Method of optically analysing a sample by directing one or more frequencies of electromagnetic radiation (EMR) through a sample and onto a partially reflective surface that allows EMR directed thereon to be both reflected and transmitted. The reflected EMR is directed back through the sample such that the pathlength through the sample is different for the transmitted EMR and reflected EMR. The transmitted EMR and reflected EMP are both detected by one or more detectors. The optical absorbance of the sample at the one or more wavelengths of EMR is calculated from the difference between the transmitted EMR and reflected EMR.