and blanks in regard to the response to the ions characteristic of the methyl esters of TCA and DBA (Figures 5 and 6). The difference between real samples and blanks was less pronounced in regard to MCA and DCA. However, it is interesting to note that, within each group of analyzed samples (one
FIGURE 7. Concentrations (ng/L) of MCA, DCA, TCA, and DBA in firn collected at different depths. Sample characteristics are given in Table 2. Desorption of haloacetates from the ion exchanger used in the preconcentration step was accomplished with Na2SO4/H2SO4 (0) or NaCl/HCl ([). When no value is shown for a sample, the response to the sample did not differ significantly from that of the associated blank.
TABLE 6. p Values in Rank Tests of the Differences in Response between Real Samples and Blanksa p value compound
NaCl desorption
Na2SO4 desorption
all samples
MCA DCA TCA DBA
0.0083 0.0083 0.0083 0.025
0.0019 0.0019 0.0019 0.0139
0.00002 0.00002 0.00002 0.00035
a Data were grouped as in Figures 5 and 6 (one blank followed by 1-4 real samples), and observations in different groups were considered to be statistically independent.
blank sample followed by 1-4 real samples), the response to the blank was invariably less than or equal to the responses to the real samples. The probability that this would occur by chance is very small (Table 6). Compilation of results obtained for samples collected at different depths showed that the methyl ester of trichloroacetic acid was present in all but one of the analyzed extracts of the firn samples (Figure 7). Three other haloacetate derivatives, i.e., the methyl esters of mono- and dichloroacetic acid and dibromoacetic acid, were found in extracts representing a wide range of sampling depths, whereas the methyl ester of monobromoacetic acid was not found in any of the firn extracts. Similar to the electrical conductivity records in Figure 2, which reflect the acidity in firn sampled at different depths (21), the haloacetate concentrations exhibited substantial sample-to-sample variation (Figure 7). The lowest concentrations were below the detection levels of the target compounds, whereas the highest concentrations of MCA, DCA, TCA, and DBA were several tens of nanograms per liter. The average concentrations of the same compounds were 17, 9, 12, and 6 ng/L, respectively. Interestingly, there were no significant relationships between concentrations and sampling depths. In particular, it can be seen that methyl esters of MCA, DCA, TCA, and DBA were also detected when
TABLE 7. Concentrations (ng/L) of Haloacetates in Snow Collected in Dronning Maud Land, Antarcticaa sampling site
MCA
DCA
TCA
MBA
DBA
site D site G site I site K site L site M
61 37 106 6 13 23
8 17 21 nd 4 4
146 318 348 58 190 108
19 16 36 3 4 5
11 4 3 1 2 4
a
The locations of the sampling sites are given in Table 1.
extracts derived from the deeper parts of the firn core were analyzed. The extracts derived from superficial snow collected at six different sites in Dronning Maud Land differed from the firn extracts in two respects. First, the concentration of TCA was considerably higher in the snow extracts (Table 7). Second, all analyzed snow samples contained MBA at concentrations ranging from 3 to 36 ng/L. The control experiments performed to detect potential sources or pathways of sample contamination showed that the blank level of the analytical procedure used was not influenced by either exceptionally strong exposure to laboratory air or any of the plastic materials used in sample handling (Tables 5 and 7). Furthermore, there was no evidence that our extensive efforts to reduce exposure of the samples to laboratory air resulted in lower concentrations of the analyzed haloacetates: MCA, DCA, TCA, and DBA were all detected in an extract of firn (M 1) that had been thawed in a glass beaker filled with purified nitrogen gas. As described in Materials and Methods, the outer part of the firn core was removed prior to chemical analysis. Normally, that part was simply discarded. However, it is interesting to note that, when both the inner and the outer part of a firn sample (M 13) were analyzed, there was no significant difference between the inner and outer part in the concentration of any of the haloacetic acids. VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 8. Amounts (ng) of Haloacetates in Milli-Q Water Exposed to Laboratory Air and Polyethylene Plastic 500 mL of Milli-Q stored 48 h in polyethylene bottle 500 mL of Milli-Q stored in open beaker for 24 h 100 mL of Milli-Q with submerged polyethylene pieces
Discussion Determination of preindustrial levels of trace organics in the environment is a demanding task. Samples must be collected in the relatively rare environments and matrixes where such compounds are practically immobile. Moreover, extreme care must be exercised to prevent and monitor contamination during all handling procedures, from field sampling to laboratory analysis, and the analytical techniques used must be optimized to enable determination of very low concentrations of the compounds of interest. It is well-known that gaseous compounds can penetrate firn and reach layers that were deposited decades ago (22). However, airborne transport of haloacetates found in firn is very unlikely because such compounds exist in the form of nonvolatile salts at the pH values that can prevail in firn (TCA has a pKa value of 0.7, and the other haloacetates under consideration have pKa values ranging from 1.5 to 2.8). Vertical transport of haloacetates with percolating meltwater can also be excluded due to the very low temperatures in the sampling area; the average temperature at a depth of 10 m was estimated to be -51 °C (17). The electrical conductivity measurements shown in Figure 2 confirmed the low mobility of ions in Antarctic firn. Major volcanic eruptions, such as that of Tambora in 1815, produced very distinct signals, which is also an indication of relatively little wind redeposition. Moreover, there was significant intra-annual variation that would have been smoothed out if ions were mobile in firn. Together, these results show that haloacetates detected in the deeper parts of the core must have been of the same age as the analyzed firn. In particular, we can conclude that haloacetates present deep in the firn core must have been deposited in the 19th century, long before a large-scale industrial production of reactive chlorine and chloroorganics was begun. The low mobility of ions in frozen firn was also an important factor in the development of the strategy that we adopted to prevent contamination of the analyzed samples. In general, during the drilling operation it is not possible to fully protect the firn from contaminants present in the air at the sampling site or on the drilling device. Transport and storage of samples represent other risks of contamination, except for the samples stored in airtight tubes. However, even if haloacetates were adsorbed on the surface of our firn core during any of these steps, such compounds would have been efficiently removed when an inner core was drilled in the original firn core immediately prior to chemical analysis. Moreover, control experiments did not reveal any contamination of the outer parts of the original firn core. The risk of sample contamination due to contact with laboratory air and polyethylene bags or bottles was investigated in control experiments, and the results were unambiguous. Samples of Milli-Q water that had been deliberately exposed to laboratory air or any of the plastic materials used in sample handling did not exhibit elevated haloacetate levels (see Table 8). Hence, we concluded that the blank levels observed for several of the analyzed haloacetates were due to impurities in the reagents used in the chemical analyses of the samples or in the Milli-Q water used to prepare blank samples. The results given in Tables 4 and 5 show that desorption with HCl and NaCl caused a higher blank level than desorption with H2SO4 and Na2SO4. At the onset of the study, HCl and NaCl were used for desorption to avoid the 244
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MCA
DCA
TCA
MBA
DBA
4 5 4
4 6 5
nd nd nd
nd nd nd
nd nd nd
formation of dimethyl sulfate in the final extracts; later, we changed to H2SO4 and Na2SO4 to further lower the blank level. GC-MS detection of the different haloacetates was found to be the least problematic part of the study. When extracts derived from firn were analyzed, the response when monitoring ions characteristic of MCA, DCA, TCA, and DBA (see Table 3) was very distinct, whereas MBA was absent in such extracts. Furthermore, the response to the ions characteristic of TCA and DBA was much greater for most of the extracts derived from firn than for any of the blank extracts. The relatively high blank levels of MCA and DCA made the interpretation of the results for these compounds somewhat less certain. However, from a statistical perspective there was also a significant difference (Table 6) between samples and blanks with regard to MCA and DCA. Regardless of the desorption method used, the response of firn extracts to the ions characteristic of MCA and DCA was invariably greater than or equal to the response of the associated blank to the same ions. The large sample-to-sample variation in the estimated concentrations of TCA (and other haloacetates) may raise questions regarding the precision of the chemical analysis. An ongoing methodological study in our laboratory has shown that the recovery of haloacetates can decrease with increasing concentrations of other ions, such as chloride, sulfate, nitrate, and bicarbonate. However, in the present study there was no indication of such matrix effects. The recovery of the internal standard was stable, and the highest concentrations of haloacetates (Figure 7) coincided with records of high electrical conductivity (Figure 2). Moreover, considering the large variation in the electrical conductivity records, the observed divergence in the concentrations of specific haloacetates is by no means unrealistic. The results of the present study are conclusive with regard to the natural occurrence of TCA and DBA, and the statistical analysis of observed data suggests a natural production of MCA and DCA. However, it has not been clarified whether the large difference in concentrations between samples of surficial snow and firn are due to degradation or temporal variation in deposition. Neither have the sources responsible for the natural occurrence of haloacetates in firn from Antarctica been identified. Nonetheless, it is intriguing that several of the major concentration peaks of MCA, DCA, TCA, and DBA coincided in time with increased electrical conductivity after major volcanic eruptions (Figures 2 and 7). It has previously been proposed that volcanic activity is a major source of volatile organohalogens, such as trichlorofluoromethane, carbon tetrachloride, and chloroform (23). We propose that further studies should be conducted to determine whether geological sources are also involved in the natural occurrence of haloacetates in precipitation.
Acknowledgments The authors are grateful for the financial support that enabled drilling of a firn core (EPICA, Environment and Climate) and chemical analysis of the collected samples (Swedish Environment Protection Agency and EUROCHLOR, Brussels). We would also thank A. Du ¨ ker for drafting Figures 1 and 3; M. Stenberg and L. Karlo¨f for kindly providing detailed information regarding the sampling; and M. R. van den Broeke, L. Conrads, T. Eiken, R. Hurlen, G. Johnsrud, L. Karlo¨f, S.
Onarheim, C. Richardson, R. Schorno, and J.-G. Winther for participating in the NARE 1996-1997 expedition. This is Contribution No. 158 of the Norwegian Antarctic Research Expedition.
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Received for review November 23, 1998. Revised manuscript received September 30, 1999. Accepted October 11, 1999. ES9812150
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