Evidence for bias in measured δ15N values of terrestrial and aquatic organic materials due to pre-analysis acid treatment methods

We investigate the effect of acid treatment methods on delta N-15 values from a range of environmental organic materials in the context of the increased application of 'dual-mode' isotope analysis (the simultaneous measurement of delta C-13 and delta N-15 from the same acid-treated sample). Three common methods are compared; (i) untreated samples; (ii) acidification followed by sequential water rinse (rinse method); and (iii) acidification in silver capsules (capsule method). The influence of capsule type (silver and tin) on delta N-15 is also independently assessed (as the capsule and rinse methods combust samples in different capsules; silver and tin, respectively). We find significant differences in delta N-15 values between methods and the precision of any one method varies significantly between sample materials and above the instrument precision (>0.3%). The delta N-15 values of untreated samples did not produce the most consistent data on all sample materials. In addition, the capsule type appears to influence the measured delta N-15 value of some materials, particularly those combusted only in silver capsules. We also compare the new delta N-15 data with previously published d13C data on the same materials. The response of delta C-13 and delta N-15 within and between methods and sample materials to acidification appears to be relatively disproportionate, which can influence the environmental interpretation of the measured data. In addition, statistical methods used to estimate inorganic nitrogen are shown to be seriously flawed. Copyright (C) 2011 John Wiley & Sons, Ltd. 1089