Abstract: A method of optically analysing a sample, which method comprises directing one or more frequencies of electromagnetic radiation (EMR) through a sample and onto a partially reflective surface (9) that allows EMR (6) directed thereon to be both reflected and transmitted, which reflected EMR (12) is directed back through the sample such that the pathlength through the sample is different for the transmitted EMR (10) and reflected EMR (12), which transmitted EMR and reflected EMR are both detected by one or more detectors (11,15), characterised in that the optical absorbance of the sample at the one or more wavelengths of EMR is calculated from the difference between the transmitted EMR (10) and reflected EMR (12).