FTIR-Spectrophotometers

A Fourier transform infrared spectrometer (FTIR spectrometer) is a spectrometer which is applied especially for infrared spectroscopy. An FTIR spectrometer does not acquire a spectrum by gradually changing the wavelength, but by performing a Fourier transform of a measured interferogram.

Architecture of an FTIR spectrometer

An FTIR spectrometer consists basically of the following components: a radiation source, the optical path, two parallel mirrors (a mobile and a stationary one), a mirror drive which can vary the distance between the mirrors, a HeNe laser, serving as a reference radiation source for determining the position of the mobile interferometer mirror, a detector converting the energy of the incoming photons into electric signals, and a computer for performing the Fourier transform of the measured electric signal. The spectral composition, i. e. the IR spectrum, is obtained as a result.

Functionality of an FTIR spectrometer

The mirrors are arranged in such a way that they make up, for example, a Michelson interferometer. The beam is split by a beam splitter into two individual beams, one of which is directed onto a stationary mirror and is reflected there. The other one is directed onto a mobile mirror. In a Michelson interferometer, the beams are then reunited in such a way that they interfere, dependent on the frequencies contained in the beam and on the reflection path. The obtained interferogram displays a maximum at that position at which both mirrors assume equal distance to the beam splitter. All frequencies are constructively interfering there. The spectral resolution of an FITR spectrometer is limited by the finite wavelength of the movable mirror. This means that with greater sampling length, spectral resolution increases. The resolution does not depend on the amount of the observed measuring points. This amount determines only the highest detectable frequency, defined as half the sampling rate, according to the 'Nyquist Shannon sampling theorem.

Application of FTIR spectrometer

FTIR spectrometers are often used for the identification of micro-organisms. The allocation to a species is made by comparing with organisms that are listed in a database. FTIR spectrometers can be also used in process analytics or in-situ spectroscopy.

Literature

•  http://www.ir-spektroskopie.de/spec/ftir-prinzip/index.html (called: 30.03.11).

• http://de.wikipedia.org/w/index.php?title=FTIR-Spektrometer&oldid=85679874 (called: 30.03.11).