Discrepancies between isotope ratio infrared spectroscopy and isotope ratio mass spectrometry for the stable isotope analysis of plant and soil waters Adam G. West 1,2 * , Gregory R. Goldsmith 1 , Paul D. Brooks 3 and Todd E. Dawson 1,3 1 Department of Integrative Biology, University of California, Berkeley, Berkeley, CA 94720, USA 2 Botany Department, University of Cape Town, Rondebosch, 7701, South Africa 3 Center for Stable Isotope Biogeochemistry, University of California, Berkeley, Berkeley, CA 94720, USA Received 7 April 2010; Revised 24 April 2010; Accepted 24 April 2010 The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotope analysis of water is increasing. While IRIS has many advantages over traditional isotope ratio mass spectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Of particular concern is the potential for contaminants in the water sample to interfere with the spectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soil samples may often contain organic contaminants. The extent to which contaminants may interfere with IRIS and thus impact upon data quality is presently unknown. We tested the performance of IRIS relative to IRMS for water extracted from 11 plant species and one organic soil horizon. IRIS deviated considerably from IRMS for over half of the samples tested, with deviations as large as 46% (d 2 H) and 15.4% (d 18 O) being measured. This effect was reduced somewhat by using activated charcoal to remove organics from the water; however, deviations as large as 35% (d 2 H) and 11.8% (d 18 O) were still measured for these cleaned samples. Interestingly, the use of activated charcoal to clean water samples had less effect than previously thought for IRMS analyses. Our data show that extreme caution is required when using IRIS to analyse water samples that may contain organic contaminants. We suggest that the development of new cleaning techniques for removing organic contaminants together with instrument-based software to flag potentially problematic samples are necessary to ensure accurate plant and soil water analyses using IRIS. Copyright # 2010 John Wiley & Sons, Ltd. The recent development of new analytical techniques for the stable isotope analysis of water has resulted in the avail- ability of several methods based on fundamentally different technologies. These can be broadly separated into two classes: (1) traditional isotope ratio mass spectrometry (IRMS) methods, during which water is converted into H 2 , CO, or equilibrated with CO 2 and thereafter analyzed in gaseous form by IRMS 1–4 and (2) more recent isotope ratio infrared spectroscopy (IRIS), where photo absorption by H 2 O molecules is measured and the isotopologues of H 2 O are calculated via spectroscopy. Two commercially available IRIS analyzers – based on off-axis integrated cavity output spectroscopy (OA-ICOS, Los Gatos Research) and wavelength-scanned cavity ring-down spectroscopy (WS- CRDS, Picarro Inc.) – are already in use. 5,6 IRIS offers many advantages over IRMS in terms of ease of use, cost and the potential of field portability. 7,8 IRIS methods have been shown to produce similar results to IRMS when analyzing pure water; 5,6,9 however, there is little information about how robust these methodologies are for waters that may contain organic contaminants. Trace amounts of contaminants are unlikely to have a large effect on the isotopic value of a water sample measured by IRMS due to the relatively small mass contribution that they make to the total amount of 1 H, 2 H, 16 O and 18 O isotopes in the sample. However, the same cannot be said for IRIS, as this is not a mass-based method of analysis. Instead, it is possible that spectral interference by contaminants may have a large effect on the calculated isotopic composition of the sample. Such an effect was shown for water/alcohol mixtures, 9 but there have been no other published studies examining other contami- nants. With the likely increase in studies using IRIS to analyze water samples from a variety of sources, it is important that IRIS techniques be tested for robustness. In the environmental sciences, stable isotope analyses are routinely performed on water samples extracted from organic or inorganic matrices. Common applications include the analysis of water extracted from plants and soils. 10 The process of water extraction from plants or soils can either introduce organic contaminants to the sample 11 or co-distil them with the desired water. 12 The majority of these organic contaminants can be removed from the extracted sample though adsorption onto activated charcoal fragments (polar carbon with a high surface area). However, trace amounts of RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2010; 24: 1948–1954 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.4597 *Correspondence to: A. G. West, Botany Department, University of Cape Town, Rondebosch, 7701, South Africa. E-mail: adam.west@uct.ac.za Copyright # 2010 John Wiley & Sons, Ltd.