reference procedure

in analysis of trace air constituents
Agreed method for determining one or more air quality characteristics where it is not practical to produce a reference material; the result obtained is defined as the measure of the air quality characteristic. For example, a gas such as NO2 may be somewhat unstable when stored in a tank at low mixing ratios, so that the use of a more stableNO standard mixture in N2 may be oxidized to NO2 by O3 in a specified manner referred to as the NO2 reference procedure.
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2210