A method comprising: emitting light from a light source towards a detector via a validation cell; receiving first light intensity data that quantifies a first intensity of light received at the detector having passed sequentially at least once along its transmission path through each of a reference gas in the validation cell and a zero gas or at least a partial vacuum, the reference gas comprising a known concentration of an analyte compound, the light source emitting the light in a wavelength range that comprises a spectral absorbance feature of the analyte compound, the zero gas or the at least a partial vacuum having at least one of a known or negligible absorbance characteristic within the wavelength range of the light source; verifying a valid state of an analytical system comprising the light source, the detector, the validation cell and the reference gas in the validation cell by determining that the first light intensity data does not deviate from a stored data set by greater than a pre-defined threshold deviation, the stored data set representing at least one previous measurement collected during a previous validation process with the zero gas or the at least a partial vacuum performed on the analytical system; replacement of the zero gas or the at least a partial vacuum by a sample gas; receiving second light intensity data that quantifies a second intensity of the light received at the detector having passed sequentially at least once along its transmission path through each of the reference gas in the validation cell and the sample gas containing an unknown concentration of the analyte compound; and determining a concentration of the analyte compound in the sample gas by correcting the second light intensity data using said first light intensity data to account for the known absorbance of the light in the validation cell and the known or negligible absorbance characteristic of the zero gas or the at least partial vacuum. emitting light from a light source towards a detector via a validation cell; receiving first light intensity data that quantifies a first intensity of light received at the detector having passed sequentially at least once along its transmission path through each of a reference gas in the validation cell and a zero gas or at least a partial vacuum, the reference gas comprising a known concentration of an analyte compound, the light source emitting the light in a wavelength range that comprises a spectral absorbance feature of the analyte compound, the zero gas or the at least a partial vacuum having at least one of a known or negligible absorbance characteristic within the wavelength range of the light source; verifying a valid state of an analytical system comprising the light source, the detector, the validation cell and the reference gas in the validation cell by determining that the first light intensity data does not deviate from a stored data set by greater than a pre-defined threshold deviation, the stored data set representing at least one previous measurement collected during a previous validation process with the zero gas or the at least a partial vacuum performed on the analytical system; replacement of the zero gas or the at least a partial vacuum by a sample gas; receiving second light intensity data that quantifies a second intensity of the light received at the detector having passed sequentially at least once along its transmission path through each of the reference gas in the validation cell and the sample gas containing an unknown concentration of the analyte compound; and determining a concentration of the analyte compound in the sample gas by correcting the second light intensity data using said first light intensity data to account for the known absorbance of the light in the validation cell and the known or negligible absorbance characteristic of the zero gas or the at least partial vacuum.