Measurement of δ18O, δ17O, and 17O‑excess in Water by Off-AxisIntegrated Cavity Output Spectroscopy and Isotope Ratio MassSpectrometryElena S. F. Berman,*,† Naomi E. Levin,‡ Amaelle Landais,§ Shuning Li,‡ and Thomas Owano†

†Los Gatos Research, 67 East Evelyn Avenue, Suite 3, Mountain View, California 94043, United States‡Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218,United States§Laboratoire des Sciences du Climat et l’Environnement, UMR 8212, IPSL, CEA/CNRS/UVSQ, 91190 Gif sur Yvette, France

ABSTRACT: Stable isotopes of water have long been used toimprove understanding of the hydrological cycle, catchmenthydrology, and polar climate. Recently, there has beenincreasing interest in measurement and use of the less-abundant 17O isotope in addition to 2H and 18O. Off-axisintegrated cavity output spectroscopy (OA-ICOS) is demon-strated for accurate and precise measurements δ18O, δ17O, and17O-excess in liquid water. OA-ICOS involves no sampleconversion and has a small footprint, allowing measurementsto be made by researchers collecting the samples. Repeated(514) high-throughput measurements of the internationalisotopic reference water standard Greenland Ice SheetPrecipitation (GISP) demonstrate the precision and accuracy of OA-ICOS: δ18OVSMOW−SLAP = −24.74 ± 0.07‰ (1σ) andδ17OVSMOW−SLAP = −13.12 ± 0.05‰ (1σ). For comparison, the International Atomic Energy Agency (IAEA) value forδ18OVSMOW−SLAP is −24.76 ± 0.09‰ (1σ) and an average of previously reported values for δ17OVSMOW−SLAP is −13.12 ± 0.06‰(1σ). Multiple (26) high-precision measurements of GISP provide a 17O-excessVSMOW−SLAP of 23 ± 10 per meg (1σ); an averageof previously reported values for 17O-excessVSMOW−SLAP is 22 ± 11 per meg (1σ). For all these OA-ICOS measurements, precisioncan be further enhanced by additional averaging. OA-ICOS measurements were compared with two independent isotope ratiomass spectrometry (IRMS) laboratories and shown to have comparable accuracy and precision as the current fluorination-IRMStechniques in δ18O, δ17O, and 17O-excess. The ability to measure accurately δ18O, δ17O, and 17O-excess in liquid waterinexpensively and without sample conversion is expected to increase vastly the application of δ17O and 17O-excess measurementsfor scientific understanding of the water cycle, atmospheric convection, and climate modeling among others.

Stable isotopic compositions of water, particularly δ2H andδ18O, have long been used to improve understanding of the

hydrological cycle,1 catchment hydrology,2 and polar climate,e.g., ref 3. The combination of the measurements, d-excess =δ2H − 8δ18O, roughly describes the degree to which δ2H valuesvary from what would be expected at low and temperate latitudesif equilibrium processes were solely responsible for the relationshipbetween δ2H and δ18O values. Kinetic fractionation processes,such as diffusive processes associated with evaporation, arepartially responsible for the variation in d-excess, which varies asa function of both evaporative temperature and humidity.1,3 Givenits sensitivity to environmental conditions of evaporation, d-excesshas been used extensively to characterize climate and hydrologicalconditions today and in the past, e.g., refs 3−6.Recently, there has been increasing interest in measurement

and use of the less-abundant 17O isotope in addition to 2H and18O. Similarly to d-excess, the deviation from an expectedrelationship between 17O/16O and 18O/16O ratios has beendefined by 17O-excess = ln(δ17O + 1) − 0.528 ln(δ18O + 1), and

variation in 17O-excess in waters is dominated by kineticfractionation effects.7 However, 17O-excess is relatively insensi-tive to temperature and thus can be used as a direct proxy forhumidity at the region of evaporation.7,8 Combined measure-ments of d-excess and 17O-excess allow researchers to understandmore fully the contributions of both temperature and relativehumidity to water isotope fractionation, thus improvingatmospheric models and understanding of climate processes,e.g., ref 8. Recent advances using fluorination coupled to isotope ratiomass spectrometry (IRMS) have made it possible to make precisemeasurements of 17O-excess in waters; these measurements havebeen applied to ice,8,9 meteoric waters,10 and African precipitation.11

The variation in 17O-excess in meteoric waters is generally avery small quantity (reported in per meg, parts per million), andmeaningful measurements require extremely high precision.

Received: July 29, 2013Accepted: September 16, 2013

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Careful studies of 17O-excess typically cite precisions for 17O-excess of less than 10 per meg, e.g., refs 8, 12, and 13, an order ofmagnitude smaller than the cited precisions for both δ18O andδ17O. While at first glance it seems impossible to measure aderived quantity to better precision than its constituentmeasurements, the improvement in precision is real and isdescribed in detail by Landais et al.14 and Luz and Barkan.10

Briefly, the definition of 17O-excess is derived from the measuredglobal meteoric water line (GMWL) for δ18O and δ17O so that17O-excess values indicate deviations from the GMWL. TheGMWL is itself naturally created by the isotopic fractionation ofwater due to vapor−liquid kinetics and equilibrium and vapordiffusion in air, and thus the slope of 0.528 in the 17O-excessdefinition12,15 is extremely close to the liquid−vapor equilibriumfractionation slope of 0.52912 and reasonably close to the kineticevaporation slope of 0.518.7 The consequence of this is thatmeasurement errors in δ18O and δ17O which result from mass-dependent equilibrium fractionation of a sample will cancelnearly perfectly on calculation of 17O-excess, provided that δ18Oand δ17O are measured on the same sample. This cancellation ofmass-dependent equilibrium fractionation errors can thus enablemeasurements of 17O-excess that are much more precise than theconstituent measurements of δ18O and δ17O.All of the previously published measurements of 17O-excess in

water have utilized fluorination of water into O2 followed byIRMS (see, e.g., Baker et al.16 and Barkan and Luz12 andreferences therein). This IRMS method requires chemicalconversion using toxic reagents and measurement on sophisti-cated and expensive equipment with highly skilled operators;only a handful of laboratories worldwide currently have thismeasurement capability. In this work we demonstrate for the firsttime the use of off-axis integrated cavity output spectroscopy(OA-ICOS) to directly measure δ18O, δ17O, and 17O-excess inliquid water. OA-ICOS involves no sample conversion and has asmall footprint, allowing measurements to be made byresearchers collecting the samples. This study demonstrates bycomparison with two independent IRMS laboratories that OA-ICOS measurements of δ18O, δ17O, and 17O-excess in liquidwater achieve accuracy and precision comparable to thefluorination and IRMS method.

■ EXPERIMENTAL SECTION

Off-Axis ICOS Analysis. We utilized a commerciallyavailable, OA-ICOS laser absorption spectrometer (Los GatosResearch (LGR) Triple Isotope Water Analyzer (TIWA-45EP))for simultaneous direct measurement of the 2H/1H, 18O/16O,and 17O/16O stable isotope ratios in liquid water. The OA-ICOSinstrument employs near-infrared tunable diode laser absorptionspectroscopy with the laser coupled off-axis to a high-finesseoptical cavity17 to provide highly accurate quantification of δ18O,δ17O, and δ2H in injected water samples. The OA-ICOSinstrument is very robust18 and more compact19 than the currentIRMS configuration. 17O-excess was calculated from measure-ments of δ18O and δ17O according to the definition 17O-excess =ln(δ17O + 1) − 0.528 ln(δ18O + 1).7 Water samples wereintroduced without sample conversion into the OA-ICOSinstrument via a PAL HTC-xt autoinjector (CTC Analytics)equipped with a heated (≈85 °C) injector block (LGR), wherethe water samples were evaporated for isotope analysis directlyon the water vapor. Liquid water samples were injected into theinjector block using a Hamilton 1.2 μL, zero dead volume syringe(P/N: 203185/01).

Simultaneous measurements of δ18O, δ17O, and δ2H werecompleted at a speed of approximately 90 s per measurement ofan individual injection. In the normal range of natural waters,LGR recommends injecting each sample six times and averagingthe last four injections to produce a single, high-throughputsample measurement; in this way 128 sample measurements canbe completed per day in addition to reference water standards.Because of the use of a large span of primary reference waterstandards and the desire to measure δ2H, as well as δ18O andδ17O, nine preparatory injections (injections which are notmeasured, designed to eliminate intersample memory) followedby five measurement injections were run for each sample in thisstudy to overcome memory effects (see the Results andDiscussion section below). The last four injections were averagedto produce a single, high-throughput (HT) sample measure-ment. Multiple HT measurements of the same sample can beaveraged to improve measurement precision as described below.δ2H values, while measured throughout this study, are outsidethe scope of this manuscript and will be discussed in a futuremanuscript. Primary isotopic reference water standards wereinterleaved with samples throughout each run; data normal-ization is described in detail below.

Off-Axis ICOS Data Processing. Processing of rawinstrumental OA-ICOS data proceeded in the following steps.First, any injection with measured water number density outsideof the manufacturer’s suggested range of 2−4.5 × 1016 H2Omolecules/cm3 was discarded. Second, any injection for whichthe water had not fully evaporated prior to measurement of theisotope ratios was discarded. Incomplete evaporation can becaused by septum pieces, salts, or other debris inside the injectorblock. Incomplete evaporation was detected by examining thestandard deviation of the measured water number density(σnmeas) as reported by the instrument. Injections with a σnmeasmore than three standard deviations higher than the averageσnmeas for the analysis were discarded. Any measurement set(typically a 20 h run sequence of interleaved samples andstandards) with more than 2% of injections rejected by thepreceding two filters was repeated.Third, a mathematical injected volume correction was applied

similar to that described by Lis et al.20 This type of correctioncommonly is called a “linearity correction” in IRMS datareductions. A single isotopic reference water was measured onthe analyzer at varying injected volumes between 600 and 1200 nL.The measured water number density (nmeas) was then normalized,N = (nmeas − n*)/n*, where n* = 3 × 1016 molecules/cm3. Foreach isotope, the measured isotope ratios of the isotopic referencewater, R(18O/16O) and R(17O/16O), were plotted versus thenormalized injected volumes and a linear least-squares fit wasutilized to determine the slope of the injected volume effect. Theslopes, one for each isotope, remained relatively constant over the6 month analysis period. For each measurement injection, themeasured water number density was normalized and thenmultiplied by the previously determined slopes to establish theratio correction. The injected volume correction was appliedseparately for each isotope to each injection in an analysis beforefurther processing. After the above processing, the isotope ratios ofthe final four injections of each sample were averaged to produceraw isotope ratios, R(18O/16O) and R(17O/16O), which were thennormalized as described below. The average of the last fourinjections of each sample produced the final, measured value forthat sample and is herein referred to as an individual high-throughput (HT) sample measurement. Multiple HT measure-ments can be averaged to improve measurement precision of all

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isotope analyses, as is described below for averages of 20 HTmeasurements, herein referred to as high-precision (HP) samplemeasurements.Isotope RatioMass Spectrometric Analysis. Isotope ratio

mass spectrometric analyses were performed at both theLaboratoire des Sciences du Climat et l’Environnement(LSCE) and Johns Hopkins University (JHU). The analyticalapproaches at both laboratories are based on methods outlinedby Baker et al.16 and modified by Barkan and Luz.12 Briefly, 2−3 μLof water was fluorinated using CoF3 under helium flow toquantitatively produce O2. O2 was separated from helium on amolecular sieve trap immersed in liquid nitrogen and finallytrapped by liquid helium for mass spectrometric analysis. MassSpectrometry was performed on a dual-inlet Thermo MAT 253mass spectrometer which allows simultaneous measurements ofm/z 32, 33, and 34.IRMS measurements at JHU used a modified fluorination line

similar to that described by Schoenemann et al.13 All JHU IRMSmeasurements were run against a working O2 reference gas duringseven sequences of 10 dual inlet runs. The working O2 referencewas routinely calibrated by analyzing the International AtomicEnergy Agency (IAEA) primary reference water standards ViennaStandard Mean Ocean Water 2 (VSMOW2) and Standard LightArctic Precipitation 2 (SLAP2)21 as part of the analytical sequence.Using this procedure, a maximum of eight total waters (includingreferences) can be processed and measured per day at JHU.IRMS measurements at LSCE were run against a working O2

isotopic reference standard during two sequences of 16 dual-inletruns. The working O2 standard was calibrated by analyzingVSMOW2 and SLAP2. Every day, one or two isotopic referencewaters, themselves also calibrated against VSMOW2 and SLAP2,were processed together with the samples. In order to avoid anymemory effects, blanks were run between samples any time theirδ18O differences exceed 3‰. Using this procedure, 4−5duplicate samples were processed and measured per day atLSCE in addition to isotopic reference waters.Data Normalization. In order to make meaningful

comparisons of isotope measurements between laboratories, acommon data normalization scheme is necessary. All threelaboratories involved in this comparison have utilized the primaryisotopic reference water standards VSMOW2 and SLAP2 with thenormalization proposed by Schoenemann et al.13 Specifically,

VSMOW2 values of δ18OVSMOW−SLAP = 0 ± 0.02‰21 andδ17OVSMOW−SLAP = 0 ± 0.02‰22 and SLAP2 values ofδ18OVSMOW−SLAP = −55.5 ± 0.02‰21 and 17O-excessVSMOW−SLAP = 0(δ17OVSMOW−SLAP≈−29.6986‰)13 were used in all calculations.The 17O-excessVSMOW−SLAP values for both VSMOW2 andSLAP2 were taken to be defined as zero and thus consideredto have no uncertainty for propagation of errors.13 Theuncertainty of the δ17OVSMOW−SLAP of SLAP2 was calculated fromthe uncertainty of δ18OVSMOW−SLAP and

17O-excessVSMOW−SLAP anddetermined to be 0.02‰. In all cases, the uncertainties of themeasured δ18O and δ17O values have been convolved with theuncertainties of the reported VSMOW2 and SLAP2 values to reportvalues on the VSMOW−SLAP scale. No convolution has been usedin the case of 17O-excess as the standard values are taken to have zerouncertainty.The OA-ICOS instrument outputs raw data as isotope ratios;

conversion to δ-values utilized the VSMOW isotope ratiosR(18O/16O) = 2005.20 × 10−6 23 and R(17O/16O) = 379.9 ×10−6.24 Note that, while these ratios were measured on VSMOW,measurements of VSMOW and VSMOW2 show that they haveindistinguishable δ18O and δ17O values,22 and thus indistinguishableR(18O/16O) and R(17O/16O) values. Normalization was completedon ratios afterwhichnormalized ratioswere converted intonormalizedδ-values versus VSMOW−SLAP. Normalization in ratio space isalgebraically identical to normalization in δ-value space; the order ofoperations is insignificant. Dual-inlet IRMS instruments output rawdata in both ratios and δ-values relative to an internal O2 workingisotopic reference standard; normalization was completed on rawδ-values to produced normalized δ-values versus VSMOW−SLAP.For OA-ICOS measurements, the primary isotopic reference

water standards VSMOW2 and SLAP2 were interleavedthroughout each measurement set, typically >10 measurementsof each standard in an approximately 20 h run. In order tominimize the effects of instrumental drift, a spline calibration wasused. Specifically, for each standard, a cubic spline function wasused to interpolate between measurements of that standard toproduce a local value of each standard for every measurement (eitherstandard or sample). For each standard or sample measurement, thelocal values of SLAP2 and VSMOW2 were fitted to the acceptedvalues for the standards using the “traditional linear” method asdescribed by Schoenemann et al.13 This produced a local slope andintercept that were used to normalize the measurement to the

Figure 1. (a) Sample-to-sample memory for the transition from SLAP2 to VSMOW2 for δ18O (black squares), δ17O (red circles), and δ2H (inset).(b) Sample-to-sample memory for the transition from GISP to VSMOW2. The solid lines show the known VSMOW2 values of 0‰; the dashed linesshow the approximate single injection 1σ uncertainty band for δ18O and δ17O of±0.2‰ and for δ2H of±1.0‰. Note that the scales on both panels arethe same to facilitate comparison.

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VSMOW−SLAP scale. This procedure was completed separately foreach measured isotope ratio.

■ RESULTS AND DISCUSSIONSample-to-Sample Memory.Many researchers have noted

with laser-based measurements of water isotopes that there is asample-to-sample memory effect between samples (e.g., refs 20and 25−27). This memory effect is likely due to a combination offactors, including mixing of waters and carryover by the samplingsyringe28 as well as the tendency of water to adhere to the internalsurfaces of the instrument.20 Sample-to-samplememory is particularlyapparent for large changes in isotope ratio and for the 2H isotope.

Figure 1a illustrates the memory observed (from all causes) with theTIWA instrument for the transition from SLAP2 to VSMOW2(−55.5‰ to 0 for δ18O), which is among the largest differencesencountered between natural waters. Fourteen injections ofVSMOW2 were performed directly after 14 injections of SLAP2.The solid line is at 0, the known value for VSMOW2, and the dashedlines are at ±0.2‰, approximately the 1σ measurement uncertaintyof a single injection (as noted above, four injections are generallyaveraged to produce a single, HT measurement with the factor of 2improvement in precision). For the SLAP2−VSMOW2 transition,for δ18O (black squares), the first four injections and for δ17O (redcircles) the first two injections fall outside of the ±0.2‰ band. In

Figure 2. Measurement precision for 514 HT OA-ICOS measurements of GISP calibrated against VSMOW2 and SLAP2 with injected volumecorrection and spline calibration. Histograms with overlaid normal distribution fit for (a) δ18OVSMOW−SLAP and (b) δ

17OVSMOW−SLAP. The average andstandard deviation of the measured values are noted along with the IAEA accepted value for δ18OVSMOW−SLAP (ref 29) and a published average value ofδ17OVSMOW−SLAP (ref 13) (IAEA does not list a value for δ17O).

Table 1. OA-ICOS and IRMS Measurements of GISP and Commercially Available USGS Isotopic Reference Watersa

aAverages are weighted by the inverse of the listed precision of the measurements. Uncertainties listed are standard errors of the mean for IRMSmeasurements, weighted standard deviations for the weighted averages of the three reported experimental measurements, the published standarddeviation for GISP, and the published 95% confidence intervals for the USGS values. Uncertainties for OA-ICOS are listed as the standard error ofthe mean for 17O-excess and as the measured limit of instrumental precision for δ18O and δ17O. bRef 29. cRef 31. dRef 34. eRef 33.fRef 32.

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contrast, for δ2H (see inset) the first six injections fall outside of the 1σuncertainty band for a single injection of 1.0‰. For smaller isotopiccomposition changes in the common range of natural waters, < 25‰change in δ18O and<13‰ change inδ17O, twoor fewer injections falloutside of the 1σ uncertainty band for all three isotopes. Figure 1billustrates this case on VSMOW2 measured immediately afterGreenland Ice Sheet Precipitation (GISP), a transition from−24.76‰ to 0‰ for δ18O.29 Note that the scales on both panelsare the same to facilitate comparison. These data suggest that formeasurements of δ18O, δ17O, and 17O-excess, depending on thelargest isotope step encountered, memory can be addressed using 1−4 preparatory injections on the TIWA instrument followed by fourmeasurement injections. If δ2H measurements are also desired, alarger number of preparatory injections may be necessary. It is alsopossible to utilize mathematical memory corrections (see, e.g., refs25−27); these are outside the scope of this manuscript and will bediscussed in a future manuscript. The accuracy and precision of theisotopemeasurements described herein demonstrate the suitability ofthis procedure to effectively handle sample-to-sample memory.Measurement Precision and Accuracy of δ18O and

δ17O. The international isotopic reference water standard GISPwas analyzed on the OA-ICOS instrument to produce 514 HTmeasurements over a 6 month period. Each instrumental analysiswas constructed as described above, with VSMOW2 and SLAP2interleaved throughout the analysis. All data was processed withthe injected volume correction and normalized with the splinecalibration as described above. Figure 2 shows the excellentmeasurement precision and accuracy for these HT measure-ments, with an average δ18OVSMOW−SLAP =−24.74± 0.07‰ (1σ)and δ17OVSMOW−SLAP = −13.12 ± 0.05‰ (1σ). The best-fitnormal distribution for δ18OVSMOW−SLAP shows a center value of−24.74‰ with a half-width half-max (HWHM) of 0.06‰.The IAEA reported value for δ18OVSMOW−SLAP in GISP is−24.76±0.09‰ (1σ).29 The best-fit normal distribution for δ17OVSMOW−SLAPshows a center value of −13.12‰ with an HWHM of 0.04‰. Forcomparison, the collected average of five previously reported IRMSmeasurements of δ17OVSMOW−SLAP in GISP was −13.12 ± 0.06‰(1σ).13 Allan deviation analyses30 (not shown) for both δ18O andδ17O show that averaging improves the precision of themeasurements to an instrumental noise floor of approximately0.02‰ in δ18O and 0.01‰ in δ17O. This precision is achieved byaveraging 20 HT measurements to produce a single HP

measurement as described below. Throughout this manuscript,these values, convolved with the uncertainties of the primaryisotopic reference water standards, are reported as themeasurementuncertainties for high-precision measurements of δ18OVSMOW−SLAPand δ17OVSMOW−SLAP in any case where using the standard error ofthe mean would result in an error lower than these noise floors.Instrumental accuracy was further demonstrated by measuring

GISP and four commercially available United States GeologicalSurvey (USGS) isotopic reference waters31−34 by OA-ICOS andby IRMS as described above. The data are summarized in Table 1.Figure 3 shows the measurement accuracy of measurements byOA-ICOS and IRMS for GISP and the four USGS isotopicreference waters. For δ18O, measurements are plotted as thedifference between each measurement and the published δ18Ovalue.29,31−34 Neither IAEA nor USGS publishes values for δ17Oof reference waters. For this reason, δ17O values for GISP areshown as the difference between each measurement and anaverage of previously reported values.13 δ17O values for theUSGS isotopic reference waters are shown as the differencebetween each measurement and the average of the threeexperimental measurements inversely weighted by the un-certainty. Error bars represent the standard error of the meanfor IRMS measurements and the measured limit of instrumentalprecision as described above for OA-ICOS. The correlation ofmeasurement errors between δ18O and δ17O described in theintroduction is clearly evident in Figure 3.

Measurement Precision and Accuracy of 17O-excess.17O-excess was calculated from the aforementioned OA-ICOSmeasurements of the international isotopic reference waterstandard GISP. In order to reach the extremely high precisionneeded for meaningful measurements of 17O-excess by eitherIRMS or OA-ICOS, averaging of multiple individual measurementsis required. The precision achieved as a function of the number ofaveraged OA-ICOS HT measurements is shown in Figure 4a. ThisAllan deviation30 plot clearly demonstrates that averaging improvesthe measurement precision; the solid line shows the case of perfectwhite noise averaging, which reduces the standard deviation by thesquare root of the number of averaged measurements. TypicalIRMSmeasurements average 70−100 individual dual-inletmeasure-ments of O2 gas from a water sample to produce a single, high-precision measurement of 17O-excess and typically report ameasurement precision of 5−10 per meg (1σ), e.g., refs 8, 12, and

Figure 3. Measurement accuracy demonstrated by measurements of GISP and four commercially available USGS isotopic reference waters by OA-ICOS and IRMS. Measurements of δ18OVSMOW−SLAP (a) are shown as differences from the values published by IAEA (ref 29) and USGS (refs 32−34) asappropriate. Neither IAEA nor USGS publishes values for δ17O of reference waters. Measurements of δ17OVSMOW-SLAP (b) for GISP are shown as thedifference between each measurement and an average of previously reported values (ref 13). Measurements of δ17OVSMOW-SLAP (b) for USGS isotopicreference waters are shown as the difference between each measurement and the average of the three experimental measurements inversely weighted by theuncertainty. Error bars represent the standard error of the mean for IRMS measurements and the measured limit of instrumental precision for OA-ICOS.

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13. Individual HT measurements on the OA-ICOS instrument areaverages of four injections; therefore, averaging of 20HTOA-ICOSmeasurements produces a HP measurement which is based on 80measured injections of the water sample, comparable to the IRMStechnique. Using this procedure, 6−7 HP sample measurements inthe normal range of natural waters can be completed per day inaddition to standards. Figure 4b shows the excellent measurementprecision achieved withHPOA-ICOSmeasurements of 17O-excess,with an average 17O-excessVSMOW−SLAP = 23± 10 permeg (1σ). Thebest-fit normal distribution for 17O-excessVSMOW−SLAP of GISPshows a center value of 23 per meg with an HWHM of 9 per meg.For comparison, the collected average of three previously reportedIRMS measurements of 17O-excessVSMOW−SLAP in GISP was22 ± 11 per meg (1σ).13

Figure 5 shows the excellent measurement agreement among17O-excess measurements by OA-ICOS and IRMS for GISP and

the four USGS isotopic reference waters. Error bars represent thestandard error of the mean for multiple HP measurements ofeach sample. The line behind the GISP columns shows thecollected average of three previously reported IRMS measure-

ments.13 These data also illustrate the utility of 17O-excessmeasurements in distinguishing among natural waters; note thatUSGS45 (Biscayne Aquifer Drinking Water) and USGS48(Puerto Rico Precipitation) have essentially identical δ18O andδ17O values, but significantly different 17O-excess values. Theexceptional precision for 17O-excess measurements amongdifferent laboratories and between methods makes it possibleto differentiate the 17O-excess composition of these two waters.Table 1 summarizes all IRMS and OA-ICOS measurements ofGISP and the USGS isotopic reference waters. Averages areweighted by the inverse of the listed precision of themeasurements. Uncertainties listed are standard errors of themean for IRMS measurements, weighted standard deviations forthe averages of the three reported experimental measurements,the published standard deviation for GISP, and the published95% confidence intervals for the USGS values. Uncertainties forOA-ICOS are listed as the standard error of the mean for 17O-excess and as the measured limit of instrumental precision forδ18O and δ17O as described above.

■ CONCLUSIONS

In this work we demonstrate for the first time the use of OA-ICOS tomeasure δ18O, δ17O, and 17O-excess in liquid water. OA-ICOS involves no sample conversion and has a small footprint,allowing measurements to be made by researchers collecting thesamples. Repeated (514) high-throughput measurements of theinternational standard isotopic reference water GISPmade over a6 month period demonstrate the precision and accuracy of theOA-ICOS instrument: δ18OVSMOW−SLAP =−24.74± 0.07‰ (1σ)and δ17OVSMOW−SLAP = −13.12 ± 0.05‰ (1σ). For comparison,the IAEA reported value for δ18OVSMOW−SLAP in GISP is −24.76± 0.09‰ (1σ)29 and the average of five previously reportedmeasurements of δ17OVSMOW−SLAP in GISP is −13.12 ± 0.06‰(1σ).13 To achieve the high precision needed for meaningful17O-excess measurement, we average 20 high-throughput OA-ICOS measurements to get a single, high-precision measurement.In this way we measure the 17O-excessVSMOW−SLAP of GISP as 23 ±10 per meg (1σ); the average of three previously reportedmeasurements of 17O-excessVSMOW−SLAP inGISP is 22± 11 permeg(1σ).13 ForOA-ICOSmeasurements of δ18O, δ17O, and 17O-excess,precision can be enhanced further by additional averaging. OA-ICOS measurements are compared with two independent IRMS

Figure 4. (a) Allan deviation plot showing precision as a function of number of HTOA-ICOSmeasurements averaged. The solid line represents perfectaveraging of white noise. (b) Histogram with overlaid normal distribution fit for HP OA-ICOS measurements (averages of 20 HT measurements) of17O-excessVSMOW−SLAP of GISP calibrated against VSMOW2 and SLAP2 with injected volume correction and spline calibration. The average andstandard deviation of the measured values are noted, along with a published average value (ref 13).

Figure 5. Measurement accuracy for 17O-excessVMSOW‑SLAP demon-strated by measurements of GISP and four commercially availableUSGS isotopic reference waters by OA-ICOS and two independentIRMS laboratories. Error bars represent one standard error of the mean.The line behind the GISP columns shows the collected average of threepreviously reported IRMS measurements (ref 13).

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measurements of commercially available GISP and USGS isotopicreference waters and shown to have comparable accuracy andprecision as fluorination followed by IRMS. The ability to measureaccurately δ18O, δ17O, and 17O-excess in liquid water inexpensivelyand without sample conversion is expected to increase vastly theapplication of δ17O and 17O-excessmeasurements for understandingof the water cycle, atmospheric convection, and climate modelingamong others.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: e.berman@lgrinc.com. Fax: 650-965-7074.NotesThe authors declare the following competing financialinterest(s): E. Berman and T. Owano disclose that they areemployed by Los Gatos Research, the manufacturer of the TripleIsotope Water Analyzer.

■ ACKNOWLEDGMENTSThis work was supported by NIH SBIR Grant No.1R43DK093362-01 and NASA SIBR Grant No. NNX13CA03C.17O-excess measurements at LSCE were funded by the ERCStarting Grant COMBINISO No. 306045. An ACS-PRF Grant(No. 52651-DNI2) and the Department of Earth & PlanetarySciences supported the work at JHU. A.L. thanks Frederic Prie for laboratory assistance. E.S.F.B. thanks Spruce Schoenemannand Tyler Coplen for helpful discussions of normalization.

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Analytical Chemistry Article

dx.doi.org/10.1021/ac402366t | Anal. Chem. XXXX, XXX, XXX−XXXG