external standard

in chromatography
https://doi.org/10.1351/goldbook.E02290
A compound present in a standard sample of known concentration and volume which is analysed separately from the unknown sample under identical conditions. It is used to facilitate the qualitative indentification and/or quantitative determination of the sample components. The volume of the external standard (standard sample) need not to be known if it is identical to that of the unknown sample.
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 837 (https://doi.org/10.1351/pac199365040819)