A method of quantitatively analysing a sample, the method comprising: irradiating the sample with radiation having a plurality of frequencies in the range from 25 GHz to 100 THz; detecting radiation reflected from and/or transmitted by said sample to obtain a frequency domain waveform of said sample; identifying at least one section of interest of said frequency domain wave-form containing spectral features due to intermolecular or other non-intramolecular excitations; and obtaining a value related to the concentration of a component of the sample from the said section.