A method of correcting for an amplitude change in a spectrometric instrument output includes: exposing a sample in a sample holder to electromagnetic radiation at a plurality of wavenumbers; detecting electromagnetic absorption intensities in the sample at the plurality of wavenumbers; providing to a computer device the detected electromagnetic absorption intensities indexed against wavenumber as spectral data; applying in the computer device a mathematical transform (Icorr) to the spectral data to correct for an amplitude change in the spectrometric instrument's output and calculated by determining a difference (Δ(SBZ)/) between first derivatives of a logarithmic transformation of spectral data (SBZ) from the zero material sample at two different wavenumber ranges (log10(SBZ(x1))/ and log10(SBZ(x2))/); and calculating the mathematical transform (Icorr) as a function inversely dependent on the determined difference (Δ(SBZ)/).