Fourier transform infrared spectroscopy

The FTIR spectrometer (abbreviation for Fourier Transform Infrared spectrometer and Fourier transform infrared spectrometer ), is a special form of a spectrometer, a measurement device for infrared spectroscopy; in this context is often spoken of FTIR spectroscopy. Unlike dispersive instruments in FTIR spectrometers, the spectrum is not absorbed by gradually changing the wavelength. Instead, it is calculated by a Fourier transform of a measured interferogram. An essential part of the spectrometer is the interferometer, such as a Michelson interferometer.

FTIR spectrometer

Construction

The FTIR spectrometer consists of at least the following components:

• Radiation source: a black body that is heated
• Projection: an array of parabolic mirrors and plan which expands the radiation from the source first, couples between two parallel mirrors, decouples and concentrated again.
• Interferometer, comprising: Beam splitter: generated from the coming of the radiation source beam two beams and recombined this again
• Mirror drive: continuously varies the distance of the interferometer
• HeNe Laser: as a reference radiation source for determining the location of or the movable interferometer mirror

Operation

The mirrors are arranged in the system such that they form, for example a Michelson interferometer. Here, the beam coming from the source is split by a beam splitter into two separate beams. One of which is directed and reflected to a fixed mirror, and the other to a movable mirror. Then the two beams are brought together again, so that they, depending on the frequencies contained in the beam and the Spiegelweg interfere. Thus one obtains an interferogram with a large maximum ( engl.: center burst ) where both levels were equidistant from the beam splitter and have thus interfere additively all frequencies, and relatively low slopes (English: wings ). The interferogram is then converted by a Fourier transform into a spectrum.

Properties

The spectral resolution of an FTIR spectrometer is essentially limited by the finite path length L of the movable mirror. It is. That is, the larger the scanning length is, the higher the spectral resolution. Furthermore, it does not depend on the number N of the recorded measurement points. This only determines the maximum measurable frequency, which is given by the Nyquist-Shannon sampling theorem by half the sample rate.

Advantages of FTIR spectrometers over dispersive devices

Compared with dispersive spectrometers operating a FTIR spectrometer by much shorter measurement times and an associated higher signal -to-noise ratio stands out. There would be three major advantages over dispersive devices:

As the Fellgett advantage suggests, the spectrum is a snapshot. This is particularly true for fast scanning FTIR spectrometer to. These permit with recording times of fractions of a second, the studies of dynamic processes.

Applications

The FTIR spectrometer have increasingly replaced the dispersive devices from the laboratories since the late 1970s. Today, they are the most commonly used spectrometer in the field of infrared spectroscopy. In addition, FTIR spectrometer for standard analyzes are available from various manufacturers already, find the comfortably on a lab bench. Also portable units that are offered in some of robust housings that can be used for mobile applications or applications in the field of on-line process analysis.

With the ability to perform significantly faster measurements compared to dispersive spectrometers, it is particularly suitable for time-dependent processes. An application example is the identification of microorganisms. By comparing the spectra of cultured microorganisms with databases can be assignment to genus, species, etc.. Another application is the process analysis, or in situ spectroscopy. The FTIR technique allows, for example, an on-line reaction monitoring in the chemical or bioreactor. Since the spectrometer or the interferometer should be mounted vibration and " relatively" high, the optical path of the spectrometer, has also into the reaction vessel and be re- directed out to the detector. This is nowadays often allows flexible fiber optic ATR probe.