Applied Geochemistry 21 (2006) 766–781

www.elsevier.com/locate/apgeochem

AppliedGeochemistry

Compound-specific chlorine isotope ratios of TCE, PCEand DCE isomers by direct injection using CF-IRMS

Orfan Shouakar-Stash *, Robert J. Drimmie, Min Zhang, Shaun K. Frape

Department of Earth Sciences, University of Waterloo, 200 University Avenue W., Waterloo, Ont., Canada N2L 3G1

Available online 23 March 2006

Abstract

A method for determining compound-specific Cl isotopic compositions (d37Cl) was developed for tetrachloroethene(PCE), trichloroethene (TCE), cis-dichloroethene (cis-DCE), trans-dichloroethene (trans-DCE) and 1,1-dichloroethene(1,1-DCE). The isotope ratio mass spectrometry (IRMS) used in this study has nine collectors, including two for m/z

50 and 52 (CH3Cl) and two for m/z 94 and 96 (CH3Br). The development of this method is based on the fact that fragmentswith mass ratios of 94/96, 95/97 and 96/98 are produced from PCE, TCE and DCE isomers during ion bombardment inthe source of a mass spectrometer. Using continuous flow isotope ratio mass spectrometry coupled with gas chromatog-raphy (GC–CF-IRMS), it is possible to separate these compounds on-line and directly measure the Cl isotopic ratios of thefragments with the specific mass ratios.

Both pure phase and aqueous samples were used for Cl isotopic analysis. For pure phase samples, a vapour phase of thechlorinated ethenes was injected directly into the GC, whereas the solid phase micro extraction (SPME) method was usedto extract these compounds from aqueous solutions. The precisions of this analytical technique were ±0.12& (1r, n = 30),±0.06& (1r, n = 30), and ±0.08& (1r, n = 15) for PCE, TCE and DCE isomers, respectively. The limits of quantification(LOQ) for analyzing Cl isotopic composition in aqueous solutions were 20, 5, and 5 lg/L for PCE, TCE and DCE isomers,respectively. This corresponds to 6–9 nano-mole of Cl, which is approximately 80 times lower than the most sensitive exist-ing method. Compared to methods previously available, this new development offers the following advantages: (1) Themuch lower LOQ make it possible to extract these compounds directly from aqueous solutions using SPME withoutpre-concentration; (2) The linking of a GC with an IRMS eliminates off-line separation; and (3) Because the fragmentsused for isotopic ratio measurement are produced during ion bombardment in the mass spectrometer, there is no needto convert chlorinated ethenes to methyl chloride. As a result, this technique greatly enhances the efficiency for isotopicanalysis by eliminating procedures for pre-concentration, off-line separation and sample preparation. In addition, it alsoreduces the potential for isotopic fractionation introduced during these procedures.

Compound-specific Cl stable isotope analysis can be used as a tool to study the sources of organic contaminants ingroundwater and their behaviour in the subsurface environments. It may also assist in understanding processes such astransport, mixing, and degradation reactions.� 2006 Elsevier Ltd. All rights reserved.

0883-2927/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.apgeochem.2006.02.006

* Corresponding author. Tel.: +1 519 885 1211; fax: +1 519 7460183.

E-mail address: orfan@uwaterloo.ca (O. Shouakar-Stash).

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 767

1. Introduction

Chlorinated solvents have been used as degrea-sers in a variety of industries such as instrumentmanufacturing, aerospace, electronic, printing anddry cleaning, and are among the most commongroundwater contaminants (McCulloch and Midg-ley, 1996). These compounds are designated aspriority pollutants by the US EnvironmentalProtection Agency (EPA) due to their toxicity andpersistence (Keith and Tilliard, 1979), and thereforeare of great environmental concern.

Over the last 10 years attempts have been madeto use Cl stable isotopes to investigate groundwatercontamination by chlorinated ethenes. Although Clisotope analysis has a potential to assist in identify-ing source and processes of contaminants ingroundwater, its application is largely limited bythe analytical techniques. The previously availabletechniques do not have on-line compound-specificcapabilities. As a result, previous studies focusedprimarily on measuring Cl isotopic signature ofvarious chlorinated ethenes from different sources.A range of d37Cl between �2.52& and +1.20&

was determined for PCE, and between �2.54&

and +4.08& for TCE (Tanaka and Rye, 1991; vanWarmerdam et al., 1995; Jendrzejewski et al., 1997;Beneteau et al., 1999; Jendrzejewski et al., 2001;Numata et al., 2002a; Shouakar-Stash et al., 2003).Numata et al. (2002a) measured the d37Cl for DCEisomers, and a range between �0.10& and +2.90&

was reported. In addition to the effort of establishinga d37Cl database for these compounds, fractionationsof Cl stable isotopes of TCE during evaporationhave also been determined (Huang et al., 1999;Poulson and Drever, 1999). Further, Numataet al. (2002b) studied the isotopic fractionations ofTCE during reductive dechlorination by anaerobicbacteria. Sturchio et al. (1998) applied Cl stableisotope analysis to evaluate natural attenuation ofTCE in an aerobic aquifer.

Using a dual inlet isotope ratio mass spectrome-try (DI-IRMS), several analytical methods havebeen developed to determine chlorine stable isotopiccompositions of chlorinated ethenes. These methodsall involve a conversion of chlorinated ethenes tomethyl chloride (CH3Cl) gas for analysis. Tanakaand Rye (1991) reacted chlorinated compounds withLi metal to produce LiCl before converting tomethyl chloride. A Parr Oxygen Bomb method(ASTM method D808-91) was introduced by vanWarmerdam et al. (1995), where chlorinated ethenes

were reacted with O2 in a pressurized bomb to pro-duce Cl� in a 5% CaHCO3 solution. The Cl� wasthen precipitated as AgCl and converted to CH3Clgas using a method modified by Long et al.(1993). The amount of Cl required by this techniquewas between 1700 and 2000 lmol with precisions of±0.1& and ±0.68&, respectively. Holt et al. (1997)reacted pure phase chlorinated compounds withCuO to produce CO2 and CuCl. The producedCO2 was analyzed for d 13C and CuCl was furthertreated with CH3I to generate CH3Cl. This methodrequired 10–70 lmol of Cl, with a precision of±0.07&. Holt et al. (2001) further developed a tech-nique to extract micromolar concentrations of chlo-rinated compounds from water using a low pressurecarrier-gas procedure and achieved a precision of±0.20&. A similar method involving wet chemistrywas proposed by Jendrzejewski et al. (1997). Sam-ples containing 33 and 78 lmol of Cl were used inthis technique, and the precision between ±0.10&

and ±0.15& was reported. Recently, Thermal Ioni-zation Mass Spectrometry (TIMS) has been intro-duced to determine Cl isotopic compositions ofchlorinated ethenes, where a conversion to CsCl isrequired. Numata et al. (2002a) treated chlorinatedethenes with sodiumbiphenyl reagent to liberateCl�. Approximately 2.8 lmol of Cl was used in thistechnique and a precision between ±0.10& and±0.40&, depending on the compound and thepreparation technique, was achieved. In the studycarried out by Holmstrand et al. (2004), organo-chlorine compounds were combusted with CuO toform CuCl, which was first converted to HCl andthen converted to CsCl by ion exchange. Only0.7 lmol of Cl was required, the smallest sample sizereported. However, this technique had a low preci-sion of ±0.46&. All of the previous techniquesinvolve lengthy preparation procedures with multi-ple steps. For a sample with a mixture of chlori-nated compounds, there is a potential to causeisotopic fractionation during separation and samplepreparation. Requirement of large sample size fur-ther limits the application of Cl isotopes in ground-water contamination.

This study explores the possibility of using massfragments of chlorinated ethenes to determine com-pound-specific Cl stable isotopic ratios. The use ofmass fragments in stable isotope analysis wasreported in an earlier study (Rees, 1978), whereSF6 was ionized to produce SFþ5 and mass frag-ments of 127 (32SF5), 128 (33SF5) and 129 (34SF5)were collected to measure d 34S. The goal of this

Table 1The source parameters of the IsoPrime, isotope ratio massspectrometer (IRMS), for different species: PCE, TCE and DCEisomers

Source parameter PCE TCE DCE isomers

Accelerating voltage (V) 2232.8 2208.2 2187.87Extraction voltage (%AV) 84 77 80Half plate differential (V) 6.4 0.0 �2.8Z plate voltage (V) �71.5 �42.0 �41.2Trap current (lA) 150 100 100Electron volts (eV) 95 90 90Ion repeller voltage (V) 0.0 10.6 10.7Magnet current (mA) 4899.9 4899.9 4899.9

768 O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781

study was to develop a new method to determine thecompound-specific Cl stable isotope compositionswith the following objectives: (1) to minimize thenumber of preparation steps, (2) to enhance the sen-sitivity, and (3) to improve the precision. The devel-opment of this technique may encourage moreresearchers to apply Cl stable isotopes to groundwa-ter contamination studies.

2. Methodology

2.1. Materials

The following chemical compounds were usedin this study: tetrachloroethene (PCE) (C2Cl4), tri-chloroethene (TCE) (C2HCl3), cis-dichloroethene(cis-DCE) (C2H2Cl2), trans-dichloroethene (trans-DCE) (C2H2Cl2), and 1,1-dichloroethene (1,1-DCE)(C2H2Cl2). These compounds are from differentsuppliers with a large variation in d37Cl values.

The following items are used during the analysisprocedure: 20 mL clear rounded base headspacevials, 23 · 75 mm (Chromatographic SpecialtiesInc., cat # RK21162); 40 mL clear vials, 28 · 95mm (Chromatographic Specialties Inc., cat #C702795); 60 mL clear vials, 27 · 135 mm (Chro-matographic Specialties Inc., cat # C85B75960);tegrabond disc (septa), 125/10 MIL silicone/teflon,20mm (Chromatographic Specialties, cat #C8812120M); tegrabond disc (septa), 125/10 MILsilicone/teflon, 22 mm (Chromatographic Special-ties, cat # C8812122M); PTEF stir bars, diameter4 mm, length 12 mm (Fisher Scientific [Fisher-brand], cat # 14-511-60A); crimp seals, magnetic,20 mm (Chromatographic Specialties, cat #SB405038); 20 mm crimper (Kimble, cat # 69901-20); open top cap, 24-400 thread, polypropylene(Chromatographic Specialties, cat # C85169624G);Viton� O-Ring (Canadian Bearing, cat # V-112).

2.2. Instrumentation

A continuous flow-isotope ratio mass spectrome-ter (CF-IRMS) (IsoPrime, Micromass, currentlyGV Instruments) was used in this study. This IRMSis designed to be used in both continuous flow (CF)and dual-inlet (DI) modes, with nine collectors. Ithas standard triple collectors and six special collec-tors, two for m/z 50 and 52 (CH3Cl), two for m/z 62and 64 (vinyl chloride) and two for m/z 94 and 96(CH3Br). The tuning parameters of the different spe-cies are presented in Table 1. The dual inlet bellows

are used as the reference gas reservoir when theIRMS is in the CF mode for analyzing toxic com-pounds, where the use of reference gas from a tankis not desirable.

An Agilent 6890 Gas Chromatograph (GC)equipped with a CTC Analytics CombiPAL SPMEautosampler is attached to the IRMS. A DB-5 MSgas chromatographic column (60 m · 0.320 mm,1 lm film thickness from J & W Scientific Inc., cat# 1235563) is used for gas separations. The auto-sampler can hold 32 vials (20 mL) and organic com-pounds in aqueous samples are extracted via aSPME fiber (75um Carboxen-PDMS for MerlinMicrosealTM, 23 gauge needle Auto holder fromSupelco, cat # 57343-U), and then desorbed intothe GC inlet. A SPME injection sleeve (0.75 mmID, Supelco, cat # 2-6375,05) is used as an inletliner. Therm-O-RingTM Seals 1/4’’ (Supelco, cat #21004-U) are used for the injection sleeve and11 mm Thermolite� septa are used for the GC inlet(RESTEK, cat # 20365).

A 4-way Valco valve with two positions isinstalled between the GC and the IRMS. This setupis important because it allows a non-stop constantflow of He into the IRMS and it provides the capa-bility to direct only desired compounds to the IRMSand all other compounds to the FID to be burned.The end part of the GC column is connected toone of the two in-ports of the valve, while the otherin-port is connected with ultra pure He gas. The twoout-ports are connected to the IRMS and to a FIDdetector mounted on the GC. In position A, flowfrom the GC is directed to the FID, while He gasis flowing into the IRMS. In position B, the out flowfrom the GC is directed to the IRMS and the Heflow is directed to the FID. A 100 lm deactivatedcapillary column is used to connect the valve portwith the IRMS. An open split is installed on this lineto lower the pressure in the capillary to (2–3) · 10�6

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 769

millibar before it flows into the IRMS. The He flowthat feeds the valve is balanced to the flow from theGC column. This is important to achieve the samebackground level when switching between the twopositions A and B. The open split is vented intothe fumehood via a Teflon tube, and all other ventports on the GC are also vented into the fumehood.Because of the toxicity of the compounds, theyshould not be vented to the laboratory atmosphere.

Fig. 1. Mass fragment spectra of PCE, TCE and DCE isomers obtainedmass spectral library, Demo Version 2.0.

2.3. Reference gas preparation

All reference gases used in this technique are mix-tures of the desired chlorinated ethenes and He.References were prepared in 15 ml vessels withPTEFE plug valves that seal with a Viton� O-ringon a tapered seat. It is attached to the inlet portof the mass spectrometer by a ball-socket (19/9)joint and a Viton� O-Ring.

from the NIST spectral search program for the NIST/EPA/NIH

770 O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781

All references were prepared on a vacuum lineequipped with an injection port and connected toa He tank through a valve. First, the vessel andthe vacuum line were evacuated for 5 min. Then aspecific volume of the solvent was injected throughthe septum port where the solvent was frozen in thevessel by liquid N2. Once the solvent was frozen,the vessel was evacuated for a couple of minutesand He gas was allowed to flow into the vessel until1 atm was reached. Then the vessel was sealed andremoved from the line.

The reference gas vessel was then attached to theinlet port of the mass spectrometer and the referencegas was introduced to the reservoir. It is recom-mended to introduce sufficient amount of referencegas to give maximum beam intensity without theneed for pressurizing the gas by decreasing the sizeof the variable volume reservoir. Even though, adependency between the volume of the referencegas and both accuracy or precision was notdetected, it is important in automated techniquesas the one presented here, to have a maximum vol-ume of the reference gas because it allows the refer-ence gas to last for a longer time without beingdepleted due to its continuous bleed.

Fig. 2. Chromatogram of TCE standard analysis in continuous-flow msample peak. The upper trace is the 95/97 ratio, the middle trace is the

Fifty microliters of PCE, 10 lL of TCE and DCEisomers were used to prepare the reference gases. Alarger volume of PCE was necessary to prepare thePCE reference gas in order to achieve maximumbeam intensity with PCE, because the 94/96 frag-ments of PCE used in the analysis are not the mostintense fragments of the compound.

2.4. Standard preparation

For the pure phase experiments, standards wereprepared by injecting specific amounts of chlori-nated ethenes into crimp sealed, evacuated 2 mLvials. A volume of 1 lL was used for TCE andDCE isomers and 5 lL was used for PCE.

For the SPME analysis of aqueous samples,stock aqueous solution standards were preparedin 60 mL vials. A certain amount of these com-pounds and ultra pure water were used to preparean approximate concentration of 230 mg/L. DCEstandards were prepared with a mixture contain-ing all three DCE isomers. These stock standardswere measured for more than 6 months and nochanges in their Cl isotopic composition wereobserved. Nevertheless, fresh stock standards were

ode. The chromatogram illustrates eight reference pulses and am/z 97 ion current and the lower trace is the m/z 95 ion current.

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 771

prepared every 1–2 months. Standards were pre-pared from the stock solution for daily analysis.Standards with a large variation of d37Cl valuesand different concentrations are included in everybatch to bracket the majority of samples in thebatch.

In order to examine the accuracy of the currenttechnique, several in-house standards were analyzedby the conventional DI-IRMS methodology. Theanalytical protocol used to prepare the standardsis described by Holt et al. (1997) with some modifi-cations (Shouakar-Stash et al., 2003). Briefly,

y = 0.9763x - 0.1076

R2 = 0.9978

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

-4.0 -3.0 -2.0 -1.0 0.037ClSMOC (

37C

l SM

OC

(‰

) (D

I-C

lass

ical

Tec

hn

iqu

e)

-4 .0

-3 .0

-2 .0

-1 .0

0. 0

1. 0

2. 0

3. 0

4. 0

5. 0

0 5 10 15 2

Run N

37C

l SM

OC(‰

)

(a)

(b)

Fig. 3. (a) Comparison between d37ClSOMC (&) values of six in-house Teffect test of six in-house TCE standards.

approximately 5lL of the standard was reacted with1 g CuO for 2 h at 850 �C in a specially constructedquartz tube. The resultant CuCl was separated fromthe CuO by redistribution in the combustion tube.This was achieved by placing one end of the tubethat contains all CuO particles in a tube furnace(midsection temperature �700 �C) and placing theother end of the tube into snug fitting holes of adouble-walled polystyrene-foam container. LiquidN2 is added to the container for 45 min to condenseall CO2 and H2O. The resultant CuCl was thenreacted with CH3I (35 lL) for 2 h at 300 �C during

1.0 2.0 3.0 4.0 5.0

‰) (CF-IRMS)

TCE-ATCE-B

TCE-CTCE-DTCE-E

TCE-F

0 25 30 35 40

umber

CE standards obtained by DI-IRMS and CF-IRMS; (b) Memory

772 O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781

which CH3Cl is produced. Later, CH3Cl and CH3Iwere separated by a gas chromatograph (GC-8AShimadzu) where CH3Cl was collected cryogenicallyusing liquid N2 in a vessel and analyzed by a DI-IRMS (IsoPrime, GV instruments) to determinethe d37Cl signature.

2.5. Sample analysis

Three steps were involved in the sample analysis:(i) sample preparation and injection, (ii) GC separa-tion, and (iii) mass spectrometry analysis. The sam-ple preparation and injection step varies accordingto the type of injection used. In the pure phase injec-tion by gas-tight syringe the autosampler has thefollowing protocol: filling volume (100 lL forTCE/DCEs and 300 lL for PCE); filling speed500 lL/s; filling strokes 3; pull-up delay 1000 ms;pre-injection delay 300 ms; injection volume 20 lLfor TCE/DCEs and 150 lL for PCE; injection speed800 ms; post-injection delay 5000 ms; post-cleanwith air four times.

For headspace SPME analysis of aqueous sam-ples, 20 mL vials were used to prepare aqueous sam-ples for the automatic run and 40 mL vials formanual analysis. In a 20 mL vial, a volume of16 mL of aqueous solution was used, leaving 4 mLof headspace. A headspace of 5 mL was createdfor the 40 mL vials. These volumes leave enough

Table 2Comparison between the d37Cl values of different chlorinated solvent stausing DI-IRMS and by the new CF-IRMS technique

Results by the DI-IRMS Resu

n Meand37ClSMOC (&)

STDV (1r) n

PCE-A 5 +0.29 0.06 5PCE-B 5 +0.11 0.10 5PCE-C 4 �0.57 0.21 5PCE-D 4 �2.52 0.15 5

TCE-A 4 +3.90 0.09 6TCE-B 4 +3.55 0.05 6TCE-C 5 +3.00 0.10 6TCE-D 4 �2.70 0.13 6TCE-E 5 �2.79 0.14 6TCE-F 4 �3.19 0.07 6

cis-DCE-A 4 +0.77 0.12 15cis-DCE-B 4 +5.94 0.11 10cis-DCE-C 4 �0.30 0.15 10trans-DCE-A 4 �1.38 0.12 10trans-DCE-B 4 �3.86 0.12 101,1-DCE-A 4 +3.26 0.13 101,1-DCE-B 4 +5.01 0.11 10

space for the SPME fiber to go into the vial withoutcontacting the solution. The fractions between thesolvent phase, aqueous dissolved phase and head-space were neglected in this study as both samplesand standards were prepared and analyzed in thesame way to eliminate any error that can be causedby the fractions between different phases. Sampleswere stirred for at least 20 min before being loadedin the autosampler. The following protocol wasemployed for the headspace SPME analysis: (1)SPME fiber conditioning for 4 min at 250 �C. (2)compound extraction for 20 min at 50 �C and stir-ring speed of 700 rpm. (3) desorption of compoundsin the injection inlet for 3 min at 270 �C. In manualinjection, the fiber is first conditioned, then the sam-ple is extracted by the manual-holder fiber devicefor 20 min and then the fiber is placed in the inletport of the GC for desorption for 3 min.

The second step is the GC separation, whichinvolves optimization of the GC settings. The fol-lowing conditions were used in this study: flowwas set at 1.8 mL/min, and kept constant duringthe run. The injection port temperature was270 �C. The oven temperature was programmed asfollows for PCE and DCEs: initial 40 �C for4 min; ramp at 150 �C/min; final 220 �C held for9 min. The TCE oven temperature program follows:initial 40 �C for 2 min; ramp at 150 �C/min; final220 �C and held for 11 min. In most cases, a suitable

ndards obtained by the conventional technique (Holt et al., 1997)

lts by the CF-IRMS

Machinemean d37Cl (&)

Correctedmean d37ClSMOC (&)

STDV (1r)

�1.83 +0.29 0.06�2.06 +0.10 0.07�3.05 �0.70 0.05�5.33 �2.54 0.12

+0.62 +3.90 0.06+0.43 +3.71 0.03+0.14 +3.43 0.09�6.09 �2.70 0.04�6.40 �3.01 0.02�6.57 �3.18 0.14

�2.27 +0.76 0.08+2.93 +5.84 0.08�3.65 �0.58 0.07�4.18 �1.18 0.11�6.85 �3.72 0.09+0.34 +3.29 0.06+2.03 +4.97 0.09

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 773

split ratio was used. When samples were analyzed ina splitless mode, purge time was set at 1.5 min witha purge flow at 80 mL/min.

The last step in the analysis involves the massspectrometer. This method is set up such that theflow from the GC was directed initially to the FIDfor a specific period of time (360 s for TCE or DCEsand 650 for PCE), during which a set of referencepulses are run by opening the dual inlet reservoirfor 40 s for each pulse. After the initial time, theflow was directed into the IRMS to allowthe desired compound/compounds to flow into thesource for the Cl isotope analysis. After 600, 650,

Table 3The d37ClSMOC (&) values obtained from a memory effect test ofdifferent in-house TCE standards (with large variations in d37Clvalues) by the new CF-IRMS technique

Order of measurement Compound Mean d37ClSMOC (&)

1 TCE-A 3.932 3.863 3.894 4.035 3.936 3.89

7 TCE-F �3.428 �3.209 �2.92

10 �3.1711 �3.2112 �3.37

13 TCE-B 3.7214 3.7015 3.7516 3.7717 3.7418 3.6819 3.89

20 TCE-E �3.1121 �3.0722 �3.1123 �3.1224 �3.0925 �3.07

26 TCE-C 3.3427 3.3728 3.5929 3.4230 3.4531 3.48

32 TCE-D �2.7933 �2.7334 �2.7835 �2.7636 �2.8637 �2.80

and 850 s for DCEs, TCE and PCE, respectively,the flow was directed back to the FID to preventany other compounds from the sample flowing intothe mass spectrometer.

The d37Cl for the selected chlorinated solvents inthis study were measured using the 94/96 Faradaycup. The measurements were performed on massfragments from the different compounds. When aspecific compound enters the source, it is bom-barded by electrons and this event produces frag-ments with different intensities (Fig. 1). Thesefragments are the result of the molecules losing elec-tron(s) or atom(s) and electron(s). A careful analysisof the mass fragments of the investigated com-pounds showed that fragments with masses of 94/96 result from PCE and TCE or fragments withclose masses such as 95/97 from TCE and 96/98from DCE isomers. PCE produces 94/96 fragmentsby losing two Cl atoms. TCE produces 94/96 frag-ments by losing one Cl atom and one H atom and95/97 fragments by losing only one Cl atom. DCEisomers can produce fragments 96/98 without losingany atoms. When these fragments lose an electron,they become positively charged, get separatedaccording to their m/z ratios and, finally, are col-lected in the corresponding Faraday cups. The rea-son for choosing masses in this range is that theIRMS used for these analyses was designed withspecific cups to measure masses 94 and 96 for Br sta-ble isotopes using methyl bromide gas. This capabil-ity allowed the use of mass fragments in analyzingthe selected chlorinated solvents for d37Cl. Initially,the work was done on TCE using masses 94 and 96,after finding the corresponding peaks, the IRMSwas tuned to achieve maximum intensity and preci-sion in the same time. More efforts were thendevoted to tuning the source parameters to accom-modate higher masses such as 95/97 and 96/98 inthe Faraday cups. This attempt was successful andallowed the examination of the validity of this tech-nique on different compounds. For analyzing TCE,mass fragments of 95/97 were used instead of 94/96because their intensities are almost ten times higher,resulting in lower limits of quantification.

The Cl isotopic compositions are reported in per-mil (&) deviation from isotopic standard referencematerial using the conventional d notation, whered = ((Rsample/Rstandard) � 1) · 1000. 37Cl/35Cl is themeasured isotopic ratio. The d37Cl values were cal-ibrated and reported relative to the reference mate-rial, Standard Mean Ocean Chloride (SMOC)(Kaufmann et al., 1984). This calibration was

774 O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781

performed by: (1) calibrating the CH3Cl referencegas used in the DI-IRMS measurements to SMOC;(2) determining the d37Cl values relative to SMOCof all PCE, TCE and DCE standards by the DI-IRMS technique after converting them to CH3Clgas (Holt et al., 1997); (3) using these standards(with known d37Cl values relative to SMOC) to cal-ibrate the results obtained by the new CF-IRMSmethodology.

Fig. 5. Chromatogram of PCE standard analysis in continuous-flow msample peak. The upper trace is the 94/96 ratio, the middle trace is the

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0 1 2 3

Peak Are

37C

l

Raw Data

Peak Area Corrected Data

Fig. 4. Relationship between the d37Cl (&) values and peak area of Tcircles represent the raw values and the open circles represent the peak

3. Results and discussion

3.1. Blanks

Different blanks were analyzed in order todetermine the contribution of masses 94 through98 from the background and the materials usedin sample preparation. Injections from evacuatedvials and from vials filled with air were made,

ode. The chromatogram illustrates eight reference pulses and am/z 96 ion current and the lower trace is the m/z 94 ion current.

y = -0.0287x2 + 0.3943x - 1.1984

R2 = 0.9109

4 5 6 7 8

a (x10-10As)

CE standard (raw data versus peak area corrected data). Closedarea corrected values.

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 775

and these masses were not detected. The sameresults were obtained in aqueous blanks whereultra pure water was used and extracted bydirect-SPME and headspace-SPME methods. Tocheck the effectiveness of backing out the fiberbetween samples and test for carry over fromsample to sample, a test was conducted on theSPME fiber after sample delivery and afterbacking out. Both tests did not show any contri-butions to the different masses. The results fromall blanks showed that there was no contributionfrom the background or the materials used duringthe analysis, and that the entire sample wasdesorbed from the SPME during the sample deliv-ery with no carry over from one sample toanother.

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

0 5Run N

37C

l SMO

C

y = 0.994x + 0.0337

R2 = 0.9979

-3 .0

-2 .5

-2 .0

-1 .5

-1 .0

-0 .5

0.0

0.5

-3 .0 -2 .5 -2 .0 -1 .5

ClSMO C

37C

l SM

OC

(D

I-C

lass

ical

Tec

hniq

ue)

(a)

(b)

Fig. 6. (a) Comparison between d37ClSOMC (&) values of four in-house Peffect test of four in-house PCE standards.

3.2. Trichloroethene (TCE)

The chromatogram in Fig. 2 illustrates a com-pound specific analysis of TCE in a continuous flowmode. The chromatogram shows 8 reference pulsesand a sample peak. The retention time for theTCE peak is around 544 s. The total length of therun is 25 min.

An accuracy test was done using a set of in-houseTCE standards with a d37Cl range between �3.19&

and +3.90&. The results achieved from the currentcompound-specific CF-IRMS method are comparedwith those from the conventional off-line techniqueusing the DI-IRMS in Fig. 3a. These results are alsosummarized in Table 2. The high R-squared (RSQ)(R2 = 0.9978) of the linear regression in Fig. 3a

10 15 20umbers

-1 .0 -0 .5 0.0 0.5

(CF-IRMS)

PCE-A

PCE-B

PCE-C

PCE-D

CE standards obtained by DI-IRMS and CF-IRMS; (b) Memory

776 O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781

indicates that the technique provides an excellentaccuracy over a range of approximately 6& for d37Cl.

The internal precision represents the precision ofthe instrument for a specific species at a specific tun-ing parameter, and was determined by analyzing thesame reference gas several times. The internal preci-sion of ±0.01& (±STDV (1r), n = 10) was achievedfor TCE. The external precision better than ±0.08&

(1r, n = 15) was determined for the head spaceinjection technique using an in-house TCE stan-dard. The external precision of the SPME technique

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

0 2 4 6

Peak Area

37C

l

Raw Data

Peak Area Corrected Data

Fig. 7. Relationship between the d37Cl (&) values and peak area of Ptriangles represent the raw values and the open triangles represent the

Table 4The d37ClSMOC (&) values obtained from a memory effect test ofdifferent in-house PCE standards (with large variations in d37Clvalues) by the new CF-IRMS technique

Order of measurement Compound Mean d37ClSMOC (&)

1 PCE-A 0.352 0.303 0.234 0.315 0.27

6 PCE-D �2.417 �2.518 �2.549 �2.57

10 �2.67

11 PCE-C �0.5512 �0.5113 �0.6014 �0.5915 �0.67

16 PCE-B 0.1817 0.1418 0.0819 0.0720 0.06

was tested in both split and splitless modes, and theprecision was better than ±0.06& (1r, n = 30).

The limit of quantification (LOQ) of TCEachieved with this technique is 10 ppb (lg/L) usingan autosampler, which can only hold 20 mL vialsfor SPME extraction; whereas samples at concen-trations as low as 5 ppb can be analyzed manuallywhen larger sample sizes were used.

A memory effect test was carried out by analyz-ing six different TCE standards with a variation ind37Cl ranging between �3.19& and +3.90&. Nomemory effect was observed, and the results are pre-sented in Table 3 and shown in Fig. 3b. During theruns, the SPME fiber was conditioned betweeninjections (4 min in the fiber conditioning stationat 250 �C) to avoid any possible memory effectcaused by residual solvent on the fiber.

A peak area dependency (linearity) test was per-formed on one TCE standard to verify the linearityof the method. The results showed a dependencybetween the peak area and the delta value of thestandard (Fig. 4). This relationship can be bestexpressed using a polynomial (second order) regres-sion line (R2 = 0.9109), showing that the depen-dency is most pronounced at peak areas smallerthan 4 · 10�10 Ampere seconds (As). Therefore, itis recommended to work with peak areas biggerthan 4 · 10�10 as in routine analysis. In order toachieve consistent results, peak area correction isnecessary when different peak areas are obtained.The precision was improved from ±0.24& (1r) to±0.07& (1r) when the peak area correction wasapplied. The correction is done by first determiningthe relationship between the deviations of the d37Clvalues of the standard against the change of the

y = -0.0885x2 + 2.047x - 9.3079

R2 = 0.9905

8 10 12

(x10-10 As)

14

CE standard (raw data versus peak area corrected data).Closedpeak area corrected values.

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 777

peak area, second applying that relationship to peakarea values of the samples in order to calculate thecorrection factor and finally adding this correctionfactor to the d37Cl values obtained for the samples.

3.3. Tetrachloroethene (PCE)

A chromatogram of a compound specific analysisof PCE in continuous flow mode is shown in Fig. 5.The chromatogram consists of eight reference pulsesand a sample peak. The retention time for the PCEpeak is around 735 s, and the total length of the runis 25 min.

An accuracy test was done using 4 in-house PCEstandards with a d37Cl range from �2.54& to+0.29&. A comparison of the results obtained bythe CF-IRMS and those by the classical off-linetechnique using DI-IRMS is illustrated in Fig. 6aand data are also presented in Table 2. Over ad37Cl range of approximately 3&, an excellent lin-ear relationship was obtained between these twotechniques (R2 = 0.9979), indicating that the newtechnique has an excellent accuracy.

An internal precision of ±0.04& (1r, n = 10) wasachieved. The external precision for the headspace

Fig. 8. Chromatogram of DCE isomers standard analysis in continuouand 3 sample peaks (1,1-DCE, trans-DCE and cis-DCE, respectively). Tcurrent and the lower trace is the m/z 96 ion current.

injection technique was better than ±0.10& (1r,n = 15), and those for the SPME techniques were±0.12& (1r, n = 30) and ±0.15& (1r, n = 30) forsplit and splitless settings, respectively.

The PCE’s LOQ for the automated and manualmodes are 40 and 20 ppb, respectively. No memoryeffect was observed when analyzing four PCE stan-dards with d37Cl values varying over approximately3& (Table 4 and Fig. 6b). Nevertheless, the SPMEfiber was conditioned between samples.

A linearity test was performed on an in-house PCEstandard, and a relationship between the peak areaand the outcome delta values is shown in Fig. 7. Thisrelationship can be described by a polynomial (sec-ond order) regression trend (R2 = 0.9905). Similarto TCE, the measured delta values of PCE alsoshowed a large dependency on peak area. The preci-sion of the raw data on the entire range was improvedfrom ±2.20& (1r) to ±0.13& (1r) by the peak areacorrection. This dependency is most pronounced atpeak areas smaller than 6 · 10�10 As. Thus, it is rec-ommended, in routine analysis, to work with peakareas that are bigger than 6 · 10�10 As. In addition,it is highly recommended to prepare all samples andstandards at similar concentrations. Furthermore, it

s-flow mode. The chromatogram illustrates eight reference pulseshe upper trace is the 96/98 ratio, the middle trace is the m/z 98 ion

778 O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781

is also important to include a set of standards with arange of concentrations that bracket the samples todetermine the relationship between peak area andthe deviation of the expected delta value of the stan-dard. This peak area correction is applied to achieveaccurate results.

3.4. Dichloroethene isomers (DCEs)

Eight reference pulses and three sample peaks inthe order of 1,1-DCE, trans-DCE, and cis-DCE areshown in Fig. 8, with a retention time of 428, 480

y = 1.0161x - 0.0153

R2 = 0.9983

-4

-3

-2

-1

0

1

2

3

4

5

6

-4 -3 -2 -1 0 1 2 337ClSMOC (CF-IRMS)

37C

l SM

OC

(Cla

ssic

al T

echn

ique

)

-6

-4

-2

0

2

4

6

8

0 10 20 30 4Run N

37C

l SM

OC

(b)

(a)

Fig. 9. (a) Comparison between d37ClSOMC (&) values of seven in-hou(b) Memory effect test of seven in-house DCE isomer standards.

and 528 s, respectively. The total length of the runwas 25 min.

An accuracy test was done using a set of 7 in-house DCE standards with a range of Cl stable iso-tope compositions of approximately 10& (�3.72&

to +5.84&). Standards were analyzed using bothcompound specific CF-IRMS and traditional off-line technique using DI-IRMS, and the results werecompared in Fig. 9a. The high correlation coeffi-cient (R2 = 0.9983) between these two techniquesindicates that the currently developed CF-IRMSmethod compares very well with the conventional

4 5 6

cis-DCE-A

cis-DCE-B

cis-DCE-C

trans-DCE-A

trans-DCE-B

1,1-DCE-A

1,1-DCE-B

0 50 60 70umber

80

se DCE isomer standards obtained by DI-IRMS and CF-IRMS;

Table 5The d37ClSMOC (&) values obtained from a memory effect test ofdifferent in-house DCE isomer standards (with large variations ind37Cl values) by the new CF-IRMS technique

Order of measurement Compound Mean d37ClSMOC (&)

1 cis-DCE-A 0.742 0.653 0.854 0.775 0.716 0.807 0.738 0.869 0.76

10 0.7211 0.8812 0.8913 0.6914 0.6815 0.74

16 cis-DCE-B 5.7217 5.8318 5.9119 5.9320 5.8121 5.8022 5.9723 5.7624 5.8225 5.89

26 cis-DCE-C �0.5527 �0.5528 �0.6629 �0.5730 �0.5031 �0.4832 �0.6433 �0.7034 �0.5535 �0.57

36 trans-DCE-A �1.3337 �1.0638 �1.3139 �1.1640 �1.3041 �1.0942 �1.1143 �1.0544 �1.1045 �1.26

46 trans-DCE-B �3.7147 �3.6048 �3.8049 �3.9050 �3.6351 �3.8152 �3.6453 �3.7854 �3.67

Table 5 (continued)

Order of measurement Compound Mean d37ClSMOC (&)

55 �3.7156 1,1-DCE-A 3.1257 3.3358 3.2959 3.3560 3.2861 3.3162 3.3063 3.3164 3.3265 3.34

66 1,1-DCE-B 5.0667 4.8568 4.8669 4.9370 4.9271 5.0272 5.1173 5.0074 5.0575 4.89

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 779

method and provides excellent accuracy over arange of approximately 10& for d37Cl.

The internal precision achieved by this techniquewas ±0.01& (1r, n = 10). The external precision forthe headspace injection technique was better than±0.09& (1r, n = 15). The external precision of theSPME technique was better than ±0.08& (1r,n = 15) for both split and splitless settings.

The achieved LOQ of the different DCE isomersby this new technique are 10 and 5 ppb for auto-mated and manual modes, respectively. A memoryeffect test was carried out using the 7 different in-house DCE standards. No memory effect wasdetected during the test (Table 5 and Fig. 9b).

The results from a linearity test conducted on anin-house DCE standard are shown in Fig. 10. Apolynomial (second order) relationship betweenthe peak area and the outcome delta value wasobserved. The high R-squared value (R2 = 0.8198)indicates that the delta value can be predicted overthe tested peak area range. A peak area correctioncan be applied to achieve accurate results, especiallywhen peak areas are different for different DCE iso-mers in a specific run. In this case, the application ofthe peak area correction improved the precisionfrom ±0.26& (1r) to ±0.10& (1r). Although theaccuracy was improved by peak area correction, itis best to work with peak areas greater than3 · 10�10 As due to large deviations below this peakarea.

y = -0.0224x2 + 0.3193x - 0.8739

R2 = 0.8198

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0 1 2 3 4 5 6 7 8 9 10

Peak Area (x10-10 As)

37C

l(‰)

Raw Data

Peak Area Corrected Data

Fig. 10. Relationship between the d37Cl (&) values and peak area of DCE standard (raw data versus peak area corrected data). Closeddiamonds represent the raw values and the open diamonds represent the peak area corrected values.

780 O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781

3.5. Mixtures

In order to save time, use smaller sample sizesand get more than one result from a single run,analyzing more than one compound from onesample was considered. Three different mixturesof standards were prepared between TCE andPCE (Mixture-1), between TCE and 3 DCEs(Mixture-2), and between PCE and 3 DCEs (Mix-ture-3). The two dual inlet reservoirs were used asreference gas reservoirs. An inlet method was cre-ated to start analyzing the mixed standard byloading the tuning parameters that correspond tothe first compound that will elute from the GCand introducing reference pulses from the reser-voir that correspond to that compound. Oncethe peak of the first compound is eluted andintroduced into the mass spectrometer a pro-grammed peak jump happens and the tuningparameter for the second peak is loaded. Refer-ence pulses from the second inlet reservoir wereintroduced after the peak(s) of the second stan-dard(s) are eluted. The mixed standards were pre-pared with optimum concentrations. The observedresults for the standards showed excellent accura-cies and precisions. However, this technique wasnot readily applicable to real samples due to largevariations in the concentrations of the differentcompounds. It is often that different sample sizesare required to analyze different compounds forthe same sample.

4. Conclusions

A new method for determining compound-spe-cific Cl isotopic compositions (d37Cl) of PCE,TCE, cis-DCE, trans-DCE and 1,1-DCE has beendeveloped in this study. The technique is based onmeasuring mass fragments of these chlorinatedcompounds, which are produced as a result of ionbombardment in the source of the mass spectrome-ter. A continuous-flow GC–IRMS was used to per-form the separation and isotopic analysis.

A linear correlation, with a high R-squared, wasfound between the results measured by this studyand those by a previously available method, indicat-ing that a high accuracy was achieved. Precisions of±0.12&, ±0.06&, and ±0.08& (1r) were obtainedfor PCE, TCE, and DCE isomers, respectively,which are better than, or at least comparable to,previous techniques. Compared to previous tech-niques, sample size was reduced greatly in the cur-rent method. The lowest concentration limits foranalyzing Cl stable isotopic composition in aqueoussolutions were 20, 5, and 5 lg/L for PCE, TCE andDCE isomers, respectively.

By using continuous-flow technology and bymeasuring mass fragments of chlorinated ethenes,this new method offers several advantages over thetechniques previously available. The much lowerlimits of quantification make it possible to extractthese compounds directly from aqueous solutionsusing SPME without involving pre-concentration

O. Shouakar-Stash et al. / Applied Geochemistry 21 (2006) 766–781 781

procedures. The combination of a GC with anIRMS eliminates off-line separation. Because thefragments used for isotopic ratio measurement areproduced during ion bombardment in the massspectrometer, there is no need to convert chlorinateethenes to methyl chloride. Procedures for pre-con-centration, off-line separation, and sample prepara-tion are all time consuming and have a potential tointroduce isotopic fractionation. This techniquegreatly enhances the efficiency for isotope analysisby eliminating these procedures and reduces thechance for isotopic fractionation.

The development of this compound-specifictechnique for chlorinated ethenes may allow Clstable isotopes to be used more extensively indetermining contaminant sources and understand-ing processes occurred in groundwater contamina-tion sites.

Acknowledgements

Funding and resources for this project in theEnvironmental Isotope Lab (EIL) were providedby Canadian Foundation for Innovation and On-tario Innovation Trust and NSERC (S.K. Frape).The authors are very grateful for the encouragementand critical comments of Dr. Henry Holmstrandand Dr. Max Coleman. We also thank Oya Albak,Mary Ellen Patton and Asma Kaghdou for theirsupport and assistance.

References

Beneteau, K.M., Aravena, R., Frape, S.K., 1999. Isotopiccharacterization of chlorinated solvents – laboratory andfield results. Org. Geochem. 30, 739–753.

Holmstrand, H., Andersson, P., Gustafsson, O., 2004. Chlorineisotope analysis of submicromole organochlorine samples bysealed tube combustion and thermal ionization mass spec-trometry. Anal. Chem. 76, 2326–2342.

Holt, B.D., Heraty, L.J., Sturchio, N.C., 2001. Extraction ofchlorinated aliphatic hydrocarbons from groundwater atmicromolar concentrations for isotopic analysis of chlorine.Environ. Pollut. 113, 263–269.

Holt, B.D., Sturchio, N.C., Abrajano, T., Heraty, L., 1997.Conversion of chlorinated volatile organic compounds to

carbon dioxide and methyl chloride for isotopic analysis ofcarbon and chlorine. Anal. Chem. 69, 2727–2733.

Huang, L., Sturchio, N.C., Abrajano Jr., T., Heraty, L.J., Holt,B.D., 1999. Carbon and chlorine isotope fractionation ofchlorinated aliphatic hydrocarbons by evaporation. Org.Geochem. 30, 777–785.

Jendrzejewski, N., Eggenkamp, H.G.H., Coleman, M.L., 1997.Sequential determination of chlorine and carbon isotopiccomposition in single microliter samples of chlorinatedsolvents. Anal. Chem. 69, 4259–4266.

Jendrzejewski, N., Eggenkamp, H.G.H., Coleman, M.L., 2001.Characterisation of chlorinated hydrocarbons from chlorineand carbon isotopic compositions: scope of application toenvironmental problems. Appl. Geochem. 16, 1021–1031.

Kaufmann, R., Long, A., Bentley, H., Davis, S., 1984. Naturalchlorine isotope variations. Nature 309, 338–340.

Keith, L.H., Tilliard, W.A., 1979. Priority pollutants, I-aperspective view. Environ. Sci. Technol. 13, 416–423.

Long, A., Eastoe, C.J., Kaufmann, R.S., Martin, J.G., Wirt, L.,Finley, J.B., 1993. High-precision measurement of chlorinestable isotope ratios. Geochim. Cosmochim. Acta 57, 2907–2912.

McCulloch, A., Midgley, P.M., 1996. The production and globaldistribution of emissions of trischloroethene, tetrachloroeth-ene and dichloromethane over the period 1988–1992. Atmos.Environ. 30, 601–608.

Numata, M., Nakamura, N., Koshikawa, H., Terashima, Y.,2002a. Chlorine stable isotope measurements of chlorinatedaliphatic hydrocarbons by thermal ionization mass spectrom-etry. Anal. Chim. Acta 455, 1–9.

Numata, M., Nakamura, N., Koshikawa, H., Terashima, Y.,2002b. Chlorine isotope fractionation during reductivedechlorination of chlorinated ethenes by anaerobic bacteria.Environ. Sci. Technol. 36, 4389–4394.

Poulson, S.R., Drever, J.I., 1999. Stable isotope (C, Cl, and H)fractionation during vaporization of trichloroethylene. Envi-ron. Sci. Technol. 30, 3689–3694.

Rees, C.E., 1978. Sulphur isotope measurements using SO2 andSF6. Geochim. Cosmochim. Acta. 42, 383–389.

Shouakar-Stash, O., Drimmie, R.J., Frape, S.K., 2003. Stablehydrogen, carbon and chlorine isotope measurements ofselected chlorinated organic solvents. J. Contam. Hydrol. 60,211–228.

Sturchio, N.C., Clausen, J.L., Heraty, L.J., Huang, L., Holt,B.D., Abrajano Jr., T.A., 1998. Chlorine isotope investigationof natural attenuation of trichloroethene in an aerobicaquifer. Environ. Sci. Technol. 32, 3037–3042.

Tanaka, N., Rye, D.M., 1991. Chlorine in the stratosphere.Nature 353, 707.

van Warmerdam, E.M., Frape, S.K., Drimmie, R.J., Flatt, H.,Cherry, J.A., 1995. Stable chlorine and carbon isotopemeasurements of selected chlorinated organic solvents. Appl.Geochem. 10, 547–552.