Waters ACQUITY UPLC Operator's, Overview And Maintenance Manual page 81

shape differences are attributable to nonideal phenomena alone and that no
evidence exists for genuine differences between the spectra. A spectral
contrast angle greater than its threshold angle indicates that the shape
differences arise from genuine differences between the spectra. When
automating the spectral contrast comparison, the maximum absorbance of the
spectra must not exceed 1 AU.
Effects of pH on the absorbance spectrum of p-aminobenzoic acid
Effect of pH
pH 6.9
pH 5.1
pH 3.1
Wavelength (nm)
Spectral contrast theory 69