FTIR spectrometer

The FTIR spectrometer (abbreviation for Fourier transform infrared spectrometer or Fourier transform infrared spectrometer ) is a special variant of a spectrometer , a measuring device for infrared spectroscopy ; FTIR spectroscopy is often used in this context . In contrast to dispersive measuring devices, the spectrum in FTIR spectrometers is not recorded by changing the wavelength in steps . Instead, it is calculated by Fourier transforming a measured interferogram . An integral part of the spectrometer is the interferometer , e.g. B. a Michelson interferometer .

Interferogram measured with an FTIR spectrometer

Single-channel IR spectrum after the Fourier transform of the interferogram

The FTIR spectrometer

construction

FTIR spectrometer with an open sample space in which a simple transmission holder is located

FTIR spectrometer without housing

The FTIR spectrometer consists of at least the following components:

Radiation source: a black body that is heated
Beam path: an arrangement of parabolic and plane mirrors which first expands the radiation from the source, couples it in between two parallel mirrors, decouples it and then concentrates it again.
Interferometer , consisting of:
- Beam splitter : generates two beams from the beam coming from the radiation source and recombines them again
- Mirror drive: continuously changes the distance between the interferometer mirrors
- HeNe laser : as a reference radiation source to determine the location of the movable interferometer mirror or mirrors
Radiation detector : a black body that converts the energy of the incoming photons into electrical signals
Computer: for performing the Fourier transformation of the measured electrical signal, the result is the spectral composition, i.e. the IR spectrum.

functionality

Basic structure of an FTIR spectrometer with Michelson interferometer

The mirrors are arranged in the system in such a way that they form a Michelson interferometer , for example . The beam coming from the source is split into two individual beams by a beam splitter . One of them is directed onto a fixed mirror and reflected, the other onto a movable mirror. Then the two beams are brought together again so that they interfere, depending on the frequencies contained in the beam and the mirror path. This gives an interferogram with a large maximum ( center burst ) where both mirrors were equidistant from the beam splitter and thus all frequencies interfered additively, and relatively flat wings . The interferogram is then converted into a spectrum using a Fourier transformation.

In order to increase the detection strength, the radiation detector is usually cooled to 77K with liquid nitrogen. Laser cooling is also currently being investigated.

properties

The spectral resolution of an FTIR spectrometer is essentially limited by the finite path length L of the movable mirror. It amounts to . That is, the longer the scan length, the higher the spectral resolution. Furthermore, it does not depend on the number N of recorded measuring points. This only determines the maximum measurable frequency , which is given by half the sample rate according to the Nyquist-Shannon sampling theorem . ${\ displaystyle {\ frac {\ nu} {\ Delta \ nu}} = 2L \ nu}$ ${\ displaystyle \ nu _ {\ mathrm {max}}}$

Advantages of FTIR spectrometers compared to dispersive devices

Compared to dispersive spectrometers, an FTIR spectrometer is characterized by significantly shorter measurement times and an associated higher signal-to-noise ratio . This results in three main advantages over dispersive devices:

Throughput or Jacquinot advantage: By eliminating the gap required in dispersive spectrometers, which determines the resolution, a larger amount of light reaches the detector. Circular diaphragms can be used which, unlike slit diaphragms, can also scatter the light as long as the next order of diffraction does not reach the interferometer. In this way, the light yield can be improved by a factor of 200 and thus the signal-to-noise ratio in turn.

Multiplex or Fellgett advantage: By using an interferometer instead of a grating monochromator , the spectrum is not measured continuously as a function of the wavelength, but all wavelengths at the same time, quasi as a snapshot over the entire defined spectral range (frequency range). This increases the signal-to-noise ratio by (for spectral elements). ${\ displaystyle {\ sqrt {N}}}$ ${\ displaystyle N}$

Connes Advantage: Using a HeNe laser as a reference results in a much higher accuracy of the frequency or wavelength axis in the IR spectrum than with dispersive spectrometers. An accuracy of the wavenumber of 0.001 cm ⁻¹ can be achieved.

As the Fellgett advantage suggests, the spectrum is a snapshot. This is especially true for the fast scanning FTIR spectrometers. With recording times of fractions of a second, these allow the study of dynamic processes.

Applications

The FTIR spectrometers have increasingly displaced dispersive devices from the laboratories since the late 1970s. Today they are the most widely used spectrometers in the field of infrared spectroscopy. In addition, FTIR spectrometers are already available from various manufacturers for standard analyzes that can be conveniently placed on a laboratory bench. Transportable devices are also offered in partly robust housings that can also be used for mobile applications or applications in the field of online process analysis.

Due to the possibility of being able to carry out significantly faster measurements compared to dispersive spectrometers, it is particularly suitable for time-dependent processes. One application example is the identification of microorganisms . By comparing the spectra of cultivated microorganisms with databases , an allocation according to genus can sometimes also be made to species . The official food control in Germany uses FT-IR for the epidemiological clarification of infection routes and works in an interdisciplinary way with doctors and veterinarians .

Another area of application is process analysis or in-situ spectroscopy. FTIR technology allows, for example, online reaction tracking in the chemical or bioreactor . Since the spectrometers or their interferometers should be stored with low vibrations and are “relatively” large, the beam path must be guided out of the spectrometer into the reaction vessel and out again to the detector. Nowadays this is often made possible by flexible fiber optic ATR probes.

Another area in which FTIR spectrometers have found widespread use is the measurement of emissions from combustion processes such as engines or power plants. This was mainly promoted by the introduction of the SCR method in vehicles, as this enables the simultaneous measurement of all parameters relevant to the method, such as NO , NO ₂ , NH ₃ , N ₂ O , H ₂ O , CO ₂ . The only exception is the measurement of hydrocarbons. Here there are major deviations between the concentrations determined with an FID . The reason for this is that the FID is used to determine a total hydrocarbon value, while the FTIR determines the concentration of specific hydrocarbons. Since the exhaust gas can contain up to several hundred different hydrocarbon compounds, the FTIR spectrometer will under-record the total hydrocarbons.

An automated FTIR spectrometer can be used to determine the formaldehyde content in the exhaust gas from combustion engines . The exhaust gas to be sampled flows through a measuring cell, which is illuminated by infrared radiation from the spectrometer. The attenuation of certain wavelengths provides information about the composition of the exhaust gas. In comparison to other emission measurement methods for formaldehyde, the measurement results are output directly.

Individual evidence

↑ Markus P. Hehlen, Junwei Meng, Alexander R. Albrecht, Eric R. Lee, Aram Gragossian, Steven P. Love, Christopher E. Hamilton, Richard I. Epstein and Mansoor Sheik-Bahae: First demonstration of an all-solid- state optical cryocooler . In: Springer (Ed.): Light: Science & Applications . tape 7 (1): 15 , 2018, doi : 10.1038 / s41377-018-0028-7 .
^ Mareike Wenning, Herbert Seiler, Siegfried Scherer: Fourier-Transform Infrared Microspectroscopy, a Novel and Rapid Tool for Identification of Yeasts . In: Applied and Environmental Microbiology . tape 68 , no. 10 , October 1, 2002, ISSN 0099-2240 , p. 4717-4721 , doi : 10.1128 / aem.68.10.4717-4721.2002 , PMID 12324312 .
↑ Herbert Seiler and Siegfried Scherer : FTIR spectra libraries for the identification of microorganisms , Institute for Microbiology, FML Freising-Weihenstephan
^ N. Mauder, J. Rau: Infrared spectroscopy - a multi-tool for microbiology , CVUA Stuttgart
^ Basil Daham, Gordon E. Andrews, Hu Li, Rosario Ballesteros, Margaret C. Bell, James Tate and Karl Ropkins: Application of a Portable FTIR for Measuring On-road Emissions . In: SAE (Ed.): SAE Technical Paper Series . tape 2005-01-0676 . SAE, 2005, doi : 10.4271 / 2005-01-0676 .
↑ VDI 3862 sheet 8 measurement of gaseous emissions; Measurement of formaldehyde in exhaust gas from internal combustion engines; FTIR method (Measurement of gaseous emissions; Measurement of formaldehyde in the exhaust gas of combustion engines; FTIR method). Beuth Verlag, Berlin, pp. 3-4.
↑ Wolfgang Schreier: Emissions measurements on gas engines. In: Hazardous substances - cleanliness. Air . 69, No. 1/2, 2009, ISSN 0949-8036 , pp. 25-30.

Web links

literature

Hans-Ulrich Gremlich, Helmut Günzler: IR Spectroscopy: An Introduction . 4th edition. Wiley-VCH, 2003, ISBN 3-527-30801-6 .

[1] Markus P. Hehlen, Junwei Meng, Alexander R. Albrecht, Eric R. Lee, Aram Gragossian, Steven P. Love, Christopher E. Hamilton, Richard I. Epstein and Mansoor Sheik-Bahae: First demonstration of an all-solid- state optical cryocooler . In: Springer (Ed.): Light: Science & Applications . tape 7 (1): 15 , 2018, doi : 10.1038 / s41377-018-0028-7 .

[2] Mareike Wenning, Herbert Seiler, Siegfried Scherer: Fourier-Transform Infrared Microspectroscopy, a Novel and Rapid Tool for Identification of Yeasts . In: Applied and Environmental Microbiology . tape 68 , no. 10 , October 1, 2002, ISSN 0099-2240 , p. 4717-4721 , doi : 10.1128 / aem.68.10.4717-4721.2002 , PMID 12324312 .

[3] Herbert Seiler and Siegfried Scherer : FTIR spectra libraries for the identification of microorganisms , Institute for Microbiology, FML Freising-Weihenstephan

[4] N. Mauder, J. Rau: Infrared spectroscopy - a multi-tool for microbiology , CVUA Stuttgart

[5] Basil Daham, Gordon E. Andrews, Hu Li, Rosario Ballesteros, Margaret C. Bell, James Tate and Karl Ropkins: Application of a Portable FTIR for Measuring On-road Emissions . In: SAE (Ed.): SAE Technical Paper Series . tape 2005-01-0676 . SAE, 2005, doi : 10.4271 / 2005-01-0676 .

[6] VDI 3862 sheet 8 measurement of gaseous emissions; Measurement of formaldehyde in exhaust gas from internal combustion engines; FTIR method (Measurement of gaseous emissions; Measurement of formaldehyde in the exhaust gas of combustion engines; FTIR method). Beuth Verlag, Berlin, pp. 3-4.

[7] Wolfgang Schreier: Emissions measurements on gas engines. In: Hazardous substances - cleanliness. Air . 69, No. 1/2, 2009, ISSN 0949-8036 , pp. 25-30.