International Journal of Wine Research Dovepress open access to scientific and medical research Open Access Full Text Article ORIGINAL RESEARCH Laser-based measurements of 18O/16O stable isotope ratios (δ18O) in wine samples Manish Gupta1 Abstract: Wine counterfeiting is an international, multi-billion dollar issue, with some estimates J Brian Leen1 suggesting that up to 5% of wines sold at auctions or secondary markets are fraudulent. Isotope Elena Simone Franklin ratio mass spectrometer (IRMS) measurements of the 18O/16O stable isotope ratio (δ18O) of water- Berman1 in-wine have been used for wine authentication; however, these analyses are time-consuming Aldo Ciambotti2 and costly. In this preliminary study, off-axis integrated cavity output spectroscopy (OA-ICOS) is used to quantify δ18O in wines. This laser-based method has been extensively used to study 1Los Gatos Research, Mountain View, water isotopes for hydrological and medical applications. Recently, the development of a CA, USA; 2Consiglio per la Ricerca e la Sperimentazione in Agricoltura, spectral contaminant identifier (SCI) has extended the application of these OA-ICOS analyzers Centro di Ricerca per l’Enologia, to contaminated water samples (eg, plant, soil, and leaf waters). Here, we utilize OA-ICOS Asti, Italy with the SCI to characterize wine samples (9%–15% ethanol), and show that the laser-based instrument provides a δ18O measurement precision of ±0.07‰ (1σ) and agrees with IRMS to within ±0.63‰ (1σ). Moreover, by training the SCI on isotopically-characterized wines, the agreement with IRMS improves to within ±0.30‰ (1σ). The utility of the instrument is demonstrated by measuring watered and mixed wines. The method presented here can be readily extended to address other food authentication applications. Keywords: wine isotopes, wine fraud, counterfeit wines, OA-ICOS Introduction Wine counterfeiting is an international, multi-billion dollar issue, with some esti- mates suggesting that up to 5% of wines sold at auctions or secondary markets are fraudulent.1 Fraudulent activities include misrepresented watering, mislabeling, wine blending, and including unauthorized additives. Researchers use a wide variety of analytical techniques to detect fraud and authenticate wine, including chromatographic separation techniques, mass spectrometry,2 emission spectrometry, and nuclear mag- netic resonance (NMR).3 Recently, measurements of the 18O/16O stable isotope ratio 18 Correspondence: Manish Gupta (expressed as δ O) of water in wine have been used for wine authentication, and Los Gatos Research, 67 East Evelyn extensive databases of wine isotope ratios are now being developed.4–7 These technolo- Avenue, Suite 3, Mountain View, 8 9 CA, USA, 94041 gies have also been extended to fruit juice extracts, concentrated spirits, and other Tel +1 650 965 7772 food authentication applications. Fax +1 650 965 7074 18 Email [email protected] Currently, measurements of δ O in wine are made using isotope ratio mass spectrometry (IRMS).10,11 The wine sample is typically equilibrated with a carbon Aldo Ciambotti dioxide gas standard at a constant temperature for 6–12 hours to permit exchange of Consiglio per la Ricerca e la Sperimentazione in Agricoltura, oxygen atoms between the water in the wine and the gas-phase carbon dioxide. The Centro di Ricerca per l’Enologia – oxygen isotope ratio of the carbon dioxide is then measured against an isotopically- via Pietro Micca 35, 14100 Asti, Italy Email [email protected] characterized carbon dioxide (CO2) gas sample using IRMS. The measurement process submit your manuscript | www.dovepress.com International Journal of Wine Research 2013:5 47–54 47 Dovepress © 2013 Gupta et al. This work is published by Dove Medical Press Ltd, and licensed under Creative Commons Attribution – Non Commercial (unported, v3.0) License. The full terms of the License are available at http://creativecommons.org/licenses/by-nc/3.0/. Non-commercial uses of the work are permitted without any further http://dx.doi.org/10.2147/IJWR.S51013 permission from Dove Medical Press Ltd, provided the work is properly attributed. Permissions beyond the scope of the License are administered by Dove Medical Press Ltd. Information on how to request permission may be found at: http://www.dovepress.com/permissions.php Gupta et al Dovepress is time-consuming and costly, usually requiring dedicated The first two injections were discarded to remove any personnel to operate the IRMS and process the samples. Due memory effects, and the final four were averaged to yield in part to these limitations, the application of isotope analysis a measured isotope value. For every five sample unknowns for wine authentication has been limited to select measure- measured, one standard was measured, yielding a net mea- ment laboratories that house the necessary expertise. surement frequency of 150 unknown samples/day. In this preliminary study, off-axis integrated cavity output The analyzer’s post processing software (Version spectroscopy (OA-ICOS) is used to quantify δ18O in wines. 2.2.0.14, Los Gatos Research, Mountain View, California, This laser-based method has been extensively applied to the USA) was used to determine the measured values of δ2H study of water isotopes for hydrological12–14 and medical15 and δ18O after calibration with the internal water standards. applications. Recently, the development of a spectral con- The analyzer’s SCI software (Version 1.0.0.75, Los Gatos taminant identifier (SCI)16 has extended the application of Research, Mountain View, California, USA) was then used these OA-ICOS analyzers to contaminated waters, including to determine broadband (mBB) and narrowband (mNB) metrics 17 plant, stem, and leaf waters. Here, we utilize an OA-ICOS for all of the measured samples. The mBB and mNB metrics analyzer and the SCI to characterize wine samples containing are described in detail elsewhere,16 and reflect contamination 9%–15% ethanol, and show that the laser-based instrument due to hydroxyl compounds, with mBB and mNB serving as provides a measurement precision of ±0.07‰ (1σ) and agrees indicators of larger (eg, ethanol and higher alcohols) and with IRMS to within ±0.63‰ (1σ). Moreover, by training smaller (eg, methanol) contaminants, respectively. Critically, the SCI on isotopically-characterized wines, the agreement previous work has shown that the values of δ2H and δ18O with IRMS improves to within ±0.30‰ (1σ). The utility of reported by the LWIA can be corrected for this contami- the instrument is demonstrated by measuring watered and nation by using the measured values of the mBB and mNB mixed wines. metrics. The functional form of these corrections is shown below and has been validated in previous publications. Note Materials and methods that the exact composition of the contaminants is not impor- A commercial, OA-ICOS liquid water isotope analyzer tant. For example, in previous studies of plant waters, there (LWIA, Los Gatos Research, Mountain View, CA, USA) was were several hundred trace contaminant compounds present used to measure natural waters, prepared ethanol-in-water (eg, acids, ketones).17 Regardless, the OA-ICOS analyzer standards, prepared methanol-in-water standards, and wines. was able to determine the correct isotope ratio as deter- All measurements were made using three commercially- mined by IRMS. Thus, mBB and mNB are measures of total available (Los Gatos Research), internal water standards contamination due to broadband and narrowband spectral with isotope values (δ2H, δ18O) of (-51.8‰, -8.02‰), absorbers respectively, and the metrics can be used to cor- (-9.6‰, -2.89‰) and (107.0‰, 12.24‰) as measured rect the measured isotope values and yield accurate results, directly against VSMOW2 (Vienna Standard Mean Ocean even in the presence of significant organic contamination. Water 2) and SLAP2 (Standard Light Arctic Precipitation 2), This is especially critical in wine studies, where there are and independently confirmed by IRMS. Measurement of numerous other compounds including organic acids, higher sample unknowns included interspersed, periodic measure- alcohols, and phenols. ments of a fourth water standard which served as an internal control and confirmed that the analyzer was accurately mea- Results and discussion suring isotope ratios. The analyzer was first tested on natural waters to gauge All samples (water, ethanol-in-water, methanol-in-water, its precision and accuracy. Note that there was no mea- and wines) were handled in an identical fashion. In order sured change in the isotope ratio due to sample handling. to remove a variety of organic compounds,18 2 mL of each A single water sample was independently measured 45 times sample was placed in an Eppendorf tube, activated charcoal in less than 8 hours to gauge the instrument’s accuracy and (∼50 mg) was added to each tube, and the tube was agitated precision. The instrument measured the δ2H and δ18O val- for 30 seconds. Subsequently, the tube was centrifuged for ues of the water sample (average of four injections) with a 2 minutes to settle the charcoal, and the top 1 mL of sample precision of ±0.37‰ (1σ) and ±0.10‰ (1σ) respectively. was transferred to an analyzer measurement vial. Six sepa- This precision improved to better than ±0.25‰ (1σ) and rate 1 µL injections from each vial (containing sample or ±0.05‰ (1σ) respectively by averaging five measurements standard) were placed into the LWIA for measurement. (eg, an average of 20 injections). Moreover, the measured 48 submit your manuscript | www.dovepress.com International Journal of Wine Research 2013:5 Dovepress Dovepress Laser-based measurements of wine isotope ratios mean δ2H and δ18O isotope values of the water sample were The dependence of the reported δ18O values on the accurate to within 0.2‰ and 0.16‰, respectively, relative narrowband metric, mNB, was determined by measuring to IRMS measurements. five methanol-in-water samples ranging from 0–130 ppm The analyzer was then used to measure twelve ethanol- by volume, three times each in a manner identical to that in-water samples ranging from 0%–17.2% by volume.