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.
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 (https://doi.org/10.1351/pac198860091449)