Dear Editor, Ignoring IUPAC guidelines for measurement and reporting of stable isotope abundance values affects us all An article published recently in Rapid Communications in Mass Spectrometry (RCM) [1] prompted us to draw attention to the subject of IUPAC guidelines and recommended terms of stable isotope abundance measurements and reporting results thereof. [2] Conditions or prerequisites for stable isotope analysis as laid down in official IUPAC publications have to be met before results of studies based upon stable isotope abundance data generated by continuous flow isotope ratio mass spectrometry (CF-IRMS) can be considered for publication. In condensed form IUPAC guidelines ask that: [2] (i) Authors should express δ values relative to current international measurement standards, avoiding out-of-date references such as SMOW, PDB, and CDT . (ii) If a second international measurement standard defines the size of a delta scale, such as L-SVEC lithium carbonate for δ 13 C measurements [3,4] or SLAP (SLAP2) water for δ 2 H and δ 18 O measurements, [5] δ values should be normalized using both standards and authors should state this clearly in their articles and reports. (iii) Authors are strongly encouraged to report δ values of (secondary) isotopic reference materials as though they had been interspersed among and used for normalization of unknowns, as appropriate for the measurement method. An isotopic reference material can be locally prepared in an individual laboratory or can be an internationally distributed isotopic reference material, e.g. USGS40 and USGS41 (L-glutamic acid), which are used for the measurement of δ 13 C and δ 15 N values. Only compliance with the aforementioned prerequisites ensures that published stable isotope abundance data are fit-for-purpose: that they might be compared with data generated by other laboratories and, more importantly, can be repeated by other laboratories and thus be externally validated. Ensuring that these standards are met should be as much the responsibility of the scientific journal as it is the authors. While for illustration purposes we refer to a specific research article, the purpose of this letter is not to single out any one research group or laboratory in particular; however, it did remind us that this is a more general problem, of which unfortunately we have seen many occurrences. Examples of CF-IRMS-based work reporting e.g. δ values relative to VPDB or VSMOW yet without formally meeting IUPAC guidelines [2] can be found not just in this journal, but in equally well- respected journals such as Food Chemistry, Forensic Science International, The Journal of Food & Agricultural Chemistry or The Journal of Paleolimnology , to name but a few. [6–11] In the following we shall use the aforementioned article [1] solely as an example how stable isotope methodology as described there falls short of IUPAC guidelines. [2] In their article the authors state the following: [1] “The isotope ratio values are expressed in δ‰ (= [(Rsample – Rstandard)/ Rstandard] * 1000, where R is the ratio between the heavier isotope and the lighter one) against international standards (Vienna-Pee Dee Belemnite (V-PDB) for δ 13 C values, and Vienna-Standard Mean Ocean Water (VSMOW) for δ 2 H and δ 18 O values). To calculate the δ‰ values, working in-house standards for proteins were used calibrated against international reference materials: L-glutamic acid USGS 40 (IAEA – International Atomic Energy Agency, Vienna, Austria), fuel oil NBS-22 (IAEA), and sugar IAEA-CH-6 (IAEA) for 13 C/ 12 C; and benzoic acid (IAEA-601) for 18 O/ 16 O in casein. The 2 H/ 1 H values of whole wood and cellulose were corrected against an internal standard with an assigned value of δ 2 H (À113‰), according to the comparative equilibration technique. [12] ” Setting aside for the moment the fact that according to IUPAC guidelines the equation defining the δ-value should no longer contain the extraneous factor 1000 nor the permil symbol, [2,4] this part of the method section on reporting δ 13 C values complies with all but point (iii) of the aforementioned condensed IUPAC guidelines if one assumes scale norma- lization of measured 13 C abundance values to the VPDB scale was indeed carried out using two of the three named international reference materials mentioned as scale end-points (NBS-22, δ 13 C VPDB = –30.031 ‰ and IAEA-CH-6, δ 13 C VPDB = –10.449 ‰) and the third (USGS40, δ 13 C VPDB = –26.389 ‰) as quality control to check on the scale normalization equation of the form “Scale Normalized = s * Measured + b” as derived from measured vs accepted δ 13 C values for NBS-22 and IAEA-CH-6. However, contrary to the requirements summarized in point (iii) the authors omitted to report the names and δ 13 C VPDB values of their locally prepared working in-house standards, i.e. the proteins used as isotopic reference materials. The calibration of the δ 18 O value of their working in-house standard casein does not comply with either point (ii) or point (iii). In analogy to SLAP (now SLAP2) that in conjunction with VSMOW (now VSMOW2) defines the VSMOW/SLAP scale for δ 18 O values, IAEA-602 (δ 18 O VSMOW = 71.4 ‰) should have been used in addition to IAEA-601 (δ 18 O VSMOW = 23.3 ‰) as the second international reference material to anchor and scale normalize measured δ 18 O values to VSMOW. [13] With this purpose in mind the difference in δ 18 O values of 48.1 ‰ between IAEA-601 and IAEA-602 is of the same order as the difference in δ 18 O values between VSMOW2 and SLAP2 (55.5 ‰). Also, while not properly scale normalized, the authors should still have reported the δ 18 O value they obtained for their working in-house standard casein. The least compliant method is that for correction of measured δ 2 H of whole wood and cellulose and this gives rise to several grave concerns. (1) The nature of the internal standard used was not stated and, more importantly, neither was any information provided as to how the assigned δ 2 H value of –113 ‰ Copyright © 2014 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2014, 28, 1953–1955 Letter to the Editor Received: 12 May 2014 Revised: 23 June 2014 Accepted: 23 June 2014 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2014, 28, 1953–1955 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6971 1953