A spectroscopic method to analyse a radiation having an unknown characteristic, the method comprising: o providing a substrate defining a first surface; o providing a liquid specimen defining a second surface, said second surface being a liquid-gas interface; o impinging the liquid specimen and the substrate with a first electromagnetic radiation having a known wavelength so that the first electromagnetic radiation is reflected on the first and on the second surface (4a, 4b); o impinging the liquid specimen and the substrate with a second electromagnetic radiation to be analysed and having an unknown characteristic, the second electromagnetic radiation having a second wavelength, so that the second electromagnetic radiation is reflected on the first and on the second surface; o varying the location of the second surface with respect to the first surface; o detecting a first plurality of data forming a first time-domain interferogram having a first plurality of fringes relative to the interference of the reflections of the first electromagnetic radiation on the first and on the second surface during the variation in location of the second surface; o detecting at the same time a second plurality of data forming a second time-domain interferogram having a second plurality of fringes relative to the interference of the reflections of the second electromagnetic radiation on the first and on the second surface during the variation in location of the second surface; o determining, from the position of the fringes in the first interferogram, a time-length transformation which transforms a time scale of the first interferogram into a length scale; o applying the time-length transformation to the second plurality of data forming the second interferogram; o determining a transformation of the points of the second interferogram so that the second interferogram becomes a signal which fast-varying component is independent from the position and shape of the second surface, and that oscillates around a constant value; and o applying such transformation to the second interferogram o applying a Fourier Transformation to the second plurality of data forming the second interferogram after the application of all the transformations to obtain the spectrum of the second electromagnetic radiation. o providing a substrate defining a first surface; o providing a liquid specimen defining a second surface, said second surface being a liquid-gas interface; o impinging the liquid specimen and the substrate with a first electromagnetic radiation having a known wavelength so that the first electromagnetic radiation is reflected on the first and on the second surface (4a, 4b); o impinging the liquid specimen and the substrate with a second electromagnetic radiation to be analysed and having an unknown characteristic, the second electromagnetic radiation having a second wavelength, so that the second electromagnetic radiation is reflected on the first and on the second surface; o varying the location of the second surface with respect to the first surface; o detecting a first plurality of data forming a first time-domain interferogram having a first plurality of fringes relative to the interference of the reflections of the first electromagnetic radiation on the first and on the second surface during the variation in location of the second surface; o detecting at the same time a second plurality of data forming a second time-domain interferogram having a second plurality of fringes relative to the interference of the reflections of the second electromagnetic radiation on the first and on the second surface during the variation in location of the second surface; o determining, from the position of the fringes in the first interferogram, a time-length transformation which transforms a time scale of the first interferogram into a length scale; o applying the time-length transformation to the second plurality of data forming the second interferogram; o determining a transformation of the points of the second interferogram so that the second interferogram becomes a signal which fast-varying component is independent from the position and shape of the second surface, and that oscillates around a constant value; and o applying such transformation to the second interferogram o applying a Fourier Transformation to the second plurality of data forming the second interferogram after the application of all the transformations to obtain the spectrum of the second electromagnetic radiation.