Reference materials are compounds which are carefully calibrated against the primary reference or a calibration material. These compounds allow for isotopic analysis of materials differing in chemical or isotopic composition from the compounds defining the isotopic scales on which measurements are reported. In general these are the materials most researchers mean when they say "reference materials". An example of a reference material is USGS-34, a KNO salt with a δN of -1.8‰ vs. AIR. In this case the reference material has a mutually agreed upon value of δN when measured relative to the primary reference of atmospheric N (Böhlke et al., 2003). USGS-34 is useful because it allows researchers to directly measure the N/N of NO in natural samples against the standard and report observations relative to N without having to first convert the sample to N gas.