A strategy for selection of reference materials in stable oxygen isotope analyses of solid materials Grzegorz Skrzypek 1 * and Rohan Sadler 2 1 West Australian Biogeochemistry Centre and John de Laeter Centre of Mass Spectrometry, School of Plant Biology, The University of Western Australia, 35 Stirling Highway (MO90), Crawley, WA 6009, Australia 2 School of Agricultural and Resource Economics, The University of Western Australia, 35 Stirling Highway (MO90), Crawley, WA 6009, Australia The propagation of uncertainties associated with the stable oxygen isotope reference materials through a multi‐point normalisation procedure was evaluated in this study using Monte Carlo (MC) simulation. We quantified the normalisation error for a particular selection of reference materials and their number of replicates, when the choice of standards is restricted to either nitrates, sulphates or organic reference materials alone, and in comparison with when this restriction was relaxed. A lower uncertainty in stable oxygen isotope analyses of solid materials performed using High‐Temperature Pyrolysis (HTP) can be readily achieved through an optimal selection of reference materials. Among the currently available certified reference materials the best performing pairs minimising the normalisation errors are USGS35 and USGS34 for nitrates; IAEA‐SO‐6 and IAEA‐SO‐5 for sulphates; and IAEA‐601 and IAEA‐602 for organic materials. The normalisation error can be reduced further – by approximately half – if each of these two analysed reference materials is replicated four times. The overall optimal selection among all nine considered reference materials is the IAEA‐602 and IAEA‐SO‐6 pair. If each of these two reference materials is replicated four times the maximum predicted normalisation error will equal 0.22‰, the minimum normalisation error 0.12‰, and the mean normalisation error 0.15‰ over the natural range of δ 18 O variability. We argue that the proposed approach provides useful insights into reference material selection and in assessing the propagation of analytical error through normalisation procedures in stable oxygen isotope studies. Copyright © 2011 John Wiley & Sons, Ltd. The range of applications for stable isotope ratio mass spectrometry (IRMS) has rapidly increased since continuous flow systems (CF‐IRMS) became an alternative to expensive and time consuming traditional dual inlet techniques. [1,2] Analysis of oxygen stable isotope composition has become a tool widely applied in biological sciences, i.e., from unravel- ling plant physiological mechanisms to tracing environmen- tal changes based on plant microfossils or tree‐rings (e.g., [3,4] ). However, the increasing demand for low‐cost analysis has been followed by increasing demand for higher analytical precision and accuracy, as experiments and models become more sophisticated. The analytical precision and accuracy depends on several instrument‐related factors, including instrument calibration, ambient conditions during analyses, and the quality and purity of the various chemicals and gases used. The analytical accuracy in stable oxygen isotope analysis is influenced by the normalisation strategy applied to the data over the VSMOW (Vienna Standard Mean Ocean Water) scale, including: (1) the quality of reference materials and accuracy of their calibration; (2) the number and selection of reference materials; and (3) the type of normalisation procedure. [5–7] Each of these two components (points 2 and 3) reflects an arbitrary choice by the analyst and therefore can be easily modified. Nine organic and inorganic stable oxygen isotope reference materials (Table 1) were recently calibrated during a program sponsored by the International Union of Pure and Applied Chemistry (IUPAC), [8] and are currently available from the International Atomic Energy Agency (IAEA, Vienna, Austria) or National Institute of Standards and Technology (NIST, Gaithersburg, MD, USA). All these materials are in a readily usable, solid form, and have been calibrated using a high‐ temperature conversion (HTC) technique versus two primary anchors defining the VSMOW scale: waters VSMOW2 and SLAP. [8] A number of other standards for δ 18 O, including IAEA‐CH6, IAEA‐C3, USGS 40 and USGS41, were also offered by IAEA and NIST. However, they were not the subject of the recent inter‐laboratory calibration by Brand et al., [8] due to potential problems with the stability of δ 18 O over time, resulting from exchanges with water or air moisture. An IRMS analyst has two options for the determination of δ 18 O in solid materials (e.g., plant tissues, extracted cellulose or lignin, precipitated phosphates or sulphates) on the VSMOW scale: (1) purchase official reference materials * Correspondence to: G. Skrzypek, West Australian Biogeo- chemistry Centre, School of Plant Biology (MO90), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. E‐mail: Grzegorz.Skrzypek@uwa.edu.au; gskrzypek@ yahoo.com Copyright © 2011 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2011, 25, 1625–1630 Research Article Received: 16 February 2011 Revised: 24 March 2011 Accepted: 26 March 2011 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2011, 25, 1625–1630 (wileyonlinelibrary.com) DOI: 10.1002/rcm.5032 1625