Gases added to a combustion mixture.
Note:
An inert diluent is non-reactive, an auxiliary gas is reactive.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 712 (https://doi.org/10.1515/pac-2019-0203)