-
Improvement of analytical method for chlorine dual-inlet
isotoperatio mass spectrometry of organochlorines
Tetyana Gilevska1, Natalija Ivdra1,2, Magali Bonifacie3 and
Hans-Hermann Richnow1,2*1Department of Isotope Biogeochemistry,
Helmholtz Centre for Environmental Research UFZ, Permoserstr. 15,
D-04318Leipzig, Germany2Isodetect GmbH Company for Isotope
Monitoring, Deutscher Platz 5b, D-04103 Leipzig, Germany3Group of
the Geochemistry of Stable Isotopes, The Institute of Earth Physics
of Paris, Sorbonne Paris Cit, Universit ParisDiderot, UMR 7154
CNRS, F-75005 Paris, France
RATIONALE: The development of compound-specic chlorine isotope
analysis (Cl-CSIA) is hindered by the lack ofinternational
organochlorine reference materials with isotopic compositions
expressed in the 37Cl notation. Thus, areliable off-line analytical
method is needed, allowing direct comparison of the 37Cl values of
molecularly differentorganic compounds with that of ocean-water
chloride, to refer measurement results to a Standard Mean Ocean
Chloride(SMOC) scale.METHODS: The analytical method includes
sealed-tube combustion of organochlorines, and precipitation
andsubsequent conversion of the formed inorganic chlorides into
methyl chloride (CH3Cl) for the determination of 37Clvalues by
Dual-Inlet Isotope Ratio Mass Spectrometry (DI-IRMS). A sample
preparation step most sensitive to thesample size dissolution of
the inorganic copper chlorides formed by combustion of -HCH was
identied.RESULTS: Recovery of 94 5% of chlorine could be reached by
applying determined optimal conditions for thedissolution, implying
good external precision of 37Cl values (0.18 0.03, 1, n = 3).
Validation of the optimizedmethod by the analysis of the produced
and initial CH3Cl sample with known 37Cl values vs SMOC resulted in
adifference of 0.11 0.04 (1), conrming the external precision and
accuracy of the entire method.CONCLUSIONS: The efciency of the
sample preparation method for CH3Cl-DI-IRMS analysis is independent
both ofthe chemical structure of the chlorinated compound and of
the amount of chlorine in the sample. This method has thepotential
to be applied to a broad range of chlorinated organic compounds,
e.g. reference material for the calibrationof methods for Cl-CSIA
against SMOC. Copyright 2015 John Wiley & Sons, Ltd.
Organochlorine pollutants are of great interest to
environ-mental scientists due to their persistence,
bioaccumulationand frequent toxicity.[1] Isotopic compositions
analysis hasfacilitated better understanding of the environmental
fate oforganic chlorinated compounds (OCs) by tracing their
sourcesand transformation processes.[2,3] Although the
determinationof chlorine stable isotope composition (37Cl values)
has thepotential to help identify the sources and degradation ofOCs
in the environment, its application is mainly limited bythe
currently available analytical techniques. In contrast tothe number
of efcient high-throughput methods developedand routinely applied
for compound-specic isotope analysis(CSIA) of carbon, hydrogen,
oxygen and nitrogen, methodsfor the CSIA of chlorine in
organochlorine compounds stillneed to be improved for routine
applications.[4,5] In theexisting analytical set-ups mixtures of
organochlorine
compounds are generally separated by gas chromatography(GC) and
then transferred by a carrier gas either to a conver-sion unit for
production of the HCl by high-temperaturecombustion (GC-HTC) which
is then used for the analysis ofchlorine isotopes,[6,7] or directly
to the mass analyzer unit formeasurement of the chlorine isotope
ratios.[8,9] Currently,Isotope Ratio Mass Spectrometry
(IRMS),[8,10] QuadrupoleMass Spectrometry (qMS)[8,1113] and
Multiple CollectorInductively Coupled Plasma-source Mass
Spectrometry(MC-ICPMS)[9] are used as mass analyzers for
thedetermination of 37Cl values. The majority of the
existingCl-CSIA methods demand comparison with molecularlyidentical
reference compounds to refer isotopic ratios of targetanalytes to
the Standard Mean Ocean Chloride (SMOC) scale,and for the quality
control following the principle of identicaltreatment.[14] Thus far
organic international standards forCSIA of chlorine are not
available. Therefore, the chlorineisotope composition of reference
materials must be previouslydetermined by off-line methods which
allow direct referen-cing with ocean-water chloride
samples.Thermal-ionization mass spectrometry (TIMS) and dual-
inlet isotope ratio mass spectrometry (DI-IRMS) are the
massspectrometric methods in current use for the determination
of37Cl values directly related to the SMOC scale. As reported
*Correspondence to: H.-H. Richnow, Department of
IsotopeBiogeochemistry, Helmholtz Centre for EnvironmentalResearch
UFZ, Permoserstr. 15, D-04318 Leipzig, Germany.E-mail:
[email protected] authors contributed equally to this
work
Copyright 2015 John Wiley & Sons, Ltd.Rapid Commun. Mass
Spectrom. 2015, 29, 18
Research Article
Received: 10 February 2015 Revised: 27 April 2015 Accepted: 28
April 2015 Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2015, 29,
18(wileyonlinelibrary.com) DOI: 10.1002/rcm.7220
1Journal Code Article ID Dispatch: 08.05.15 CE:
R C M 7 2 2 0 No. of Pages: 8 ME:
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by Rosenbaum et al. for seawater samples,[15] DI-IRMS is
bestsuited for samples containing >10 mol Cl, allows 37Clvalues
to be measured with a precision of 0.1 (2). TIMSis more sensitive
and applicable for samples containing~0.10.3 mol of Cl with
achievable uncertainties of 0.2.However, these uncertainties do not
directly refer to the37Cl values of OCs since both methods require
off-lineconversion of organochlorine compounds into
inorganicchloride, which then can be processed in parallel
withstandard samples of ocean-water chloride for the analysis.Prior
to DI-IRMS analyses, chlorine from OCs is rst
transformed into CuCl via combustion with CuO (SchemeS1 1,path
a)[16,17] or into NaCl by trapping Cl into a sodiumcarbonate
solution after an oxygen combustion bomb(Scheme 1, path b).[18]
Alternatively, organic chlorine can betransformed into NaCl via
reductive dehalogenation.[19,20]
The obtained inorganic chlorides are then converted intoAgCl
(Scheme 1, paths c and d) and subsequently into CH3Clfor DI-IRMS
(Scheme 1, path e).[16] Alternatively, CuCl,obtained in the
combustion reaction, can be directlyconverted into CH3Cl for
37Cl measurements by DI-IRMS(Scheme 1, path f).[2123]
Different physical and chemical processes during theconversion
of OCs could generate isotopic fractionation,which needs to be
thoroughly quantied. It has been reportedthat losses in the
conversion of CuCl into CH3Cl and duringthe purication of CH3Cl
could bias the obtained
37Cl value,leading to a correlation between the recovery of
chlorine in thesample preparation procedure and the accuracy of the
37Clvalues.[17,21] Many attempts have been made to
optimizedifferent sample preparation steps in order to reach
thehighest possible recoveries of chlorine and the
reproducibilityof 37Cl values for the entire procedure.[17,22,23]
However, noclear relationship between losses at each separate
conversionstep and changes in the isotopic compositions has
beenreported up to now. In addition, all previously reportedmethods
were optimized for specic amounts of combustedchlorinated compound,
so the applicability and efciency ofthese procedures for samples
containing different amountsof chlorine stayed unrevealed.
In this study we present a sample preparation method priorto
CH3Cl-DI-IRMS, which holds the potential to be used forthe
determination of 37Cl values of organochlorines asreference
material for the calibration of the Cl CSIA methodsagainst SMOC. We
selected off-line conversion of OCs intoCH3Cl via CuCl and AgCl,
which allows one to obtain isotopicprecisions of 0.15 and high
overall recoveries of chlorine(>97%).[16,24] The rst step of the
procedure combustion ofOCs to CuCl is applicable to a broad range
of organiccontaminants from small molecules, such as
chlorinatedmethanes and ethenes,[21,23] to chemically persistent
complexmolecules, such as chlorinated pesticides.[17,22]
We have chosen -hexachlorocyclohexane (-HCH, Lindane)as a model
compound for the optimization of the samplepreparation method. -HCH
and other isomers of HCH wereused worldwide as agricultural
insecticides until they werebanned or restricted to specic
applications by the StockholmConvention on persistent organic
pollutants.[25,26]
In the course of this study, we investigated in detail
eachtransformation from -HCH into CH3Cl to determine theinuence on
the accuracy of obtained 37Cl values of chlorinerecoveries at
different sample preparation steps. The leastefcient steps, leading
to changes in the isotopic compositions,were identied and
optimized. Our sample preparationmethod is applicable for different
sample sizes, as we proposethe optimal water/chlorine ratio for the
dissolution step withthe possibility of recalculating the necessary
volume of waterfor each particular amount of chlorine. We applied
theoptimized procedure to a CH3Cl sample with a known
37Clvalue relative to SMOC. Finally, we compared the
obtained37Cl value of the recovered CH3Cl and that of
non-processedCH3Cl to conrm the accuracy and precision of the
optimizedmethod.
EXPERIMENTAL
Solvents and chemicals
Acetone (99.5%), potassium nitrate (KNO3; 99%), citric
acidmonohydrate (C6H8O7*H2O; 99.5%), silver nitrate (AgNO3)and
copper(I) chloride (CuCl; 99.9%) were purchased fromCarl Roth
(Karlsruhe, Germany). Ultra-high purity (uhp)-water (resistivity of
18 M) was prepared with a MerckMilli-Q A+ system from Millipore
(Billerica, MA, USA).-HCH (99.1%) was purchased from HiMEDIA
(Mumbai,India); CH3Cl (99.90%, #N30) was obtained from Air
Liquide(Paris, France); copper oxide (CuO; >98%) and
potassiumphosphate dibasic dihydrate (Na2HPO4*2H2O; 98%)
wereobtained from Sigma Aldrich, (Steinheim, Germany). Nitricacid
(HNO3; 69%) was purchased from Merck (Darmstadt,Germany). Quartz
glass (Schott) tubes (20 mm o.d.) wereprepared at the glass
workshop of the Helmholtz Centre forEnvironmental Research UFZ
(Leipzig, Germany).
Initial procedure for conversion of -HCH into CH3Cl
-HCH was converted into CH3Cl prior to Cl DI-IRMS. Weup-scaled
the method to 250 mol of Cl to ensure reliableDI-IRMS analysis over
all optimization steps and tominimize the inuence of a blank on the
chlorine sample.The sample preparation procedure consisted of the
following
Scheme 1. Chlorine from OCs is rst transformed into CuClvia
combustion with CuO (path a)[16,17] or into NaCl bytrapping Cl into
a sodium carbonate solution after an oxygencombustion bomb (path
b).[18] Alternatively, organic chlorinecan be transformed into NaCl
via reductive dehaloge-nation.[19,20] The obtained inorganic
chlorides are thenconverted into AgCl (paths c and d) and
subsequently intoCH3Cl for DI-IRMS (path e).
[16] Alternatively, CuCl, obtainedin the combustion reaction,
can be directly converted intoCH3Cl for
37Cl measurements by DI-IRMS (path f).[2123]
T. Gilevska et al.
wileyonlinelibrary.com/journal/rcm Copyright 2015 John Wiley
& Sons, Ltd. Rapid Commun. Mass Spectrom. 2015, 29, 18
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steps: step 1: high-temperature combustion of -HCH to CuClin an
evacuated sealed quartz ampule with excess of CuO(Eqn. (1)); step
2: disproportionation of Cu(I)Cl to formCu(II)Cl2 and dissolution
of the formed soluble CuCl2 inwater (hereafter called dissolution)
(Eqn. (2)); step 3:precipitation of chlorides as AgCl (Eqn. (3));
and step 4:halogen exchange reaction of AgCl with an excess of CH3I
toform CH3Cl (Eqn. (4)). The obtained gaseous CH3Cl was
thenchromatographically separated from excess CH3I and puriedbefore
DI-IRMS at the Institut de Physique du globe de Paris(IPGP
Laboratoire de Gochimie des Isotopes Stables, Paris,France)
following the procedure routinely used to performDI 37Cl
measurements (see section 2 of the SupportingInformation).[2732] In
step 1 the ratio of CuO to the mass ofCl in the sample was 200 mg
CuO per 1 mg of Cl (that is~7.1 mg for 1 mol of Cl); in accordance
with Holmstrandet al.,[17] the combustion temperature was 620
C[16,17] andthe volume of water for the dissolution process in step
2 was25 mL.[16] The amounts of chemicals for the precipitation
ofAgCl in step 3 were calculated proportionally to the amountof Cl
reported by Jendrzejewski et al.[16] (detailed amountsand further
experimental details of the procedure can befound in section 1 of
the Supporting Information).
step 1 : HCH 24 CuO9 Cu2O 6 CuCl 6 CO23 H2O (1)
step 2 : 2Cu I ClCu Cu II Cl2 aq (2)
step 3 : CuCl2 aq 2 AgNO3 aq Cu NO3 2 aq 2 AgCl (3)
step 4 : AgCl CH3I excess CH3ClAgI (4)
Optimized procedure
The entire sample preparation procedure after
optimizationconsisted of the following steps: step 1: organic
sample,containing 250 2 mol of Cl was transferred to the
quartzampule (pre-heated at 700 C for 1 h), containing 1.8 g ofCuO
(pre-heated at 800 C for 1 h). The ampule was thenevacuated to
~1200 mbar, and sealed, while the lower partwas immersed in liquid
nitrogen. The ampule wasafterwards heated in the furnace (with a
temperatureincrease of 5 C/min towards 620 C and kept at 620 C for1
h), before being allowed to cool to room temperature (r.t.),washed
with 10% HNO3 and uhp-water. Step 2: the ampulewas broken and all
solid residues together with quartz culletwere transferred to a
screw-capped bottle, containing 80 mLuhp-water, followed by
vortex-mixing for 1 min and 1 h ofsonication at 50 C. The
solutionwas decanted from the solidresidues, ltered through a nylon
lter (0.22 m) and theltration residues were rinsed with an
additional 20 mL ofwater to obtain a total volume of 100 mL for the
rst extract(H2O/Cl ratio 0.4 mL/mol). Subsequently, 80 mL of
freshuhp-water were added to the solid residues, the
dissolutionprocedure was repeated, nished with washing of
theltration residues with an additional 20 mL of water, and thetwo
extracts were combined to obtain 200 mL of the nalsolution. Step 3:
20 g of KNO3 and a pH 2.2 buffer (4.5 g ofcitric acid*H2O+140mg of
Na2HPO4*2H2O) in dry formwereadded to the obtained CuCl2 solution
and heated at 80 C until
complete dissolution of KNO3 and buffer reagents.Afterwards,
3.75 mL of 1 M AgNO3 solution were added.The solution was left in
darkness to cool down for 1 h andthe suspension was ltered on a
glass microber lter anddried in the darkness at r.t. The obtained
AgCl on the lterwas split into 24 subsamples to give approximately
50 molof chlorine on each part of the lter. Step 4: each lter
partwas then placed in a borosilicate glass tube, excess of
CH3I(100 L) was added, the tube sealed and left at 80 C for 72 hto
form CH3Cl. CH3Cl was then separated from excess CH3Iby preparative
gas chromatography using two Porapak-Qlled columns and analyzed by
DI-IRMS as described byEggenkamp[32] and Bonifacie et al.[31]
Dual-inlet isotope ratio mass spectrometry (DI-IRMS)
The 37Cl measurements of CH3Cl gas were performed usinga triple
collector gas-source dual-inlet mass spectrometer(Delta plus XP;
Thermo Fisher Scientic, Bremen, Germany)at IPGP. The 37Cl values
were obtained by determining thesignal intensity of m/z 52 (CH3
37Cl+) and m/z 50 (CH335Cl+)
using two different collectors with resistances of 1109and 3108,
respectively. One measurement consisted of aseries of 10 individual
comparisons of the ratio 52/50 in thesample CH3Cl to that of the
CH3Cl gas used as a laboratorystandard. The reference gas is
compared with CH3Clprepared from seawater chloride at least twice a
day, andtypically following each 5 to 6 samples. This
procedurechecks for instrumental drift during the day, and allows
directreferencing of the 37Cl values of unknown samples to theSMOC
scale.[2830] The chlorine isotope composition of eachproduced CH3Cl
sample was determined by DI-IRMS twice.
Gas chromatography/mass spectrometry (GC/MS)
The concentration of the organic compounds was determinedby
GC/MS to test the conversion of -HCH into copperchloride. Details
of the GC/MS method are available insection 3 of the Supporting
Information.
Ion chromatography (IC)
IC analyses of chlorine concentration were performed at
theDepartment of Analytical Chemistry at UFZ on a DionexICS-2000
ion chromatograph (Thermo Fisher Scientic).Detailed information can
be found in section 4 of theSupporting Information.
Chloride blanks
Chloride blanks were analyzed to investigate the total
addedamount of chlorine at the dissolution step and through
theentire conversion of OCs into AgCl. Two experiments
wereperformed, using 1.8 g subsamples of commercially availableCuO.
The rst subsample was suspended in 200 mL of uhp-water (as for two
subsequent extractions of 250 mol of Cl).The optimized dissolution
procedure was then applied andthe concentration of the chlorine
extracted from the mixturewith CuO was measured by IC. The second
subsample ofCuO was heated in a sealed evacuated ampule
andsubsequently extracted with 200 mL of uhp-water asdescribed
above for the optimal procedure. A 1 mL aliquotof the ltered
solution was taken for the determination of
Chlorine stable isotope analyses of organic compound
Rapid Commun. Mass Spectrom. 2015, 29, 18 Copyright 2015 John
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chlorine concentration. The rest of the solution was subjectedto
the AgCl precipitation procedure. The mass of the driedAgCl
precipitates and the chloride ion concentration in theltrate were
determined. From these experiments the amountof chlorine which may
be added over all steps of the entireprocedure was determined.
RESULTS AND DISCUSSION
Chloride blanks
The chlorine concentration, measured by IC, in both
preparedblank solutions was determined as 0.11 mg/L,
whichcorresponded to 0.1% of the amount of Cl in the
initialorganochlorine compound. Furthermore, there was nomeasurable
amount of AgCl recovered on the lter after theprecipitation
procedure. Thus, the chloride blanks both ofthe dissolution step
(Eqn. (2)) and of the entire optimizedconversion procedure showed
no signicant amount ofexternal chlorine.
Initial procedure: efciency and accuracy
The initial procedure for the conversion of -HCH into AgClshowed
low recovery yields of chlorine (23 3% (n= 2)calculated from the
mass of precipitated AgCl) (TableT1 1). Thisis in sharp contrast to
the nearly complete conversion yields
reported by previous studies.[16,17] Such low recovery yieldsof
chlorine suggest that the methods previously developedcannot be
applied to the higher amount of -HCH withoutadjustments. The
measured 37Cl value of CH3Cl, producedfrom -HCH by the initial
procedure, was 1.15 0.34(n = 2) (Table 1). -HCH is generally
produced by thechlorination of benzene with Cl2 gas, derived from
brines,with 37Cl values typically between 0.5 and 0.[30,33]
The signicantly lower 37Cl value of -HCH that weobserved in
experiments with only 23% yield, than thevalue of brine, could thus
result from chlorine isotopicfractionation due to preferential loss
of 37Cl isotopes. Inaddition, the external precision of 0.34 is
much largerthan usually achieved for the sample preparation, i.e.
fromseawater (0.08, 2), indicating that the samplepreparation
procedure used was not optimal under theapplied conditions and for
the amount of chlorine taken.Thus, a revision of all conversion
steps with respect to the37Cl precision and accuracy was conducted,
as presentedbelow.
Revision and optimization of sample preparation steps
The efciency of each individual step (Eqns. (1) to (4))
wastested by determining the recovery yields (dened as therecovered
amount of Cl, measured by weighing AgClprecipitates or
determination of the chlorine concentration
Table 1. Optimization of sample preparation method with -HCH
Optimization parametersWater ratio,mL/mol Cl
Chlorinerecoveries, %
37Cl valuea
(n = 2),
Average 37Clvalue from multiple
ampulesb,
Initial procedure 0.10 22.2 1.39 1.150.34 (n = 2)21.6 0.91
Sonication at the dissolution procedureVortex mixing for 1 min
0.06 1Sonication for 2 h 0.06 32Sonication for 1 day 0.06
38Sonication for 2 days 0.06 34Repeated extraction 0.10 57.1 0.60
-0.630.10 (n = 2)
44.9 0.70Enhanced oxygen supply 0.06 31Elevated temperature 0.06
49Water volume 0.28 53
2.00 695.00 73 0.12 0.170.08 (n = 2)
76 0.22Conversion of -HCH under optimized conditions
82 0.291st extraction 0.40 93 0.25 0.260.02 (1,n= 3)
90 0.257 0.66
2nd extraction 0.40 6 0.89 0.920.28 (1,n= 3)6 1.22
Combinedc 94 5 0.180.03 (1,n= 3)aDifference between duplicate
measurements of the same prepared CH3Cl sample was typically below
0.02.b external precision of sample preparation procedure from
different subsamples of -HCH.cCombined extract represents the sum
of recoveries from two subsequent extractions and calculated 37Cl
values, based onthe mass balance.
T. Gilevska et al.
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in the solution by HPLC/the expected amount of Cl). Thepotential
impact of these losses on the obtained 37Cl valueswas then taken
into consideration.Step 4 transformation of AgCl, obtained from
seawater
standard solutions, into CH3Cl (Eqn. (4), following exactlythe
protocol of Jendrzejewski et al.[16] and routinely used
inIPGP[2830] (Supporting Information, section 2)) proved tobe
complete (overall yields close to 100%, including thepurication of
CH3Cl from CH3I by preparative GC) and thusshould not inuence the
accuracy of the isotope values in theinvestigated sequence of
steps.Therefore, for the further revision of the previously
suggested procedure and its adaptation for large samples of-HCH,
we took into account the following parameters: (1)incomplete
combustion of -HCH in step 1; (2) incompleteprecipitation of AgCl
in step 3; (3) inefcient dissolutionprocess in step 2.
(1) Incomplete combustion of -HCH may lead to theformation of
partly dechlorinated organic by-productsassociated with lower
yields of obtained CuCl andsubstantial changes in the 37Cl values
of CH3Cl. We testedthe conversion of -HCH into CuCl by combusting
-HCHas described in the initial procedure. After reaction,
theampule was slowly cooled down to +5 C, then crackedand the
copper oxide particles were dispersed in 10 mL ofacetone to
dissolve any unreacted organic materialpotentially remaining in the
ampule. GC/MS analysis ofthe residues after combustion showed that
0.01% of theinitially loaded compound was recovered,
conrmingcomplete conversion of -HCH into inorganic
reactionproducts. No traces of other organic products of
-HCHdegradation were detected. These results were in the
goodagreement with reports on complete combustion forDDTs[17] and
polychlorinated hydrocarbons[16,34] andconrm that the selected
combustion conditions aresuitable for structurally different
OCs.
(2) Efciency of the AgCl precipitation procedure (Eqn. (3))was
tested by applying it to a solution of NaCl in 25 mLof water with
chlorine ion concentration equal to theCuCl concentration after
100% conversion of 250 molof Cl from -HCH. A test of the
precipitation efciencyresulted in 36.6 mg of AgCl being
precipitated from theNaCl solution, corresponding to 102% (the
efciencyhigher than 100% could result from the uncertainty
ofweighting NaCl and/or AgCl). This completeprecipitation of
chloride is reinforced by the fact that notraces of remaining
dissolved chlorine were detected byIC analysis of the ltrate after
precipitation.
(3) Dissolution of CuCl. After conrmation that both
thecombustion and the precipitation processes are completeat
selected conditions, both in agreement with previouslyreported
results for entire procedures,[16,17] we identiedthat CuCl2
formation and dissolution is the least efcientstep of the
conversion of OCs into AgCl. We testedseveral modications of this
particular step: (i)sonication, (ii) repeated extraction, (iii)
elevatedtemperature, (iv) oxygen supply, and (v) water volume.
(i) As previously suggested by Jendrzejewski et al.,
insufcientdissolution time may lead to incomplete extraction
ofwater-soluble chlorine species.[16] Thus, introduction of a
sonication step allowed us to signicantly increase thetotal
recoveries of chlorine in the dissolution process andto shorten the
overall time needed for this step. Wedetermined the concentration
of dissolved chlorine in thesolution after 1 min, 2 h, 1 day and
nally after 2 days ofsonication, corresponding to 1, 32, 38 and 34%
of theinitially loaded chlorine, respectively (Table 1). This
testsuggests that increasing the sonication time does
notsignicantly increase the chlorine recovery yields.
(ii) Subsequently, we tested if the yield of the
dissolutionprocess can be increased by repeated extraction.
Aftercombining two extracts and washings after 2 h ofsonication and
subsequent ltration, 51% of the chlorinewas recovered in
precipitated AgCl and then convertedinto CH3Cl (Table 1). A 13%
increase of the efciency inthe combined extract relative to the
result achieved in theprevious experiment by a single extraction
after 2 h ofsonication showed that it might be necessary to
repeatan extraction from the same solid residues aftercombustion.
DI-IRMS analysis of combined extractsshowed an elevated 37Cl value
of 0.63 0.10 (n=2)compared with the value of 1.15 0.34 (n=2)
obtainedthrough the initial procedure and associated with morethan
two times lower overall yields (23%). These resultssuggest that the
isotopically heavier fraction of chlorinecan still remain
undissolved in the residues after anincomplete dissolution process,
leading to biased lower37Cl values of extracted chlorine. This
observationconrms that it is crucially important to achieve
completetransformation of the dissolution process for the
accuratedetermination of the 37Cl values.
(iii) During the dissolution step (Eqn. (2)) the
water-insolubleby-product copper oxychloride (Cu3Cl2(OH)4) can
beformed by the oxidation of CuCl by atmospheric oxygenin the
presence of water (Eqn. (5)).
4 CuClO2 2 H2OCu3Cl2 OH 4 CuCl2 aq (5)
On the other hand, copper formed in the
disproportionationreaction at the dissolution step (Eqn. (2)) may
be thenoxidized with oxygen, shifting the reaction (Eqn.
(2))equilibrium towards the formation of the desired
product(water-soluble CuCl2). Thus, by introducing a gentle
oxygenow for 2 h into the CuOCuCl water suspension aftercombustion,
we tested whether an enhanced oxygen supplyaffects the prevalence
of the disproportionation process (Eqn.(2)) over the competing
oxidation reaction (Eqn. (5)) to obtainmaximum possible recoveries
of chlorine. An enhancedoxygen supply caused only insignicant
improvement ofthe efciency of dissolution, allowing us to recover
31% ofchlorine in the 15 mL of the extraction solution (Table
1).Thus, the negative effects of the possible side-reaction
(Eqn.(5)) were negligible or compensated for by the
improvedkinetics of the dissolution (Eqn. (2)) due to the better
mixingof the solution. Thus, an enhanced oxygen supply was
notconsidered as an optimization measure.
(iv) We tested the impact of elevated temperature on theefciency
of the dissolution process by heating thesuspension of CuOCuCl,
formed in step 1, at 90 C for2 h in a water bath. Formation of HCl
in a further reactionof the combustion products CuCl and water
(Eqn. (6)),
Chlorine stable isotope analyses of organic compound
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followed by its volatilization, was reported by Holt
andSturchio,[21] as probably being the major source of thelosses of
chlorine, associated with more variable 37Clvalues.
2 CuClH2OCu2O 2 HCl (6)
To prevent any losses of chlorine in the formofHCl, the
heatingwas performed in screw-capped bottles. Elevated
temperatureat the dissolution step allowed us to reach 49%
conversion ofchlorine (compared with 34% conversion, obtained with
thesame water/Cl ratio by 2 days of sonication at the
roomtemperature), showing the temperature-enhanced kinetics ofthe
dissolution process (Eqn. (2)). Thus, we used a temperatureof 50 C
in the ultrasonic bath during the sonication forfurther
experiments.
(v) The most signicant effect on the efciency of thedissolution
process was caused by changing the volumeof water during extraction
of the soluble chlorine species(Eqn. (2)) after combustion. By
increasing the water/Clratio from 0.1 to 5 mL/mol 76% Cl recovery
was reached.The recovered Cl had a 37Cl value of 0.17 0.08 (n=
2),that is 0.98 higher than the value of 1.15 0.34obtained for the
lowest recoveries of chlorine in the initialprocedure, and thus
exhibiting the same trend towardsincreasing 37Cl value with
increasing chlorine recoveryyields. In an additional set of
experiments with dissolutionof commercially available crystalline
CuCl in differentvolumes of uhp-water (ratio mL/mol Cl from 0.36
to8.00, section 5, Supporting Information) we determinedthe optimal
water/Cl ratio as 0.4 mL/mol, resulting in96% chlorine recovery
(Supplementary Table S-1,Supporting Information). Thus, a water/Cl
ratio of0.4 mL/mol was used for the further optimization steps.
Conversion of -HCH under optimized conditions
We applied the total set of optimized parameters
(repeatedextraction by sonication for 2 h at 50 C with two times100
mL of water, corresponding to a water/Cl ratio of0.4 mL/mol) to
prove the efciency of the full procedureboth in terms of complete
chlorine recoveries and in thereproducibility of the determined
37Cl values (Table 1). Totest if the remaining residues of chlorine
species after a non-complete dissolution process have the same
chlorine isotopiccomposition as initially dissolved CuCl2,
fractions from tworepeated extractions were separately analyzed for
chlorinecontents and 37Cl values. The optimized extractionprocedure
was performed in triplicate from three combustedsubsamples of -HCH.
Chlorine concentration and DI-IRMSmeasurements showed that 88% of
the chlorine, with a 37Clvalue of 0.26 0.02 (n = 3), can be
recovered in the rstextraction of CuCl2, but the second recovered
fraction ofsoluble chlorine salt (6%) was signicantly enriched in
37Cl,exhibiting a 37Cl value of 0.92 0.28 (n = 3). We
havecalculated the combined 37Cl value of the two extracts withthe
94 5% of recovered chlorine (sum of recoveries from 1st
and 2nd fractions), based on the mass balance. The obtained
37Cl value of 0.18 0.03 (n = 3) more accurately representsthe
isotopic composition of the organochlorine compoundstudied here. We
hence conclude that incomplete chlorineextraction leads to
analytically biased too low 37Cl values ofthe studied
organochlorine compounds. Therefore, werecommend applying repeated
extractions to obtain completechlorine recovery and thus achieve
accurate and precisedeterminations of the Cl isotopic compositions
of organo-chlorine compounds.
Conversion of CH3Cl under optimized conditions
The determined optimal conditions of the dissolution processwere
incorporated into the full procedure for the conversionof
commercially available CH3Cl gas with known isotopiccomposition
(Table T22). The optimized conversion procedurethat we dened here
allowed CH3Cl to be transformed intowater-soluble CuCl2 and back to
CH3Cl with a slight off-setof 0.11 0.04 (1, n = 3) (Table 2), which
shows animprovement in comparison with a decrease of 0.23 0.05 (1)
for the entire sample preparation procedurewith 89% of recoveries
reported by Holt et al.[21]
Isotopic precision of the method
The external precision of all the 37Cl values of
seawaterindependently prepared and analyzed over the course of
thisstudywas0.07 (standard deviation of 15measurements);the shift
within 1 day was in the range from 0.02 to 0.07(n = 2). The
difference between duplicate DI-IRMS analysesof two introduced
subsamples of the same produced CH3Clwas 0.02. The external
precision of the method,determined from different ampules with
identical startingmaterial (-HCH), was 0.03% (1, n = 3) (Table 1).
Similarly,the precision of the 37Cl values, obtained through
the
Table 2. Validation of the overall optimized procedurewith
CH3Cl
37Clvalue ofcommercialCH3Cl,
After the entire sample preparationprocedure
37Clvaluea
(n = 2),
Average37Cl
value 1b
from 3ampules, 37Clc,
1.181.060.02 1.13 1.170.04 0.11 0.04
1.20aDifference between duplicate measurements of the
sameprepared CH3Cl sample, produced by subsequenttransformation of
commercial CH3Cl to soluble CuCln,precipitation of AgCl and
transformation back to theCH3Cl for the DI-IRMS analysis, was
typically below 0.01.bExternal precision of sample preparation
procedure fromdifferent subsamples of CH3Cl.cDifference between
37Cl value of unprocessed CH3Cl andthat of CH3Cl after the entire
sample preparationprocedure.
T. Gilevska et al.
wileyonlinelibrary.com/journal/rcm Copyright 2015 John Wiley
& Sons, Ltd. Rapid Commun. Mass Spectrom. 2015, 29, 18
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sample preparation procedure from CH3Cl, was 0.04(1, n = 3)
(Table 2). This external precision for OCs is ofthe order of the
uncertainty related to the preparation ofsea water standards (0.08,
2), when only inorganicchloride is processed to CH3Cl.Such a good
external precision for the studied compounds
(-HCH and CH3Cl) proves the applicability of the optimizedmethod
for the chlorine isotope analysis of organochlorines.
CONCLUSIONS
We present a method for the determination of the chlorinestable
isotope composition of -HCH by DI-IRMS in 37Clnotation vs SMOC,
using seawater for direct referencing. Inthe course of this study
the links between losses of chlorine atevery separate conversion
step and accuracy of the measured37Cl values were quantied.
Dissolution of water-solublechlorine salts, formed by combustion of
-HCH, was identiedto be the critical step,most sensitive to the
amount of chlorine inthe sample and associated with signicant
losses of chlorine.Incomplete chlorine extraction at this
dissolution step led toinaccurate lower 37Cl values of the studied
organochlorinecompound. Optimization of this step resulted in
completechlorine recoveries through the entire sample
preparationprocedure (945%) and more accurately determined
theisotopic composition of -HCH (0.18 0.03, 1), whichwas enriched
in 37Cl by 0.97 in comparison with the non-optimized initial
procedure. Thus, we recommend thedeveloped optimized chlorine
extraction method for thedetermination of the 37Cl compositions of
organochlorinecompounds. The optimized sample preparation
procedureshould be applicable for different amounts of chlorine in
thesample. Validation of the procedure with a CH3Cl sample
withknown isotopic composition proved the method to be accurateand
precise and showed a total deviation of 37Cl results of lessthan
0.11 0.04 (1, n = 3).As the most critical step is not related to
the oxidation of
the chlorinated compound and, thus, is independent of
itschemical structure, the presented optimized procedure holdsthe
potential for 37Cl determination of a broad range ofchlorinated
organic compounds.
AcknowledgementsWe gratefully acknowledge H. G. M. Eggenkamp,
ThomasGiunta, Gerard Bardoux and Michaela Wunderlich foranalytical
support and helpful discussions on the improve-ment of applied
methods, as well as Kristina Hitzfeld, JulianRenpenning and Angela
Woods for critical comments. Weacknowledge nancial support from the
European Unionunder FP7-People-ITN-2010 (Grant Agreement No.
264329).
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SUPPORTING INFORMATION
Additional supporting information may be found in theonline
version of this article at the publisher's website.
T. Gilevska et al.
wileyonlinelibrary.com/journal/rcm Copyright 2015 John Wiley
& Sons, Ltd. Rapid Commun. Mass Spectrom. 2015, 29, 18
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