New reference materials for nitrogen-isotope-ratio measurements
Three new reference materials were manufactured for calibration of relative stable nitrogen‐isotope‐ratio measurements: USGS25 (ammonium sulfate) δ615″=−30 per mil USGS26 (ammonium sulfate) δ615″=+54 per mil USGS32 (potassium nitrate) δ615″=+180 per mil where δN′, relative to atmospheric nitrogen, is an approximate value subject to change following interlaboratory comparisons. These materials are isotopically homogeneous in aliquots at least as small as 10 pot N, (or about 1‐2 mg of salt). The new reference materials greatly extend the range of δ15N values of internationally distributed standards, and they allow normalization of δ15N measurements over almost the full range of known natural isotope variation on Earth. The methods used to produce these materials may be adapted to produce homogeneous local laboratory standards for routine use.