![]() ![]() Today, most research and development grade mid-IR instruments are of the FT type. The FT method produces spectra that show a much better signal-to-noise ratio and, as the wavelength scale is calibrated with a very precise reference laser, provides higher wavelength accuracy than IR.įigure 3: Schematic diagram showing the layout of a dispersive IR spectrophotometer. The acquisition of FTIR spectra is much faster than by conventional dispersive instruments. FTIR and FT-NIR are potentially complementary techniques, but usually the analyst must make a choice of which to use for a particular application, so it is worth considering their relative strengths and weaknesses. However, FT instrumentation is available for both UV and NIR spectral forms. The signal must first be Fourier-transformed (FT) to produce the intensity as a function of wavenumber.īy convention when we speak of FTIR, we think of it as operating in the mid-IR region. This represents the light intensity as a function of the position of a mirror inside the interferometer, not as a function of wavelength (as occurs in dispersive instruments). The difference between IR and FTIR is that the latter is constructed from an interferogram as the raw signal. What is FTIR spectroscopy and what’s the difference between FTIR vs IR spectroscopy? These movements cause the IR spectral absorbance bands we observe. The reader is also directed to useful additional introductory material available from the Royal Society of Chemistry.įigure 2: Animation showing the 3-dimensional movements that can occur for molecular atomic bonds when excited by IR light. In contrast, NIR spectroscopy is comprised of combination bands of “overtones” produced from those fundamental vibrations. The higher-energy mid-IR radiation excites fundamental vibrations when energy is absorbed by molecules, raising them from ground state to the first vibrational state. These covalent bonds are often considered to act like stiff springs that can stretch, bend, rotate and scissor (Figure 2). Photon energies in the IR region induce vibrational excitation of covalently bonded atoms. IR light is absorbed by molecules at specific frequencies based on the molecular bonds between atoms and the types of atoms present at the end of the bonds. Far-IR is typically defined as radiation between 500 and 20 cm -1, mid-IR between 4,000 and 500 cm -1 and NIR as typically between ~ 10,000 and 4,000 cm -1. Thus, a shorter wavelength will have a larger wavenumber, referring to the fact that more waves would be able to fit into a given distance. Historically, by convention, IR tends to be described in “wavenumber”, where any wavenumber is inversely proportional to its wavelength. IR spectroscopy is the study of the interaction of IR light with matter, where IR light is characterized by the wavenumber range spanning 12,800 to 10 cm -1. Figure 1: The electromagnetic spectrum, with inset indicating the sub-region typical for infrared spectroscopy types. ![]()
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