The above calculation assumes that the sample spectra are quantified by linear least
squares scaling of reference spectra. If the analyser quantifies determinands by fitting
reference spectra by some other method (perhaps involving non-linear fitting) this shall be
repeated here in place of the above to create a residual. In addition, if the analyser software
automatically reports a residual that is the result of the difference between the sample
spectrum and the fitted reference spectra this shall be used in place of the above.