In FTIR spectroscopy, there are two modes , which are reflectivity and transmissivity measurements respectively. what is the difference between them? Will the peak be different?
In reflectivity, you measure the loss of intensity in the IR beam which is reflected upon the sample surface. In transmittivity, you measure the loss of intentity in the IR beam which is transmitted through the sample.
They give the same FTIR spectrum. In transmittivity mode, the peak are usually more intense (the absorption is more important).
Transmittivity is usually used for samples which have low absorptions or that are deposited on transparent plates. Reflectivity is used in the oposite case.
Dear Chao Shen,
i think Kirchhoff law can clarify your doubt,
absorption(A)+transmittance(T)+reflectance(R)=1
if your sample is opaque (T=0), then Kirchhoff law becomes R=1-A
if your sample is more transparent(consider, R=0), then Kirchhoff law becomes T=1-A
In these two cases R &T depends on your sample absorption
Also, it may be important to remember what IR wavelength range your source is emitting and whether your sample or sample holder reflect the wavelength range that you are interested in. For instance, gold reflects wavelengths of >800 nm nearly 100%, but not all metals do that. Also, there are reflective IR slides that are useful if you are interested in using a glass substrate to take your measurements in reflective mode.
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