difference absorption spectroscopy

A highly concentrated analyte in an analytical sample can be determined with better precision by replacing the blank (reference) cell by one containing a solution of the analyte or other absorber of known concentration; this is known as difference absorption spectroscopy. Difference spectra can also be obtained by computer or other subtraction methods.
Source:
PAC, 1988, 60, 1449 (Nomenclature, symbols, units and their usage in spectrochemical analysis - VII. Molecular absorption spectroscopy, ultraviolet and visible (UV/VIS) (Recommendations 1988)) on page 1455