Reference material USGS35 was aimed at δ15N calibrations.
USGS35 was produced by J.K. Böhlke, S.J. Mroczkowski and T.B. Coplen, U.S. Geological Survey, Reston, USA,  from natural nitrate ore deposits of the Atacama Desert in Chile.
(R/I/C) Classification assigned to the property value for analyte (Recommended/Information/Certified)
BÖHLKE, J.K., MROCZKOWSKI, S.J., COPLEN, T.B., Oxygen isotopes in nitrate: new reference materials for 18O:17O:16O measurements and observations on nitrate water equilibration, Rapid Commun. Mass Spectrom. 17 (2003) 1835–1846.