Infrared spectroscopy (
IR spectroscopy or
Vibrational Spectroscopy) is the
spectroscopy that deals with the
infrared region of the
electromagnetic spectrum, that is light with a longer
wavelength and lower
frequency than
visible light. It covers a range of techniques, mostly based on
absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study
chemicals. For a given sample which may be solid, liquid, or gaseous, the method or technique of infrared spectroscopy uses an instrument called an
infrared spectrometer (or spectrophotometer) to produce an
infrared spectrum. A basic IR spectrum is essentially a graph of infrared light
absorbance (or
transmittance) on the vertical axis vs. frequency or wavelength on the horizontal axis. Typical
units of frequency used in IR spectra are
reciprocal centimeters (sometimes called
wave numbers), with the symbol cm
-1. Units of IR wavelength are commonly given in
micrometers (formerly called "microns"), symbol µm, which are related to wave numbers in a
reciprocal way. A common laboratory instrument that uses this technique is a
Fourier transform infrared (FTIR)
spectrometer. Two-dimensional IR is also possible as discussed below.