Evidence for bias in measured [delta]15N values of terrestrial and aquatic organic materials due to pre-analysis acid treatment methods

We investigate the effect of acid treatment methods on δ15N values from a range of environmental organic materials in the context of the increased application of ‘dual-mode’ isotope analysis (the simultaneous measurement of δ13C and δ15N 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 δ15N is also independently assessed (as the capsule and rinse methods combust samples in different capsules; silver and tin, respectively). We find significant differences in δ15N values between methods and the precision of any one method varies significantly between sample materials and above the instrument precision (>0.3‰). The δ15N 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 δ15N value of some materials, particularly those combusted only in silver capsules. We also compare the new δ15N data with previously published δ13C data on the same materials. The response of δ13C and δ15N 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.