Stable carbon isotope analysis of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in natural waters - Results from a worldwide proficiency test
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Received: 17 March 2013 Revised: 23 May 2013 Accepted: 21 June 2013 Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2013, 27, 20992107Stable carbon isotope analysis of dissolved inorganic carbon(DIC) and dissolved organic carbon (DOC) in natural waters Results from a worldwide proficiency test
Robert van Geldern1*, Mahendra P. Verma2, Matheus C. Carvalho3, Fausto Grassa4,Antonio Delgado-Huertas5, Gael Monvoisin6 and Johannes A. C. Barth11GeoZentrum Nordbayern, Applied Geosciences, Friedrich-Alexander-University Erlangen-Nuremberg, Schlossgarten 5, 91054Erlangen, Germany2Geotermia, Instituto de Investigaciones Elctricas, Reforma 113, Col. Palmira, Cuernavaca, Mor. C.P. 62490, Mexico3Centre for Coastal Biogeochemistry Research, Southern Cross University, Lismore 2480, NSW, Australia4Istituto Nazionale di Geofisica e Vulcanologia Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy5Laboratorio de Biogeoqumica de Istopos Estables, Instituto Andaluz de Ciencias de la Tierra IACT(CSIC-UGR), Avda. de lasPalmeras 4, 18100 Armilla, Granada, Spain6Laboratoire Interactions et Dynamiques des Environnements de Surface, Btiment 504, Universit Paris Sud, 91405Orsay, France
RATIONALE: Stable carbon isotope ratios of dissolved inorganic (DIC) and organic carbon (DOC) are of particularinterest in aquatic geochemistry. The precision for this type of analysis is typically reported in the range of 0.1 to0.5. However, there is no published attempt that compares 13C measurements of DIC and DOC among differentlaboratories for natural water samples.METHODS: Five natural water samples (lake water, seawater, two geothermal waters, and petroleum well water) wereanalyzed for 13CDIC and 13CDOC values by five laboratories with isotope ratio mass spectrometry (IRMS) in aninternational proficiency test.RESULTS: The reported 13CDIC values for lake water and seawater showed fairly good agreement within a range of about1, whereas geothermal and petroleum waters were characterized by much larger differences (up to 6.6 betweenlaboratories). 13CDOC values were only comparable for seawater and showed differences of 10 to 21 for other samples.CONCLUSIONS: This study indicates that scatter in 13CDIC isotope data can be in the range of several per mil forsamples from extreme environments (geothermal waters) and may not yield reliable information with respect todissolved carbon (petroleum wells). The analyses of lake water and seawater also revealed a larger than expecteddifference and researchers from various disciplines should be aware of this. Evaluation of analytical procedures of theparticipating laboratories indicated that the differences cannot be explained by analytical errors or different datanormalization procedures and must be related to specific sample characteristics or secondary effects during samplestorage and handling. Our results reveal the need for further research on sources of error and on method standardization.Copyright 2013 John Wiley & Sons, Ltd.
(wileyonlinelibrary.com) DOI: 10.1002/rcm.6665Dissolved carbon in natural waters is of particular interest ingeological, biological, and environmental studies. In aquaticgeochemistry the concentration measurements of carbonspecies (H2CO3, HCO3
and CO32) are a well-established and
widely used tool, which allows the evaluation of thebuffering capacity of water (alkalinity and acid neutralizingcapacity). These analyses are performed mainly by acid-basetitrations and provide information on the suitability of waterfor its different uses, efficiency of wastewater treatment,anthropogenic pollution and ecosystem health.[1,2] In addition* Correspondence to: R. vanGeldern, GeoZentrumNordbayern,AppliedGeosciences, Friedrich-Alexander-University Erlangen-Nuremberg, Schlossgarten 5, 91054 Erlangen, Germany.E-mail: firstname.lastname@example.org
Rapid Commun. Mass Spectrom. 2013, 27, 20992107
209to species distribution the source of carbon has also receivedincreasing interest in several geological and ecologicalstudies. The stable carbon isotope ratio (13C value) ofdissolved inorganic (DIC) and dissolved organic carbon(DOC) provides a natural label, which indicates the originand migration pathway of the carbon sources.
Numerous stable isotope laboratories worldwide conduct13CDIC and
13CDOC measurements on a routine basis. Atpresent, no soluble international isotope reference materialis available for 13C analyses of DIC and DOC. Despite thismajor drawback, this type of analysis is considered as astandard analytical technique. This might be true as long asdry pure chemicals, which are dissolved in deionized waterby the participating laboratories, are compared during aproficiency test. The analytical results from such freshlyprepared synthetic solutions might identify some analyticalCopyright 2013 John Wiley & Sons, Ltd.
R. van Geldern et al.
2100problems but will not provide any information about otheraspects that are valid for real natural water with realbacteriological and geochemical background matrices fromdifferent environments. The samples of this study weretherefore selected to represent realistic every-day samples inenvironmental research studies.In contrast to water stable isotope ratios (18O and 2H
values) no published attempt exists that compares 13Cmeasurements of DIC and DOC among different laboratories.This study presents the results of a first inter-laboratorycomparison study for the determination of carbon stableisotope ratios of DIC (13CDIC) and DOC (
13CDOC). Theexercise was announced through the Isogeochem mailing liston 30 April 2012 and involved five laboratories fromdifferent countries that reported values for 13CDIC; two ofthem also reported values for 13CDOC.EXPERIMENTAL
Water samples often originate from various environments suchas freshwater, seawater, or brines. Therefore, five natural watersamples were selected for this study instead of syntheticsamples prepared from deionized water and chemicals(Table 1). There are no reference 13C values that are regardedas correct (or true) for the samples as the key interest of thisstudy was on the relative differences between the laboratories.To ensure correct data normalization of the isotope ratio valuesto the Vienna Pee Dee Belemnite Scale (VPDB) everyparticipant reported details on its international and in-housereference materials and calibration procedures (Table 2).For the 13C inter-laboratory comparison, the samples
were distributed among the participating laboratoriesin 125 mL Nalgene (high-density polyethylene, HDPE)bottles. The new bottles were rinsed with deionized waterand the sample before filling. No preservatives (e.g. HgCl2)were added. Samples were filtered by a laboratory vacuumfiltration unit (pore size 0.45 m; Merck Millipore,Billerica, MA, USA) into a large container, homogenizedand subsequently filled into the individual bottles forshipping. All samples are natural waters collected fromdifferent environments with different DIC and DOCconcentrations. These water samples were also analyzedfor their carbon species distribution during anotherinternational proficiency test on species concentrations bychemical laboratories (M. P. Verma and colleagues, 2013,personal communication).Table 1. Natural water samples that were distributed for13C analyses and corresponding pH values (1) asdetermined by an international proficiency test on carbonspecies distribution (M. P. Verma and colleagues, 2013,personal communication)
Sample Water type pH
IIE30 lake water 8.290.15IIE33 geothermal water 7.380.11IIE34 geothermal water 7.170.26IIE35 seawater 7.760.23IIE36 petroleum well water 6.720.54
wileyonlinelibrary.com/journal/rcm Copyright 2013 John WileEach laboratory reported its methodology and theinstrumentation that was used to determine 13CDIC and13CDOC values and DIC concentrations (Table 2). Inprinciple, DIC is extracted from the solution by addingphosphoric acid that converts the different carbon speciesinto CO2 that is released into the headspace of the samplevessel (e.g. by agitation). Subsequently, the CO2 is analyzedfor its stable carbon isotope ratio by gas isotope ratio massspectrometry (IRMS). This method also allows the amountof released CO2 to be quantified either from the isotoperatio chromatogram or by a separate detector. Theconcentration of DIC in the samples can then bedetermined by a calibration with standards of knownconcentration prepared from DIC-free water and a readilysoluble carbonate (generally sodium bicarbonate).
In this study, four participating laboratories used IRMSsystems in continuous flow mode with helium carrier gas,and one used offline sample preparation and IRMS analysiswith a dual inlet technique (high vacuum sample inlet port)(Table 2). Laboratories no. 1 and 2 used a Gasbench II device(Thermo Fisher Scientific GmbH, Bremen, Germany) for theconversion of the DIC into CO2.
 This device uses 12 mLLabco exetainer (Labco Ltd, Lampeter, UK) with gas-tightbutyl rubber septa that are pre-loaded with a few drops of100% phosphoric acid, capped and filled with helium. Thesamples were transferred from the Nalgene bottles to thevials by disposable syringes. A comparable transfer techniquewas used by laboratory no. 4 with the difference that theyused smaller sample vials (5.9 mL) and a 10 mL gas-tightsyringe with stopcock (Hamilton Bonaduz AG, Bonaduz,Switzerland). Laboratory no. 3 filled the samples into 40 mLglass vials that meet the standards of the US EnvironmentalProtection Agency, so-called EPA vials, which weresubsequently capped by pierceable caps with septa. The vialswere filled gently to avoid air bubbles and to the brim withno headspace. The sample were transferred from theEPA vials by an autosampler to a fully automated Aurora1030W total organic carbon (TO