Question

Why is Fourier transform an excellent method for data collection in the IR range but not as useful for spectroscopy in the UV range? b) What form does the raw data take in FT methods, i.e. what is on the x and y axes? c) How is the raw data converted into the IR spectrum?

Answer 1

• FT method has extremely high resolving power and all signal reaches detector simultaneously, so can obtain entire spectrum essentially simultaneously. This multiplex advantage means many scans can be completed and averaged in a shorter time. In the IR the chief source of noise is ‘flicker’ in the detector and its circuitry. And the multiplex advantage does great things to get rid of this noise. In the visible and the UV, the chief source of noise is in the ‘flicker’ in the source illumination. Here the signal averaging doesn’t help, in fact this noise tends to get distributed throughout the spectrum. This is why FT machines aren’t used in the visible and UV.
• The raw data is called an interferogram which shows light absorption for each mirror position by plotting light absorption on the x axis and mirror position on y axis.
• The raw data (light intensity for each mirror position) is converted into the desired result (light intensity for each wavelength) by the Fourier transform algorithm, which gives the name to the entire spectrometer. This produces a “single beam” spectrum. A reference or “background” single beam is collected without a sample; the sample single beam is collected with the only change being the presence of the sample. The ratio of these two leads to the spectrum. The final spectrum can be presented as transmittance (%T) or absorbance; the computer easily performs this conversion.