Anal. Chem. 1997, 69, 4259-4266
Sequential Determination of Chlorine and Carbon Isotopic Composition in Single Microliter Samples of Chlorinated Solvent N. Jendrzejewski,*,† H. G. M. Eggenkamp, and M. L. Coleman
University of Reading, Postgraduate Research Institute for Sedimentology, Whiteknights, P.O. Box 227, Reading, Berkshire RG6 6AB, UK
Chlorinated hydrocarbons, significant environmental pollutants, may be characterized by stable isotopic compositions of carbon and chlorine. Previously published analytical methods needed separate preparations for carbon and chlorine and were not ideal for environmental studies because of low sensitivity and precision. This method quantitatively extracts both carbon and chlorine from a single aliquot of chlorinated solvent of a size practically applicable to natural systems. Samples of 1-2 µL of chlorinated solvent are combusted in sealed tubes with cupric oxide at 620-820 °C. After separation of CO2 for isotopic analysis of carbon, the residue contains all the chloride in water-soluble form. It is then processed by conventional methods for chlorine isotopic analysis and gives a yield of 97 ( 27% (SD) on standard trichloroethylene (TCE) and 97 ( 4% on standard perchloroethylene (PCE) for the whole of the Cl preparation process. TCE and PCE laboratory standards show reproducibilities (SD) for both products of 0.08‰ for δ13C and 0.15 and 0.10‰, respectively, for δ37Cl. The technique is applicable to all five types of chlorinated solvents investigated (TCE, PCE, dichloromethane, 1,1,1-trichloroethane, and chloroform) and was tested by application to an environmental sample of a vacuum-extracted mixture of chlorinated solvents from a polluted site, giving excellent reproducibilities for both carbon and chlorine isotopic compositions (respectively (0.05 and (0.08‰). Chlorinated hydrocarbons (e.g., trichloroethylene, TCE; perchloroethylene, PCE; dichloromethane, DCM; 1,1,1-trichloroethane, TCA; chloroform, CFM; chloromethane, CM; carbon tetrachloride, CTET; and dichloroethane, DCA) are extensively used in many industries. Examples include use in dry cleaning, degreasing metal after machining or before painting in the car manufacturing industry, organic synthesis, and the paint industries. Extensive and often careless use, for more than a century in some cases, has resulted in seepage into the ground of large amounts of chlorinated solvents at many sites. From the vadose zone, the solvents may permeate to the water table. For example, in one TCE plume at the Savannah River Site, South Carolina, it was estimated that 31 kg of chlorinated solvent was present as dissolved phase.1 Since the 31 kg of chlorinated solvent represents only a small fraction of what had undergone flow through † Present address: Universite ´ Paris 7, I.P.G.P., Laboratoire de Ge´ochimie des Isotopes Stables, T54-64 1er e´tage, 2, place Jussieu, 75 251 Paris cedex 05. E-mail:
[email protected].
S0003-2700(97)00447-2 CCC: $14.00
© 1997 American Chemical Society
approximately 15 m of overlying sands, silts, and clays without being degraded yet, it can be seen that this may be a significant problem. Although these chlorinated solvents can contaminate water supplies, predicting the potential pollution migration and biochemical evolution is hampered by insufficient definition of the relevant processes. Tracing sources and monitoring the processes that reduce concentrations of these contaminants are essential precursors to making decisions on methods of treatment or predicting possible access to water resources. Cl isotope compositions may differentiate pollutants and natural sources. Small variations in natural abundances of stable isotopes characterize sources and reactions of H, C, N, O, and S. Isotope compositions are expressed in the usual δ notation, defined as parts per thousand, or per mill (‰), variation of the isotopic ratio of interest, relative to that ratio in an international standard material, e.g.,
δ13C ) (Rsample/Rstandard - 1) × 1000
where, Rsample and Rstandard are respectively 13C/12C ratios of the sample and standard. The international standard for carbon is the Pee Dee Belemnite (PDB). More recently, methods have been developed for Cl stable isotope analysis,2-4 where the ratio of interest is 37Cl/35Cl and the results are presented as δ37Cl, but there is no international standard for chlorine. However, it has been shown that chlorine in seawater and deep ocean water is isotopically homogeneous on a worldwide scale,2,5 with variations less than analytical precision, and standard mean oceanic chloride (SMOC) has been proposed as the reference standard. With few exceptions, the range of variation of chlorine isotopic compositions in sedimentary materials is very small (classically from -2 to +1‰ SMOC). Kaufmann et al.2 showed that ancient evaporites had δ37Cl values similar to those of present-day seawater. More recently, Eggenkamp et al.6 analyzed ancient evaporites which gave a mean (1) Nichols, R. L.; Looney, B. B.; Huddleston, J. E. Environ. Sci. Technol. 1992, 26, 642-649. (2) Kaufmann, R. S.; Long, A.; Campbell, D. J. Am. Assoc. Petrol. Geol. Bull. 1988, 72, 839-844. (3) Long, A.; Eastoe, C. J.; Kaufmann, R. S.; Martin, J. G.; Wirt, L.; Finley, J. B. Geochim. Cosmochim. Acta 1993, 57, 2907-2912. (4) Eggenkamp, H. G. M. δ37Cl: The geochemistry of chlorine isotopes. Ph.D. Thesis, University of Utrecht, The Netherland, 1994. (5) Kaufmann, R. S.; Long, A.; Bentley, H.; Davis, S. Nature 1984, 309, 338340. (6) Eggenkamp, H. G. M.; Kreulen, R.; Koster van Groos, A. F. Geochim. Cosmochim. Acta 1995, 59, 5169-5175.
Analytical Chemistry, Vol. 69, No. 20, October 15, 1997 4259
value of -0.27‰ (σ25 ) 0.19‰). Exceptions to this small range of variation are deep subsurface brines associated with oil reservoirs.7 Thus, most natural sources of chlorine in natural environments at risk from pollution have a characteristic small range of chlorine isotopic compositions, offering the possibility of differentiating them from chlorine produced from degradation of chlorinated solvents. Only two recent studies report chlorine isotopic compositions of chlorinated solvents.8,9 Tanaka and Rye8 reported chlorine isotopic compositions of four chlorinated solvents but did not determine carbon isotopic compositions. The analytical method involved heating the solvent with lithium metal in a sealed tube in order to form LiCl which, therefore, can be put into aqueous solution. The chloride is recovered by precipitation as silver chloride. This method was applied to samples of the size of 100 µmol of Cl. Unfortunately, this technique would require an additional δ13C determination for fuller characterization of the chlorinated solvents, but no carbon isotopic compositions were reported nor details of the manufacturers. This pioneering work is, nevertheless, very interesting, as it suggests a very large range of variation of industrial solvent δ37Cl, from -6.82 to +2.61‰, interpreted as the result of kinetic isotopic fractionation of chlorine during the synthesis of the chlorinated solvents. More recently, Van Warmerdam et al.9 investigated chlorine and associated carbon isotopic compositions of three different chlorinated solvents produced by identified manufacturers. The chlorine extraction from the chlorinated solvents was performed via combustion in a Parr 1901 bomb under an O2 pressure of 20 atm and in the presence of a 5% calcium carbonate solution in order to absorb the combustion products. The CO2 was obtained via another combustion of the chlorinated solvent with copper oxide and silver wire in borosilicate glass sealed tubes at 550 °C following the procedure of Boutton et al.11 This CO2 was then cryogenically purified for isotopic measurements. δ37Cl, determined on TCA, TCE, and PCE, varies from -2.90 to +4.08‰, and the δ13C data also show a relatively large range of variation, from -37.20 to -23.19‰. Moreover, their data suggest that each manufacturer and chlorinated solvent type has distinct ranges of δ37Cl and δ13C. Thus, the isotopic data could potentially be used as tracers of specific contaminants. In both of these previous studies, the analytical techniques involved still require separate experiments for the extraction of carbon converted to CO2 and of chlorine converted to CH3Cl. Moreover, both methods described by Van Warmerdam et al.9 for carbon and for chlorine analyses are potentially flawed or contain some omissions (see Discussion). The principal concern is with determinations of the chlorine isotopic compositions for which very low yields (65-75%) are reported, associated with a large range of reproducibilities (from (0.10 to (0.68‰). An additional weakness of this technique is the relatively large amount of chlorinated solvent required, making this method difficult to apply to environmental samples. Aliquots of up to 50 µL of chlorinated solvent were used, although the exact volumes were not stated. Moreover, this technique still requires two different aliquots of chlorinated solvent, one for carbon extractions and the (7) Eggenkamp, H. G. M.; Coleman, M. L. Bull. Am. Assoc. Petrol. Geol. 1993, 77, 1620. (8) Tanaka, N.; Rye, D. M. Nature 1991, 353, 707. (9) Van Warmerdam, E. M.; Frape, S. K.; Aravena, R.; Drimmie, R. J.; Flatt, H.; Cherry, J. A. Appl. Geochem. 1995, 10, 547-552.
4260 Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
Table 1. Results Obtained from Diluted Solutions without Reconcentration run
Cl (ppm)
δ37Cla (‰/SMOC)
MS precision (std error)
1 2 3 4 5 6 7
76 57 57 38 19 7.6 7.6
-0.06 +0.05 +0.07 -0.01 +0.01 +0.35 +1.02
0.022 0.023 0.017 0.010 0.013 0.021 0.004
a δ37Cl are blank-corrected from the deionized water chloride measured independently: 4 ppm Cl (equivalent to 3 ( 1 µmol of Cl contribution in 25 mL) with an isotopic composition of δ37Cl ) -2.60‰. The MS precision given is within-run mass spectrometer precision.
other for chlorine (double extraction from natural samples). Average levels of chlorinated solvents in ground water and soils vary from fractions of a microgram per liter to several micrograms per liter. However, they can reach a few thousands of micrograms per liter in extremely polluted areas. Investigation of water polluted with 500-1000 µg/L of chlorinated solvent would involve processing 50-100 L of water to give the 50 µL necessary for a single analysis of chlorine. This demonstrates the need for a more sensitive but accurate method. EXPERIMENTAL SECTION Chlorine in Aqueous Solutions, in Small Amounts and in Low Concentrations. In tracking pollution and degradation of chlorinated solvents in waters, it is necessary to characterize chloride in aqueous solution and relate it to a possible chlorinated solvent precursor. Therefore, it was necessary to determine the lowest concentration of chloride that can be measured in solution. The limiting step in the analytical procedure appears to be the precipitation of AgCl (see refs 3 and 4 for a detailed description of the analytical procedure). The mass spectrometer used in our laboratory is a dual-inlet isotope ratio VG SIRA 12 and allows isotopic analysis of samples down to 4.5 µmol. Two types of limits were determined: first, the lowest concentration of chloride in solution that can be analyzed without preconcentration and the associated uncertainty in δ37Cl; second, in the case of large volume availability, the lowest concentration of chloride in solution that can be analyzed after preconcentration (by evaporation) and its δ37Cl uncertainty. (i) Running Small Samples. Chloride solutions were prepared by mixing various quantities of our internal standard seawater (slightly more concentrated in Cl compared to average seawater, 22 500 ( 500 ppm Cl determined independently by ion chromatography) with 25 mL of deionized water. Five different concentration solutions were prepared (Table 1). The AgCl precipitation step was performed using dry chemicals (for ionic strength adjustment and pH buffer, see the Chlorine Extraction section) instead of aqueous solution to avoid enlarging the final volume of solution. Two milliliters of (0.2 M)AgNO3 was added in solution to precipitate the AgCl. After the precipitation of AgCl, the solution was filtered over a Whatman GF/F 0.7 µm glass filter, which was subsequently dried overnight in an oven at 75 °C. The direct comparison of the filter weight with and without the precipitate allowed a quick estimate of the yields of extraction of chlorine from the solution. This determination was accurate enough for chlorine solution of
Table 2. Comparison of δ37Cl Obtained on Standard Seawater (Undiluted) and on Diluted Solutions (1 and 0.5 ppm Cl) after Evaporitic Reconcentration volume of seawater (µL) 10 10 10 12 20 20 mean ( SD a
δ37Cl (‰/SMOC) undiluted -0.45 -0.07 -0.11 +0.19 -0.11 ( 0.26
diluted -0.01a +0.22a +0.28b +0.02b +0.03a -0.20a -0.06 ( 0.17
1 ppm Cl. b 0.5 ppm Cl.
g50 ppm Cl. The yields obtained were typically 103 ( 3% on both low-concentration solutions and reference seawater standards. These yields, slightly over 100%, probably reflect the small uncertainty in the seawater standard chlorine concentration and the potential addition on the filter of small particles (dust, fiber, silver, etc.). Solutions down to 20 ppm Cl could be analyzed satisfactorily without preconcentration (Table 1). Taking into account a constant blank of 3 ( 1 µmol of chlorine from the deionized water with a measured isotopic composition of -2.60‰, the isotopic compositions obtained on these standard solutions were not significantly different from the isotopic composition of the starting seawater standard (especially for solutions g20 ppm Cl). (ii) Evaporation Experiments. Experiments were performed to estimate the lowest concentration that could be analyzed for chlorine isotopic composition when large volumes of samples were available (Table 2). Dilute chloride solutions were prepared by mixing our laboratory seawater standard with HPLC-quality deionized/filtered water. No blank corrections were applied to the data reported in Table 2 because the HPLC-quality water contained no detectable levels of chloride. Various volumes of solutions from 190 to 570 mL were run, together with control experiments of equivalent quantities of pure seawater (Table 2). The diluted solutions were evaporated (without boiling) down to 25 mL. The AgCl precipitation was performed using dry chemicals instead of aqueous solution to avoid enlarging the final volume of solution. Three milliliters of (0.2 M)AgNO3 were added for the final step of the precipitation. Solutions down to chloride concentrations of 0.5 and 1 ppm Cl could be successfully analyzed after evaporitic preconcentration. Yields were determined by weighing the filter with its AgCl precipitate and ranged between 64 and 128%. It must be noted that apparent yields lower than 100% do not necessarily mean a smaller recovery of chlorine. The glass fiber filter pad may be damaged as it is removed from its holder, leaving part of it behind. δ37Cl and the associated standard deviation measured on the 0.5 and 1 ppm Cl solutions are not significantly different from the δ37Cl of the standard seawater (-0.06 ( 0.17‰). New Technique to Extract C and Cl from Chlorinated Solvents. (i) Specification of a New Method. The main concern was to develop a method giving yields for extraction of chlorine as close as possible to 100% (65-75% for the previous study of Van Warmerdam et al.9). Moreover, to be easily and routinely used, the method should be able to use no more analyte
than can be extracted from 5 L of water. Given the average levels of contamination (see introduction section), this is equivalent to a required sensitivity, for chlorine analysis, of 1-2 µL of analyte. If one wants to associate chlorine with carbon isotopic analysis, a technique allowing for the extraction of both carbon and chlorine from a single aliquot would be preferable: first to keep the same limits of water volume to work with and second to determine both isotopic compositions on strictly the same aliquot of analyte, thus minimizing any possible subsampling artefacts. Many sample combustion methods use borosilicate or silica tubing, so we tried both. Finally, our objective was to set up a technique easily adaptable to any stable isotope laboratory. The first possibility would have been to adapt to smaller analyte volumes the method of Tanaka and Rye,8 which involves the use of lithium metal. However, this method still would have required the extraction of twice the minimum amount of chlorinated hydrocarbons from a contaminated water and two different processes for extracting both carbon and chlorine. Moreover, the reagents used in Tanaka and Rye’s technique8 are potentially hazardous: lithium metal reacts with both air and water and requires rigorously maintained laboratory conditions which are not easily available. The method presented here allows the extraction of both carbon and chlorine from a single aliquot of analyte of the size required to be applicable to natural systems. All the chemicals and apparatus to be used are easy to handle and to set up. This method is thus easily adaptable to any laboratory. After optimization of all the experimental conditions (sample introduction, combustion tube material, temperature of combustion), it gives yields very close to 100% and reproducible chlorine and carbon isotopic compositions. In the absence of standard reference materials for isotopic compositions of chlorinated hydrocarbons, we used three criteria to judge the accuracy of results. If the preparation method gives low blanks, 100% yields for sample preparation, and reproducible results, then all of the element of interest will have been converted to the species to be measured in the mass spectrometer. Reproducibility of isotopic results confirms the efficiency of the method. Low yields often give more negative isotopic values as a result of kinetic isotopic fractionation effects: the first-formed product of a reaction is depleted in the heavier isotope. However, even consistent low yields tend to give poorer reproducibility. Therefore, low yields give worse precision and accuracy. (ii) New Method Description. Major steps of the method are summarized schematically in Figure 1. The basis of the method is that used for preparation of CO2 from organic matter by sealed-tube combustion with cupric oxide as the oxidant.10,11 However, the conditions of the preparation have been optimized so that all the chlorine from the analyte is trapped as soluble compounds (probably copper chlorides) and none is lost in the gas phase when the CO2 is separated. Cleaning and Preparation Procedures. Silica (6 mm o.d.) or borosilicate (9 mm o.d.) glass tubes, sealed at one end, are heated in air at 920 and 620 °C, respectively, prior to use. The analytes, kept in amber glass bottles in a refrigerator, are allowed to settle at room temperature for 1 h before injection under vacuum. The analyte is introduced in a gas-tight syringe that is rinsed 10 times (10) Sofer, Z. Anal. Chem. 1980, 52, 1389-1391. (11) Boutton, T. W.; Wong, W. W.; Hachey, D. L.; Lee, L. S.; Cabrera, M. P.; Klein, P. D. Anal. Chem. 1983, 55, 1832-1833.
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Figure 1. Diagrammatic summary of the procedure for stable isotopic analysis of chlorine and carbon from chlorinated solvent samples. After injection of the sample, the products of the combustion at 620-820 °C are treated first under vacuum (white domain) and subsequently in aqueous solution (gray-shaded area) (for details, see New Method Description section).
with acetone and degassed in an oven at 75 °C for more than 30 min and finally prerinsed with the analyte of interest. The copper oxide is analytical grade (BDH, Analar), wire form, heated to 900 °C in air, and kept in a closed container before use. Sealed Tube Preparation and Heating. Sample combustions are performed in either borosilicate or silica glass tubes. Approximately 1 g of copper oxide is loaded into a precleaned tube and evacuated. A known analyte volume (1-3 µL) is injected through a septum directly under vacuum using a gas-tight syringe (Teflon plunger) and trapped with liquid nitrogen (-196 °C) on the copper oxide in the bottom of the tube. After the analyte is completely trapped and the potential residual pressure pumped away (vacuum better than 1 × 10-9 bar), the other end of the tube is sealed under vacuum with an oxygen gas flame. The sealed tubes in which the CuO is regularly distributed are placed in individual stainless steel protecting tubes and introduced into the furnace at 400 °C (in a few of the earliest experiments, tubes were introduced at 300 °C). They are then slowly heated to the final temperature, at which they are kept for at least 1 h. Testing the optimum combustion temperature for carbon and chlorine led to 620 °C for borosilicate glass tubes and 720-820 °C for silica glass tubes. The tubes are then cooled slowly to room temperature before extraction of CO2 under vacuum. The pressure of oxygen released by the copper oxide to allow the combustion is a function of the temperature. Both the amount of oxidant (copper oxide) and the temperature for a given amount of analyte thus control the combustion yields. Combustion temperatures between 620 and 820 °C gave optimum chlorine yields on a series of TCE, PCE, and DCM samples from four different manufacturers. For volumes of analytes corresponding to an equivalent 40 µmol of chlorine, the best yields are achieved for a minimum of 0.8 g of copper oxide. This is particularly critical as far as dichloromethane is concerned but does not seem to be of the same importance for PCE combustion. CO2 Extraction for Carbon Isotope Analysis. The combustion tube is scored with a file, put in a flexible stainless steel tube cracker,12 and evacuated to e1 × 10-9 bar. The tube is cracked, and all the condensable gases are extracted by trapping with liquid (12) DesMarais, D. J.; Hayes, J. M. Anal. Chem. 1976, 48, 1651-1652.
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nitrogen. The CO2 is isolated and purified cryogenically; other gases potentially present (H2O, O2) are pumped away. The pure CO2 gas is then transferred cryogenically to a vessel fitted with a vacuum stopcock and analyzed isotopically for δ13C. In our study, the carbon isotopic analyses were performed on a triple-collector VG SIRA Series II isotope ratio mass spectrometer. Graphite samples, either laboratory standards or international standards (NBS 21 or USGS 24), were combusted in silica glass tubes at 950 °C and analyzed isotopically at the same time as the chlorinated hydrocarbons. Chlorine Extraction for Chlorine Isotope Analysis. All the residues of the combustion (glass + CuO and other solid residues) are recovered and transferred to a 25 mL beaker as soon as the vacuum is broken. They are then immediately rinsed with HPLCquality water and left in 15 mL of water for at least 15 h (a shorter time led to poorer chlorine yields). The rest of the procedure is similar to that used for preparation of chloride solutions for isotopic analyses.3,4 Quantitative recovery of chlorine by precipitation as AgCl requires controlled ionic strength and pH. Therefore, the solution is poured into another 25 mL beaker containing (in the dried form) 1.5 g of KNO3 and a pH 2.2 buffer (0.515 g of citric acid + 0.0175 g of Na2HPO4‚ 2H2O, ref 13), leaving most of the solid particles behind. The AgCl precipitation step is performed using dry chemicals instead of aqueous solution to avoid enlarging the final volume of solution. The solid residue and the first beaker are rinsed again several times with HPLC water, pouring the rinsings into the precipitation beaker to give ∼25 mL of solution finally. This solution is then heated slowly to 80 °C and kept at this temperature for approximately 1 h. When the chemicals are completely dissolved, 3 mL of 0.2 M AgNO3 solution is added to precipitate AgCl (ref 3) and the solution filtered on a glass fiber filter pad. One hundred microliters of CH3I is added to the dried filter pad with AgCl in a 12 mm o.d. borosilicate glass tube. This tube is then sealed under vacuum and kept out of the light at 70-80 °C for 48 h to produce CH3Cl. The CH3Cl formed is then separated from the excess CH3I and purified by gas chromatography using a double column filled with Porapak Q for subsequent isotopic analysis. For this work, the resultant, pure chloromethane gas was then analyzed isotopically on a triple-collector isotope ratio mass spectrometer (VG SIRA12) using a slightly modified positive ion method.4,14,15 The δ37Cl was determined using the ratio of masses 52 (CH337Cl+) and 50 (CH335Cl+); the effects of D/H, 13C/12C variation, and interference of CH37Cl+ on mass 50 are negligible. Safety Considerations. The different chlorinated hydrocarbons and the iodomethane analyzed or used in this study are toxic and may be carcinogenic. They should be handled in an appropriate ventilated device and disposed of accordingly. Liquid nitrogen used in cryogenic seperations and purifications may cause severe burns and should be handled with care. The reaction of conversion of AgCl to ClCH3 produced AgI as a byproduct, which is light sensitive and can, therefore, decompose into Ag and I (irritating for skin and eyes). Finally, vacuum lines and combustion tubes can implode or explode (in case of an excessive pressure or increase of pressure) and should be protected accordingly. (13) McIlvaine, T. C. J. Biol. Chem. 1921, 49, 183-186. (14) Kaufmann, R. S. Chlorine in groundwater. Stable isotope distribution. Ph.D. Thesis, University of Arizona, 1984. (15) Taylor, J. W.; Grimsrud, E. P. Anal. Chem. 1969, 41, 805-810.
Table 3. δ13C, δ37Cl, and Chlorine Yields of Laboratory Standards TCE and PCE Extractionsa T (°C)
CuO weight (g)
620 620 620 720 720 720 820 820
0.86 0.96 0.89 0.85 0.90 0.98 0.91 0.91
δ13C (‰/PDB)
δ37Cl (‰/SMOC)
CH3Cl (µmol)
yield (%)
-33.65 -33.56 -33.43 -33.49 -33.42 -33.44 -33.47 -33.44
+3.80 +3.53 +4.07 +3.80 +3.79 +3.88 +3.92 +3.79
66.2 46.4 46.4 66.2 49.7 46.4 64.6 46.4
67.2 83.9 119.8 53.4 107.9 127.2 91.3 123.9
-33.49 0.08 -33.40 0.15
+3.82 0.15 +3.85 0.27
-23.95 -24.15 -24.07 -24.08
+0.91 +1.06 +1.15 +1.01
-24.06 0.08 -24.07 0.08
+1.03 0.10 +0.96 0.16
TCE S S S S S S S S
mean (620-820 °C) SD (n ) 8) average of combustions at all temperatures (570-920 °C) SD (n ) 17)
96.8 27.4 93.0 25.6
PCE B S S S
620 620 720 820
0.71 0.84 0.82 0.89
mean (620-820 °C) SD (n ) 4) average of combustions at all temperatures (570-920 °C) SD (n ) 7)
38.9 77.8 77.8 77.8
93.6 102.0 95.9 97.2 4.3
a Combustion temperatures, key factors of the procedure, are indicated. B, borosilicate glass; S, silica glass. All samples were injected under vacuum.
RESULTS AND DISCUSSION Optimization and Validation Tests. (i) C Blanks - Cl Blanks. As previous published work did not report different blanks for the two different types of tubes (borosilicate or silica glass), blank experiments were performed only on silica glass tubes. They were conducted by running the tubes containing only copper oxide through the whole procedure (heating, carbon and chlorine extraction). Both carbon and chlorine were below measurable levels: e0.05 µmol for both C and Cl. (ii) Reproducibility. For the recommended conditions as stated in the method description (particularly temperature between 620 and 820 °C) and on standard TCE and PCE, the reproducibility of carbon isotopic compositions determined on eight and four measurements, respectively, is (0.08‰ on both products, whereas that of chlorine is (0.15 and 0.10‰ respectively (Table 3), where uncertainty (() is defined as standard deviation of the mean (SD ) σn-1). For these laboratory standards and 11 different manufacturers’ products (TCE, PCE, TCA, DCM, and CFM) analyzed subsequently,16,17 the reproducibility of δ37Cl varies from 0.04 to 0.32‰. The δ37Cl analytical reproducibilities given by Van Warmerdan et al.9 for the three different chlorinated solvents they analyzed are from (0.10 to (0.68‰ (SD). The improvement of reproducibility on chlorine isotopic compositions compared to the technique recommended by Van Warmerdan et al.9 is thus more than a factor of 2. (iii) Sample Introduction. Samples were introduced into the silica or borosilicate glass tubes in two ways: either directly by injection under vacuum or by loading the chlorinated solvent in a weighed capillary tube outside the vacuum line, putting this capillary into the tube, adding the CuO, trapping in liquid nitrogen, (16) Jendrzejewski, N.; Coleman, M. L.; Eggenkamp, H. G. M., Unpublished data (in preparation, to be submitted to Appl. Geochem.). (17) Jendrzejewski, N.; Coleman, M. L.; Eggenkamp, H. G. M. Terra Nova 1997, 9, (Suppl. I), 658.
and then evacuating the tube. Injection under vacuum was preferred because it allowed a better estimate of the chlorinated solvent volume effectively introduced in the combustion tube and thus a more accurate estimate of conversion yield. (iv) Chlorine Extraction Yield Measurements. Three different techniques for measurement of yield were tested on seawater and chlorinated solvent standards. One method was calibration and measurement of the CH3Cl peak area on the gas chromatograph. Another was calibration of the major ion beam signal in the mass spectrometer (mass 50) after a very strict and systematic procedure of gas sample introduction. Both of these techniques were rejected as being too inaccurate and inconsistent with time. The only satisfactory method was measurement of the volume of gas produced for isotopic measurement (CO2 or CH3Cl) using a capacitance manometer (Baratron). The gas was transferred cryogenically to a calibrated volume where its absolute pressure was measured. This volume was calibrated previously using CO2 samples obtained from the combustion in sealed silica glass tubes of weighed amounts of standard graphite (950 °C) (Figure 2, 9). Subject only to a small temperature correction, the pressure is a function of gas volume. The yield for CH3Cl was checked by preparing known amounts of laboratory standard seawater using the standard method (Figure 2, O), which gave consistent and accurate yields ((3%) for seawater very close to 100%. Measured yields of CH3Cl from chlorinated hydrocarbons depend, therefore, solely on those aspects of the method specific to the analyte preparation procedure and, in particular, on the efficiency of conversion of chlorine in the analytes to soluble chloride. (v) Effect of the Combustion Temperature on Carbon Isotopic Compositions. The procedure for analysis of carbon isotopic compositions used by Van Warmerdam et al.9 is questionable, and relevant details of the procedure are missing. Van Warmerdam et al.9 refer for their carbon extraction method to that of Boutton et al.,11 which compares the results of combustion Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
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Table 4. Comparison of δ13C (‰/PDB) and Associated Standard Deviations of Different Chlorinated Solvents Obtained by Combustions in Borosilicate or in Silica Glass Tubesa δ13C (‰/PDB) ( SD (n)
trichloroethylene (std) trichloroethylene A trichloroethylene B trichloroethylene C perchloroethylene (std) perchloroethylene A perchloroethylene B dichloromethane A dichloromethane B dichloromethane C 111-trichloroethane A chloroform B
total average
borosilicate + silica glass (570-820 °C)
silica glass (880-920 °C)
∆low-high temp (°C)
-33.40 ( 0.15 -31.52 ( 0.06 -27.84 ( 0.09 -29.91 ( 0.15 -24.07 ( 0.08 -27.06 ( 0.08 -35.23 ( 0.11 -31.47 ( 0.58 -40.21 ( 0.45 -31.33 ( 0.26 -31.57 ( 0.20 -51.66 ( 0.23
-33.46 ( 0.08 (12) -31.54 ( 0.01 (3) -27.90 ( 0.08 (4) -29.93 ( 0.19 (4) -24.06 ( 0.08 (4) -27.07 ( 0.09 (3) -35.27 ( 0.12 (4) -31.80 ( 0.52 (3) -40.38 ( 0.47 (4) -31.45 ( 0.26 (3) -31.55 ( 0.23 (6) -51.66 ( 0.40 (2)
-33.24 ( 0.18 (5) -31.51 ( 0.09 (3) -27.77 ( 0.03 (3) -29.87 ( 0.04 (2) -24.07 ( 0.09 (3) -27.06 ( 0.09 (2) -35.17 ( 0.08 (3) -30.98 ( 0.06 (2) -39.88 ( 0.09 (2) -31.16 ( 0.15 (2) -31.63 ( 0.12 (2) -51.67 ( 0.02 (2)
-0.22 -0.03 -0.13 -0.06 0.01 -0.01 -0.10 -0.82 -0.50 -0.29 0.08 0.01
weighted mean
-0.12
a Numbers in parentheses indicate the number of replicates. ∆low-high refers to the difference in isotopic composition between the borosilicate and the silica glass experiments; ∆ ) (δ(low temp in borosilicate or silica glass) - δ(high temp in silica glass)).
Figure 2. Chlorine yield from standard seawater (O) compared to CO2 calibration from graphite combustion (9).
of organic samples for stable isotopic analysis in both silica and borosilicate glass tubes. Organic combustions are usually performed in silica glass tubes at high temperatures (>900 °C). Sofer10 and Boutton et al.11 tested low-temperature combustions (550 °C) for carbon isotopic analysis on various organic compounds. Although chlorinated hydrocarbons were not specifically tested in the Boutton et al.11 study, Van Warmerdam et al.9 used borosilicate glass combustion tubes for their experiments. This is a very important point, since Boutton et al.11 reported 100% combustion only for silica glass tubes. Incomplete combustion, associated with more variable δ13C and poorer reproducibility, resulted from use of borosilicate glass tubes at 550 °C.11 The nonquantitative extraction of carbon in borosilicate glass tubes was interpreted to be partly due to charring on the tube surface.11 It was then suggested that the problem is minimized by preheating the furnace to the required temperature before introducing the tubes.10,11 Unfortunately, Van Warmerdam et al.9 did not give sufficient details of their procedure to judge the validity of their results, and, in particular, no information on the temperature of introduction of sample in the furnace or duration and efficiency of the combustion was given. 4264 Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
The method presented in this new contribution requires combustion at temperatures between 620 and 820 °C to ensure optimum recovery of chlorine. Even a slightly less than complete combustion can lead to significant carbon isotopic discrepancy. It is thus important to assess the quality of the combustion in borosilicate or silica glass tubes at these low to intermediate temperatures. All the analytes involved in this study were tested using combustion both at higher (880-920 °C) and lower temperatures (570-820 °C). Table 4 presents a comparison of the δ13C obtained in both cases. There were no significant isotopic differences between preparations in silica or borosilicate glass tubes combusted at 620 °C, the only common temperature (comparison not shown). The main deduction from the data presented in Table 4 is that the δ13C obtained from lower temperature experiments (570-820 °C) was generally slightly lower than those obtained on the same product after combustion between 880 and 920 °C. The weighted mean measured difference of isotopic compositions was -0.12‰. This negative factor is what would be expected from an incomplete combustion. This difference was significantly higher for dichloromethane (maximum -0.82‰). Thus, in general, higher temperature silica glass experiments led to more reproducible results than lower temperature experiments. Again, this was quite critical for DCM and, to a lesser extent, for TCA and chloroform. These three compounds are the most volatile species studied (DCM being the single most volatile), and this poorer reproducibility might be partly the result of isotopic fractionation by evaporation during the syringe sampling or injection, for example. The difference between the δ13C obtained at lower and higher temperatures is, however, small, and the precision is still acceptable. But the results of this comparison indicate the potential danger of performing combustions at too low temperatures. (vi) Effect of the Combustion Temperature on the Chlorine Yields and Isotopic Compositions. The principal concern with the techniques used by Van Warmerdam et al.9 is for chlorine extractions. The Parr bomb combustion method used by Van Warmerdam et al.9 gave very low yields, ranging from 65 to 75%. These systematically low yields are very troubling in stable isotope preparation methods, where the fraction lost may be of different
Table 5. Analysis of Environmental Sample from Savannah River Site yield T (°C) CuO (g) ((2%) S S S B
720 720 720 620
average
0.96 1.01 0.94 0.97
119.1 86.3 87.7 91.0
δ13C (‰/PDB) -25.49 -25.46 -25.49 -25.57
δ37Cl (‰/SMOC) +0.59 +0.48 +0.43 +0.59
-25.50 ((0.05) +0.52 ((0.08)
Figure 3. δ37Cl of laboratory standard chlorinated solvent (TCE, PCE) compared with their respective accepted values, prepared at different temperatures of combustion. A temperature between 620 and 820 °C gives the most accurate and precise results.
a All injected samples were 51.3 µmol of CH Cl. Conversion yields 3 are measured on a capacitance manometer. Uncertainties given are standard deviations.
isotopic composition from the sample material, leading to an inaccurate and imprecise result. Moreover, a specific yield was not given for each individual result. In the present study, two laboratory standard chlorinated solvents (TCE and PCE) were sampled to give approximately 50 µmol of CH3Cl and heated in borosilicate glass tubes between 580 and 620 °C or in silica glass tubes between 620 and 920 °C. For a weight of copper oxide of around 1 g, results of yields and isotopic compositions were consistent in the temperature range 620-820 °C recommended in the methods section (Table 3). For the two laboratory standards TCE and PCE (Table 3) and the set of 11 other chlorinated hydrocarbons (not shown16,17), the variation of yields, outside the 620-820 °C temperature range, had a significant effect on the chlorine isotopic composition (Figure 3). For temperatures lower than 620 °C, as the yields of chlorine tended to be erratic and often low, the mean δ37Cl of the extracted fraction was slightly higher than the mean δ37Cl of experiments between 620 and 820 °C (Figure 3, dashed line), but the results were very erratic. For temperatures above 820 °C, yields were consistently lower (∼70%) and sometimes very low. In parallel, δ37Cl tended to be lower than the mean 620-820 °C value and associated with poorer reproducibilities. The comparison of δ37Cl obtained for combustions between 570 and 820 °C and above 880 °C showed an almost constant difference of 0.46‰; the higher temperature combustions led to systematically isotopically lighter chlorine compositions. The δ37Cl shift of 0.46‰ might represent the isotopic fractionation factor between the chlorine species quantitatively recovered when combustion was performed at 620-820 °C (probably CuCl2 and CuCl) and a chlorine compound that might have been formed at 880-920 °C (Cl2, Cl2O, ClO2?) but not recovered. Although there are many possible causes, the crucial factor might have been the pressure of oxygen, which increases with the temperature. Over a certain partial pressure of oxygen, it may induce the oxidation of the otherwise stable chlorine species, perhaps forming chlorine oxides (Cl2O, ClO2?), which might be noncondensable or might form copper compounds from which chlorine would not be recovered subsequently. It is possible that a similar effect was the cause of the low yields reported by Van Warmerdan et al.9 in the Parr bomb, where the oxygen pressure is ∼10 bar. For the recommended 620-820 °C temperature range, the yields of chlorine recovery on standard TCE and PCE were 97% on average (Table 3). This was confirmed on a set of 88 measurements performed over eight different temperatures on 11 additional chlorinated hydrocarbon samples (data not shown;16,17 no difference in yield was noted between similar temperature
experiments performed in either silica or borosilicate glass). Over the 620-820 °C temperature range, although the standard deviation of the chlorine yields was sometimes large (especially for standard TCE), the standard deviation of the chlorine isotopic compositions was always very small. It is then probable that these apparent low yields were not the results of incomplete recovery of chlorine or incomplete combustion, which, as shown above and illustrated in Figure 3, would give an isotopic shift, but more likely due to the fact that the injected volume was less precisely achieved than expected. Equally, the various, apparently high yields are unlikely to be the result of blank problems, because of the reproducibility of the isotopic results. It seems likely that there were occasional problems with injection of precise volumes of analyte. Further Validation/Test on a Mixture of Chlorinated Solvents Extracted from Contaminated LandsSavannah River. The Savannah River Ecology Laboratory provided us with a sample of vacuum-extracted chlorinated hydrocarbon. This sample is a mixture of different chlorinated hydrocarbons that has been analyzed by GC.MS, using a Supelco VOCOL column for the separations and a Hewlett Packard GC.MS system (PCE, 73.9%; TCE, 11.3%; 1,1,1-TCA, 7.3%; CM, 3.4%; CTET, 1.7%; Freon11, 1.2%; 1,1-DCA, 0.9%; 1,2-DCA, 0.3%). The GC.MS analyses are semiquantitative and should only be used to gauge the presence and relative concentration of chlorinated solvent components in the bulk sample. However, the main objective of this exercise was to establish that the analytical techniques could be applied to real environmental samples, and from this point of view, it provided a good test on a sample that was not simply a pure chlorinated solvent. For the isotopic analysis, the chlorinated solvent kept in a sealed bottle was sampled with a syringe, and 1.4 µL aliquots were directly injected under vacuum in CuO-loaded silica glass tubes. The sealed tubes were combusted at 620 and 720 °C. Carbon and chlorine extractions were performed following the analytical technique previously described. The analytical results are shown in Table 5. Four replicate analyses were performed, and the yields of recovery of chlorine ranged from 86.3 to 119.1%. Because of the excellent reproducibility in both carbon ((0.05‰) and chlorine ((0.08‰) isotopic compositions, and the absence of a relationship between yields and isotopic compositions, the discrepancy in yields is believed to be dependent mainly on the uncertainty in the volume injected. Thus, we have demonstrated that the carbon and chlorine extraction method gives very precise results on a mixture of chlorinated solvents vacuum-extracted from a polluted site. Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
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CONCLUSIONS A new method of extraction of C and Cl from chlorinated solvents has been developed, based on the classical CuO sealedtube organic combustion technique. Unlike that of Tanaka and Rye,8 the new method does not offer any unusual laboratory hazard and is easy to adapt to any isotope laboratory. Unlike any other method,8,9 both carbon and chlorine isotopic compositions are determined on the same, single aliquot of chlorinated solvent. Sample size requirements are reduced by at least a factor of 20 compared to the more recent technique proposed9 and will allow the method to be applied to real environmental problems. More importantly, in comparison with this latest approach, yields from the preparation process are near to 100%, as opposed to 6575%, and give confidence that the chlorine isotopic data produced are accurate values. Probably as a result of the improved yield, precision is improved by a factor of 2.9 There is a significant effect of the temperature of combustion on carbon and chlorine isotope compositions. We recommend a combustion temperature in the range 620-820 °C. Thus, there may be some doubt over the quality of previously published carbon isotope results from chlorinated solvents combusted at the relatively low temperature of 550 °C. The method was shown to be applicable not only to manufacturers’ products but also to a sample from a polluted site. Data quality similar to that obtained for pure chlorinated solvents was
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achieved from a chlorinated solvent mixture vacuum-extracted from the site. ACKNOWLEDGMENT We thank Akzo Nobel, Dow Chemicals, Elf Atochem, and ICI for kindly providing us with samples of chlorinated solvents. The environmental sample of vacuum-extracted chlorinated solvent and its GC.MS analysis were kindly provided by the Savannah River Technology Center, a research unit of the Westinghouse Savannah River Co., which manages the Savannah River Site for the U.S. Department of Energy. N.J. thanks the Socie´te´ de Secours des Amis des Sciences (French Acade´mie des Sciences) for financial support during the writing of this article and M. Javoy, Director of the Laboratoire de Ge´ochimie des Isotopes Stables of the Universite´ Paris 7/Denis Diderot. This study was supported by a grant to M.L.C. from the UK Natural Environment Research Council, No. GR9/1721, and all experiments were performed in the University of Reading at the Postgraduate Research Institute for Sedimentology. This paper is P.R.I.S. Contribution No. 606. Received for review April 30, 1997. Accepted July 28, 1997.X AC970447Z X
Abstract published in Advance ACS Abstracts, September 1, 1997.
