reference procedure

in analysis of trace air constituents
https://doi.org/10.1351/goldbook.R05232
Agreed method for determining one or more air quality characteristics where it is not practical to produce a @R05230@; the result obtained is defined as the measure of the air quality characteristic. For example, a gas such as NO2 may be somewhat @U06569@ when stored in a tank at low mixing ratios, so that the use of a more @S05900@NO standard mixture in N2 may be oxidized to NO2 by O3 in a specified manner referred to as the NO2 reference procedure.
Source:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2210 [Terms] [Paper]