difference spectroscopy 

https://doi.org/10.1351/goldbook.08234
Measurement method of spectroscopy in which spectral subtraction is used to help the study and identification of individual species in a mixture.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 655 [Terms] [Paper]