A method of compensating for amplitude drift in a spectrometer generating optical spectral data (SB S) from an unknown sample in a sample holder the method comprising: performing in the spectrometer a standardisation process including: measuring in the spectrometer a single beam spectrum of a standardisation sample (SB SS) and a single beam amplitude spectrum of a zero material (SB Z); deriving in an arithmetic unit associated with the spectrometer a dual beam spectrum relating substantially only to the standardisation sample (DB SS) by dividing in the arithmetic unit the single beam spectrum of the standardisation sample (SB SS) by that of the zero material (SB Z) at the same wavelengths; and determining in the arithmetic unit a mathematical transform (T Y) from a comparison of amplitude values of the derived dual beam spectrum relating substantially only to the standardisation sample (DB SS) with previously defined desired spectral amplitude values (DB SSR), which transform describes a transition of the amplitude values of the derived dual beam spectrum relating substantially only to the standardisation sample (DB SS) to the desired amplitude values; performing the standardisation process periodically throughout the lifetime of the spectrometer; acquiring in the spectrometer a single beam reference spectrum for a zero material (SB ZR) at the time of a preceding performance of a standardisation process; and acquiring a single beam spectrum (SB Zt) for the zero material at least once in the time interval between performances of the standardisation process; characterised in that the method further comprises the steps of, in the time interval between performances of the standardisation process: modifying in the arithmetic unit the mathematical transform (T Y) with a function (F(SB Zt,(SB ZR); F(SB Zt / SB ZR)) describing a change in optical path length (ΔP) through the sample holder, the function being dependent on the single beam reference spectrum (SB ZR) for the zero material acquired at the time of a preceding performance of the standardisation process and the spectrum (SB Zt) for the zero material acquired at least once in the time interval between performances of the standardisation process; and applying in the arithmetic unit the modified mathematical transform to generated optical spectral data (SB S) of an unknown sample. performing in the spectrometer a standardisation process including: measuring in the spectrometer a single beam spectrum of a standardisation sample (SB SS) and a single beam amplitude spectrum of a zero material (SB Z); deriving in an arithmetic unit associated with the spectrometer a dual beam spectrum relating substantially only to the standardisation sample (DB SS) by dividing in the arithmetic unit the single beam spectrum of the standardisation sample (SB SS) by that of the zero material (SB Z) at the same wavelengths; and determining in the arithmetic unit a mathematical transform (T Y) from a comparison of measuring in the spectrometer a single beam spectrum of a standardisation sample (SB SS) and a single beam amplitude spectrum of a zero material (SB Z); deriving in an arithmetic unit associated with the spectrometer a dual beam spectrum relating substantially only to the standardisation sample (DB SS) by dividing in the arithmetic unit the single beam spectrum of the standardisation sample (SB SS) by that of the zero material (SB Z) at the same wavelengths; and determining in the arithmetic unit a mathematical transform (T Y) from a comparison of amplitude values of the derived dual beam spectrum relating substantially only to the standardisation sample (DB SS) with previously defined desired spectral amplitude values (DB SSR), which transform describes a transition of the amplitude values of the derived dual beam spectrum relating substantially only to the standardisation sample (DB SS) to the desired amplitude values; performing the standardisation process periodically throughout the lifetime of the spectrometer; acquiring in the spectrometer a single beam reference spectrum for a zero material (SB ZR) at the time of a preceding performance of a standardisation process; and acquiring a single beam spectrum (SB Zt) for the zero material at least once in the time interval between performances of the standardisation process; characterised in that the method further comprises the steps of, in the time interval between performances of the standardisation process: modifying in the arithmetic unit the mathematical transform (T Y) with a function (F(SB Zt,(SB ZR); F(SB Zt / SB ZR)) describing a change in optical path length (ΔP) through the sample holder, the function being dependent on the single beam reference spectrum (SB ZR) for the zero material acquired at the time of a preceding performance of the standardisation process and the spectrum (SB Zt) for the zero material acquired at least once in the time interval between performances of the standardisation process; and applying in the arithmetic unit the modified mathematical transform to generated optical spectral data (SB S) of an unknown sample.