Environ. Sci. Techno/. 1995, 29, 1297-1302
Enantioselective Breakdown of a-Hexachlorocyclohexane in a Small Arctic bike and Its R E N E E L. FALCONER,*,+ TERRY F. BIDLEMAN,t DENNIS J. GREGOR,$ RAY SEMKIN,§ AND C A M I L L A T E I X E I RAS Atmospheric Environment Service, Downsview, Ontario M3H 5T4, Canada, Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada, and National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario L7R 4A6,Canada
Water and snow samples were collected at Amituk Lake on Cornwallis Island t o investigate the enantioselective degradation of a-hexachlorocyclohexane (a-HCH) in the Arctic. The two enantiomers were separated by gas chromatography on permethylated cyclodextrin capillary columns. The enantiomeric ratio (ER = (+)a-HCH/(-)a-HCH) for an a-HCH standard was 1.00 f 0.005, which is in excellent agreement with a theoretical ER of 1.00 for unmetabolized a-HCH. ERs of snow samples were racemic (0.98 f0.03). Degradation was found in Amituk Lake at 15-21 m where ERs were 0.77 f 0.004; however, stream runoff and lake outflow ERs varied considerably during the study. ERs of the outflow traced the meltwater running over the surface of the lake, being close to streamwater values during peak runoff and returning to deep lake water values during low flow. Streamwater ERs decreased within a few weeks of snowmelt and showed a large variability(0.970.62), which may be due to the differences in temperature and amount of suspended sediments. The rapid enantioselective breakdown of a-HCH suggests that the ability of arctic microbial systems to degrade organic contaminants is greater than commonly thought.
Introduction Long-range transport and deposition of organochlorine (OC) compounds have resulted in widespread contamination of arctic ecosystems (1-3). The most abundant OC pesticide in air and water in northern latitudes is hexachlorocyclohexaneor HCH (4-6). The technical HCH mixture, heavily used in Asian countries, contains 60-70% a-HCH, 5-12% P-HCH, and 10-15% yHCH (6) along with minor percentages of other isomers. Once introduced into an aquatic ecosystem, HCHs are subject to several removal processes: volatilization, photolysis, sedimentation, hydrolysis, and microbial degradation. The first two are operative only in the upper water column. Slow sedimentation ( 7 ) and hydrolysis (8)result in residence times in the mixed lgyer on the order of years to decades. Microbial degradation may compete with these other processes for removing HCHs, but little is known about its importance in arctic waters. Indeed, it has been suggested that the persistence of OCs in the polar environment is increased due to low temperatures, limited biological activity, and relatively small incidence of sunlight (9).
Several authors have shown the feasability of distinguishing between enantioselective microbial degradation and nonenzymatic degradation (nonenantioselective) by measuring enantiomeric ratios (ERs) of chiral pollutants using derivatized cyclodextrin capillary gas chromatography columns (10-15). Enantioselectivebreakdown of a-HCH, the only chiral isomer in technical HCH, has been found in surface waters of the North Sea (10,11,14),in laboratory cultures of marine bacteria (121, and in birds and marine mammals ( 1 3 , 1 4 , 1 6 ) . This study shows the first evidence of this phenomenon in a small arctic lake and influent streams.
Experimental Section Amituk Lake and Its Watershed. Amituk Lake (Figure 1) is on the east side of Cornwallis Island (75’02’57’’ N, 93”45’51”W). The lake has an area of 37.84 ha and mean and maximum depths of 19.4 and 43.0 m. The overall basin of Amituk Lake is approximately 26 km2 and contains six small watersheds of which Gorge, Cave, and Mud Creeks account for 78% of the drainage area. Snowmelt begins in mid- to late-June, and in 1992, peak discharge occurred during the first week in July (Figure 2). Thereafter streamflow diminished and ceased altogether in mid- to lateAugust with freeze-up. Flow was first detected on June 26 in Mud and Camp Creeks, which, because of their southern exposure,were several days ahead of Gorge Creek. Between June 26 and July 8, ice thickness on the lake ranged from 2.4 to 1.9 m. Through peak runoff, temperature profiles revealed a stratification in the upper 5 m with water temperatures approaching 0 OC beneath the ice cover and warming to 2.0 “C at depth. On July 8, the surface temperature increased with the influx of relatively warm * Address correspondence to this author at her present address: Department of Chemistry,Youngstown State University,Youngstown, OH 44555. t Atmospheric Environment Service. University of Waterloo. 5 Canada Centre for Inland Waters.
*
0013-936)(/95/0929-1297$09.00/0
@ 1995 American Chemical Society
VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1297
k= 7
/ b
i
Cornwallis Island
FIGURE 1. Amituk lake and its watershed.
streamwater (Mud Creek recorded a temperature of 14.8 "C on July 71, higher air temperatures, and increased solar radiation. Sample Collection. Water samples (40 L) were collected through the ice at 20 m into metal containers by means of a submersible, magnetic drive pump. Lake outlet and streamwater were surface samples. Shallow (30-40 cm thick) and deep (2 m thick) sections of the snowpack were collected and shipped in sealed40-Lmetalcases. Collection dates and locations are given in Table 1. Extraction and Cleanup. Lake, stream, and melted snow samples were processed by passing 40 L of unfiltered water through a Goulden liquid-liquid extractor (17, 18) and concentrating the HCHs into 500 mL of dichloromethane (DCM). The snowpack samples were melted inside the sealed cases at the field in a circulating water bath at 17 "C and extracted by the Goulden method. Extracts were reduced into 20 mL of hexane, and a base wash was performed using 0.1 M potassium carbonate solution. The extracts were then run through a 10-g silica gel column, and the HCH fraction was eluted with 85 mL of 35% DCMhexane. Samples were reduced to 1 mL in isooctane, shaken with 0.5 mL of 18 M sulfuric acid, and then adjusted to a suitable volume for analysis by nitrogen blowdown. Analysis. Determination of a-HCH enantiomers was done using a Hewlett Packard 5890 gas chromatograph with 63Nielectron capture detection on Gammadex-120 and 1298 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5,1995
Betadex-120 columns. Both columns were 20% permethylated y- or .B-cyclodextrinin phenyl polysiloxane (30 m length, 0.25 mm i.d., 0.25 pm film thickness; Supelco). The samples (1 pL) were injected splitless (split opened after 1 min) at an oven temperature of 90 "C, held 1 min, ramped 20 "C min-l to 130 "C, ramped 2 "C min-l to 170 "C, 15 min hold, and ramped 15 "C min-' to 210 "C. The carrier gas was hydrogen at 60 cm s-l, injector temperature was 220 "C, and detector temperature was 300 "C. Chromatographic data were collected using a Hewlett Packard chemstation. Confirmationwas done for selected samples by electron impact (EI/MS) or negative ion (NI/MS) mass spectrometry. EIlMS was done on a Hewlett Packard 5890/ 5790 GC-mass selective detector using the Gammadex-120 column. The same injection technique was used at an oven temperature of 100 "C, held 1 min, ramped at 20 "C min-I to 150 "C, ramped 2 "C min-l to 190 "C, 15 min hold, and ramped 30 "C min-' to 210 "C, 5 min hold. The carrier gas was helium at 40 cm s-l, injector temperature was 220 "C, and transfer line temperature was 200 "C. HCH ions 181 and 183 were monitored. NUMS was done on a HewlettPackard 5890/5989B MS engine using the Betadex column. Samples were injected splitless at an oven temperature of 90 "C, held 1min, ramped at 15 "C min-' to 140 "C, ramped 1 "C min-' to 220 "C, and ramped 20 "C min-' to 240 "C. The carrier gas was helium at 40 cm s-l. The injector and transfer line temperatures were 250 "C, and the ion source temperature was 150 "C. Methane at 1.8 Torr was used as the reagent gas. HCH ions 255 and 257 were monitored. Quantitative analysis for a-HCH and y-HCH was done using a Hewlett Packard 5890 GC equipped with dual ECD detectors on SPBl and SPB5 columns (0.25 mm i.d., 0.25 pm f ilmthickness; Supelco). Samples were injected splitless (split opened after 30 s) at an initial temperature of 80 "C, held for 2 min, ramped 4 "C min-' to 280 "C, and held 12 min. Injector and detector temperatures were 220 and 350 "C, respectively. The carrier gas was helium at 30 cm s-l. Octachloronaphthalene was used as an internal standard. Concentrations of Ca'', Mg2+,alkalinity, and pH were measured in lake and streamwater (Table 21 using standard water quality methods (19).
Results and Discussion Quality Control. Three deionized (Milli-Q)water samples were extracted as blanks, and mean f sd concentrations were 0.030 f 0.011 ng L-' a-HCH and 0.042 f 0.042 ng L-' y-HCH. Recoveries were assessedby spikingthe DCM used to extract each sample with 2,3,4,5-tetrachlorobiphenyl. Mean yields of this surrogate were 76 f 34%. Triplicate snow sampleswere extracted on five occasionsto determine analytical precision. The pooled coefficients of variation for these replicates were 38% for a-HCH and32%for y-HCH. Concentrationsof HCHs in Amituk Lake and Snow. A summary of quantitative results for HCHs in various water types is given in Table 2. The average concentration of ay-HCH in Amituk Lake (2.4 ng L-l) was about half that in seawater from Resolute Bay off Cornwallis Island (20). Snowmelt is the main source of HCHs to Amituk Lake, and levels in the snowpack varied greatly with snow depth and time of sampling. Deep snow samples collected before the beginning of melting contained 4.3 & 3.3 ng L-' a-HCH and 1.8 It 1.1ng L-l y-HCH. A single snow sample taken during post-melt conditions contained 0.46 ng L-l a-HCH and 0.23 ng L-' y-HCH. Lower levels of HCHs were also found in shallow snow before melting: 0.22 f 0.10 ng L-l
+
3
-.-
2.5 2
I
n
n
i
:i_
n
n
0.5 0
177 178
179 180 '181 ' 182 ' 183 ' 184 ' 185 ' 186 ' 187 '188 ' 189 190 191
Julian Day 1992
FIGURE 2. Discharge summary of streams influent to Amituk lake for 1992. TABLE 1
Collection Data and Enantiomeric Ratios (ER) of a-HCH in Snow and WateP date (Julian date)
location
web
Batadex ECD
May 10 (1311 May 11 (132) June 20 (172) June 23 (173) June 23 (173) June 23 (173) June 29 (179) May 12 (1331 May 12 (1331 May 12 (1331 July l ( 1 8 3 l
Amituk Lake Golden Pond Golden Pond Rock Creek Rock Creek Rock Creek Rock Creek Amituk Lake Amituk Lake Amituk Lake Amituk Lake
ss ss ss
1.01 1.00 1.00 0.98 0.99 1.oo 0.91
July 8 (1901 June 30 (1821 July 4 (1861 J u l y 7 (1891 J u l y 9 (1911 Aug. 1 5 (2281 Aug. 16 12291 June 26 (178) July 2 (1841 July 8 (1901 June 30 (182)
Amituk Lake outflow outflow outflow outflow outfiow outflow Mud Creek Mud Creek Mud Creek Gorge Creek
July 4 (186) July7 (189) June 30 (1821
Gorge Creek Gorge Creek Cave Creek
W
July 3 1185) July 8 (190)
Cave Creek Cave Creek
W W
DS DS DS
ss W W W W
Cammadex ECD
Gammadex EIiMS
Beladex NIWS
1.02
0.77 0.78 0.77 0.77 0.76c
W
0.77 0.74
W
W W W W W W W
W W
0.77 0.86
0.85 0.85
0.74 0.77 0.95 0.82 0.62 0.97
0.79 0.926
W W
0.92 0.84 0.88
0.8Ee
0.88 0.74
0.76
* ER = l+)-o-HCH/i-)-a-HCH. determined on different columns with electron capture detection IECD). electron impact mass spectrometry IEUMSI, ornesativeionmassspectrometryINI/MSi. ~SS=shallowsnow,DS=deepsnow. W=water. EC~mbinedsamplesfromJulyland& *Combined Samples from June 30 and July 4. *Combined sampler from June 30 and July 3.
a-HCH and 0.25
+ 0.16 ng L-' y-HCH.
suggest t h a t volatilization
These differences of HCHs takes place from t h e
I
snowpackastemperaturesriseabovefreezingandalsofrom exposed shallow s n o w layers even before melting begins. VOL. 29. NO. 5.1995 IENVIRONMENTAL SCIENCE &TECHNOLOGY. 1299
TABLE 2
Concentrations of HCHs and Major Ions in Snow, Amituk Lake, and Streamsa snowC shallow (4) deep ( 13 Amituk Lake 15-21 m (6) lake outflow (4) Cave Creek ( 4) Gorge Creek ( 3)
M u d Creek ( 3)
a-HCH (ng/L)
y-HCH (ng/L)
0.16-0.34 0.22 f 0.10 0.92-12.7 4.3 k 3.3 1.3-3.4 2.1 ?c 0.8 0.9-2.7
0.12-0.43 0.25 f 0.16 0.50-4.4 1.8 f 1.1 0.21 -0.60 0.32 f 0.15 0.1-0.5
1.9-3.9 2.7 k 0.9 3.2-9.0 5.2 f 3.3 0.35-5.8 2.5 f 2.9
0.33-0.85 0.55 f 0.23 0.56-2.1 1.1 f 0.8 0.06-1.3 0.51 f 0.65
Ca (mg/L)
Mg (mg/L)
alkalinity* (mequiv/L)
22.1
4.3-4.5
0.85- 1.8
14.6-21.0
3.0-3.9
0.86- 1.3
15.3-20.6
2.2-2.9
0.92- 1.2
2.1-2.8
0.68-0.73
2.5-2.7
1.1-1.3
9.9-1 1.3 20.9-22.2
a Range, mean f standard deviation, number of samples for HCH analysis in parentheses. bThe pH of all lake and stream samples was 8.1-8.2. Collected during premelt conditions.
This phenomenon has been previously observed in the Canadian Arctic. Snow sampled in 1986 from the Agassiz Ice Cap on Ellesmere Island contained 6.6 ng L-' a-HCH and 4.1 ng L-' y-HCH, whereas residues in the same snow layer a year later were only 3-8% of these values (21).New snow falling on the Canadian Ice Island in 1987 contained higher levels of HCHs than exposed surface snow (5). The lower concentrations of HCHs in Amituk Lake compared to Resolute Bay may be due to volatilization losses from the snowpack and to increased degradation in the lake and watershed. Another factor may be air-water gas exchange, which is a major contributor of OCs to ocean surface waters but of minor importance for Amituk Lake due to ice cover, even during summer. Determination of Enantiomeric Ratios. The enantiomeric ratio (ER) is defined here as the area ratio of the (+)a-HCH/(-)a-HCH peak eluting on the permethylated cyclodextrin columns. Confirmation of elution order was done by injecting a standard enriched by ~ 3 0 % in the (-)enantiomer, prepared by dehydrochlorination of racemic a-HCH with brucine (10,22). The elution order is reversed on the two columns, with (-)a-HCH eluting first on the Gammadex and (+)a-HCH first on the Betadex column, shown in Figure 3 for a brucine-treated standard. Chromatograms of a-HCH enantiomers in air and water from Amituk Lake and Resolute Bay are shown in ref 20. Faller et al. (10) used a B-cyclodextrin column and found (+)aHCH to elute first. Assignment of (+)- and (-)-a-HCHs has been achieved by chiroptical detection by Buser and Muller (23),who also found elution reversals on different chiral GC columns. a-HCH occurs in the technical mixture as a racemic mixture of the two enantiomers. If no metabolism occurred, the ER would be 1.00. Our a-HCH standard showed an average ER of 1.00 f0.005 on both the Betadex and the Gammadex columns with baseline separation. At different stages of this work, separations were done on Betadex or Gammadex columns. In a few samples, an interference was noted as a small shoulder on one of the enantiomer peaks on Gammadex, and in these cases, peak height rather than area was used in calculating ERs. A comparison of ERs using different analytical methods is given in Table 1. Although the same samples were usually not run on both columns, ERs of the same water types agreed within 1.3%on the two columns. Several combined 1300 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5, 1995
20.0
21.o
20.5 (-)
Beta-dex
I
1
I
22.0
22.5
23.0
Minutes
FIGURE 3. Chromatograms of an a-HCH standard partially dehydrochlorinated with brucine, showing the elution order of the two enantiomers on Gammadex and Betadex columns.
samples were analyzed by EIlMS or NI/MS for confirmation, and the ERs compared favorably with ECD results. Enantiomeric Ratios in Amituk Lake Basin. The ERs of a-HCH in selected samples representing various water types are given in Table 1. Snowpack ERs (0.98 =k 0.03) were racemic with no difference between deep and shallow samples. ERs for streamwater started out high and decreased over the period of the study (Figure4), indicating that enantioselective breakdown of a-HCH occurred to some extent before reaching the lake. This degradation may be related to the warming of the streams and the thawing ofthe sediments. For the three biggest contributors of flow (Gorge,Cave, and Mud Creeks),ERs were measured during the last week of June and the first and second weeks of July. The change in ER was lowest in Gorge Creek, which accounted for 53% of total inflow to the lake. Mud Creek
-20
0.9
0.9 25
0.8
0.8
E
0.7
2 W
O c a + Mg
mER 0.85
$ E
0 .0.7
t
;"
E
20
0.80
15
0.75
+
-0.6 -L
0.6
+
8
I 0 I
b
Y
0.5
June 26-30
July 2-4
1 =Gorge Ck. 0 Cave Ck.
July 7-8 Mud Ck.
0.5
I
FIGURE 4. ERs for the three major tributaries from latedune io mid-July.
showed the widest range of ERs starting at 0.95 Dune 26) and decreasing to 0.62 Uuly 8). It also had the widest temperature range over this period (0.8-14.8 "C) and the highest amount of suspended sediment of the three streams. ThetemperaturerangeforCaveandGorgeCreekswasO-3.9 "C. These differences may explain the more extensive degradation of a-HCH in Mud Creek. The Amituk Lake ERs (at 15-21 m) were essentially constant over the time periodofthework (range=0.77-0.78), whereaslakeoutilow ERs varied with stream runoff. Chemical weathering of limestone and dolomite in the Amituk Lake basin is reflected in the high concentrations of Cazf, Mg2+,and alkalinity (HC03-I in the water (Table 21, which on an equivalent basis constitute 95% and 90% of the cation and anion content of the lake water, respectively. The streahwater major ion composition is also indicative of limestone weathering of the watershed withCaz+,Mg2', and alkalinitydominatingtheioniccontent at all sites. Gorge Creek contained lower levels of these ions, suggestingdecreasedweatheringrates in this subhasin. Iackofverticalmixingbetweenlakewater andstreamwater during peak runoffwas indicated by the temperature profile and by lower Ca2+and Mg2+concentrations at the surface. The outflow ERs traced the cold meltwater running over the surface of the lake. In late-June, when stream input was low, the outflow ER was 0.74, similar to that from the deep lake, indicating that the lake was well mixed with respect to HCHs. As streamwater input increased, outflow ERsincreasedtovaluesclosertothoseofthestreams (0.850.86). Using the volume-weighted flows of the streams at peak runoff and their ER values, the ER of the outflow was predicted to be between 0.81 and 0.91. By mid-August, after stream inflowhad ceased, the oumow ERs were once again indicative of lake water (0.74-0.77). Other species, such as Ca2+ M$', show this same dilution of lake surface water by meltwater (Figure 5). Concentrations of these ions in the lake outflow were lower during periods of high runoff, reflectingtheir lower concentrations in streamwater than in the deep lake. Little is known about the mechanisms of these transformations. Measurements of ERs in ambient air, where chances for metabolism are low, were 1.00 5 0.04 on Cornwallis Island (20)and 1.020 inNorway (13). Seawater samples in Resolute Bay off Cornwallis Island showed a 10-15Wdepletionof (+I-a-HCHrelativeto (-)-a-HCH (20). Samples from the North Sea showed degradation of the
+
."
rn
n 7"
".I"
June 30
July 4
July 7
July 9
August 15
FIGURE 5. Dilution of lake water by meltwater at the outflow. Note that lower concentrations of Ca2++ Mgf+ in stream runoff coincide with higher ERs.
(+I-enantiomerinsome regions andofthe (-)-enantiomer in others (10.1 I ) . Reasons for these differences are unclear; however, depletionofthe (-I-enantiomeris seen in marine mammals and birds (13,14,16')as well as some soils (13). In the case of biota, these differences may result not only from selective metabolism but also from the ability of one enantiomer to cross membranes more easily than the other (24). Ludwig et al. (12) investigated chiral breakdown of a-HCH in aerobic laboratory cultures of bacteria from the North Sea and found a slow loss of the (+)-enantiomer along with production of chiral pentachlorocyclohexenes. No evidence for enan>ioselective breakdown other than biological-mediated reactions has been reported.
Conclusions Although enantioselective breakdown of a-HCH has been reportedinseawater(l0,ll.14,20),AmitukLakeisthefirst freshwater system where chiral degradation has been investigated. Differences in ERs up to 35% found in the lake system are much greater than the 5-1596 changes in seawater. The rapid chiral breakdown that occurs in the creeks within a few weeks of snowmelt suggest that the ability of arctic microbial systems to degrade organic contaminants is greaterthan commonlythought. Because ERs can be determined more preciselythan concentrations, they areuseful for followingwater movements from streams into Amituk Lake and possibly in other aquatic systems.
Acknowledgments We thank Bob Rowsell, Roy Neureuther, Peter Amarualik, and Ipeelee Itorcheak, National Water Research Institute, EnvironmentCanada, for sample collection. This workwas supported by Indian & Northern Affairs Canada and Polar Continental Shelf Project.
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(6) Iwata, H.; Tanabe, S.; Sakai, N.; Tatsukawa, R. Enuiron. Sci.
Technol. 1993, 27, 1080-1098. (7) Tanabe, S.; Tatsukawa, R.J. Oceanogr.Soc. Jpn. 1983,39,53-62. (8) Ngabe, B.; Bidleman, T. F.; Falconer, R. L. Enuiron. Sci. Technol. 1993, 27, 1930-1933. (9) Wania, F.; Mackay, D. Ambio 1993, 22, 10-18. (10) Faller, 1.; Huhnerfuss, H.; Konig, W. A.; Krebber, R.; Ludwig, P. Enuiron. Sci. Technol. 1991, 25, 676-678. (11) Faller, J.; Hiihnerfuss, H.; Konig, W. A.; Ludwig, P. Mar. Pollut. B d l . 1991, 22, 82-86. (12) Ludwig, P.; Hiihnerfuss, H.; Konig, W.A.; Gunkel, W.Mar.Chem. 1992, 38, 13-23. (13) Muller, M. D.; Schlabach, M.; Oehme, M. Enuiron. Sci. Technol. 1992, 26, 566-569. (14) Pfaffenberger, B.; Hiihnerfuss, H.; Kallenborn, R.; KohlerGunther, A.; Konig, W. A.; Kriiner, G. Chemosphere 1992, 25, 719-725. (15) Buser, H.R.;Muller, M. D.Enuiron. Sci. Techno[.1993,27,12111220. (16) Mossner, S.; Spraker, T. R.; Becker, P. R.; Ballschmiter, K. Chemosphere 1992, 24, 1171- 1180. (17) Neilson, M. A,; Stevens, R. J. I.; Biberhofer, J.; Goulden, P. D.; Anthony, D. H. J. Tech. Bull. 1988, 157. Y
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(18) Foster, G. D.; Rogerson, P. F. Int. 1. Enuiron. Anal. Chem. 1990, 41, 105. (19) Anonymous. Unpublished report for National Water Research Institute,Canada Centre for Inland Waters, Environment Canada, Burlington, Ontario, Contribution 85-121, 1987. (20) Falconer, R. L.; Bidleman, T. F.; Gregor, D. J. Sci. Total Enuiron. 1995, 1601161, 65-74. (21) Gregor, D. J. In Pollution ofthe Arctic Atmosphere; Sturges, W. T., Ed.; Elsevier Applied Science: 1991; pp 217-254. (22) Cristol, S. J. J. Am. Chem. SOC.1949, 71, 1984. (23) Buser, H. R.; Muller, M. D. Enuiron. Sci. Technol. 1995,29,664672. (24) Moller, K.; Hiihnerfuss, H.; Rimkus, G. 1. High Resolut. Chromatogr. 1993, 672-673.
Received for review August 8, 1994. Revised manuscript received January 17, 1995. Accepted Ianuary 26, 1995.@
ES9405078 @Abstractpublished in Advance ACS Abstracts, March 1, 1995.
