Fourier transform infrared spectroscopy - Revision history

Shawndouglas: Updated to transclusion

2020-03-10T23:16:38Z

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	{{Reflist\|2}}		{{Reflist\|2}}

			==References==
			<references />

			<!---Place all category tags here-->
			[[Category:Scientific techniques]]
	[[Category:Spectroscopy]]		[[Category:Spectroscopy]]

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Added attribution.

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	*Emission spectra. Instead of recording the spectrum of light transmitted through the sample, FTIR spectrometer can be used to acquire spectrum of light emitted by the sample. Such emission could be induced by various processes, and the most common ones are [[luminescence]] and [[Raman scattering]]. Little modification is required to an absorption FTIR spectrometer to record emission spectra and therefore many commercial FTIR spectrometers combine both absorption and emission/Raman modes.<ref>{{cite book\|url=http://books.google.com/?id=QBoTvW_h1FQC&pg=PA263\|page=263\|title=Luminescence spectroscopy of minerals and materials\|author=Michael Gaft, Renata Reisfeld, Gérard Panczer\|publisher=Springer\|year= 2005\|isbn=3540219188}}</ref>		*Emission spectra. Instead of recording the spectrum of light transmitted through the sample, FTIR spectrometer can be used to acquire spectrum of light emitted by the sample. Such emission could be induced by various processes, and the most common ones are [[luminescence]] and [[Raman scattering]]. Little modification is required to an absorption FTIR spectrometer to record emission spectra and therefore many commercial FTIR spectrometers combine both absorption and emission/Raman modes.<ref>{{cite book\|url=http://books.google.com/?id=QBoTvW_h1FQC&pg=PA263\|page=263\|title=Luminescence spectroscopy of minerals and materials\|author=Michael Gaft, Renata Reisfeld, Gérard Panczer\|publisher=Springer\|year= 2005\|isbn=3540219188}}</ref>
	*Photocurrent spectra. This mode uses a standard, absorption FTIR spectrometer. The studied sample is placed instead of the FTIR detector, and its photocurrent, induced by the spectrometer's broadband source, is used to record the interferrogram, which is then converted into the photoconductivity spectrum of the sample.<ref>{{cite book\|url=http://books.google.com/?id=SvVYBK6YAxAC&pg=PA189\|page=189\|title=Thin film solar cells: fabrication, characterization and applications\|author=Jef Poortmans, Vladimir Arkhipov\|publisher=John Wiley and Sons\|year= 2006\|isbn=0470091266}}</ref>		*Photocurrent spectra. This mode uses a standard, absorption FTIR spectrometer. The studied sample is placed instead of the FTIR detector, and its photocurrent, induced by the spectrometer's broadband source, is used to record the interferrogram, which is then converted into the photoconductivity spectrum of the sample.<ref>{{cite book\|url=http://books.google.com/?id=SvVYBK6YAxAC&pg=PA189\|page=189\|title=Thin film solar cells: fabrication, characterization and applications\|author=Jef Poortmans, Vladimir Arkhipov\|publisher=John Wiley and Sons\|year= 2006\|isbn=0470091266}}</ref>

	~~==References==~~
	~~{{Reflist\|2}}~~

	==External links==		==External links==
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	*[http://www.piketech.com/technical/application-pdfs/SemiconductorApplOverview.pdf Semiconductor applications] FTIR Sampling Techniques Overview.		*[http://www.piketech.com/technical/application-pdfs/SemiconductorApplOverview.pdf Semiconductor applications] FTIR Sampling Techniques Overview.
	*[http://infrared.als.lbl.gov/content/web-links/58-irwindows infrared materials] Properties of many salt crystals and useful links.		*[http://infrared.als.lbl.gov/content/web-links/58-irwindows infrared materials] Properties of many salt crystals and useful links.

			==Notes==

			The bulk of this article is reused from [http://en.wikipedia.org/wiki/Fourier_transform_infrared_spectroscopy the Wikipedia article].

			==References==
			{{Reflist\|2}}

	[[Category:Spectroscopy]]		[[Category:Spectroscopy]]

Shawndouglas: /* Michelson interferometer */ Removed image link.

2011-10-27T20:57:16Z

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	==Michelson interferometer==		==Michelson interferometer==

	~~[[File:FTIR Interferometer.png\|thumb\|374 px\|Schematic diagram of a Michelson interferometer, configured for FTIR]]~~
	In a Michelson interferometer adapted for FTIR, light from the [[polychromatic]] infrared source, approximately a [[black-body]] radiator, is [[collimated]] and directed to a [[beam splitter]]. Ideally 50% of the light is reflected towards the fixed mirror and 50% is transmitted towards the moving mirror. Light is reflected from the two mirrors back to the beam splitter and (ideally) 50% of the original light passes into the sample compartment. There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the retardation. An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of the retardation. The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength. This results in a maximum at zero retardation, when there is [[constructive interference]] at all wavelengths, followed by series of "wiggles". The position of zero retardation is determined accurately by finding the point of maximum intensity in the interferogram. When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample.		In a Michelson interferometer adapted for FTIR, light from the [[polychromatic]] infrared source, approximately a [[black-body]] radiator, is [[collimated]] and directed to a [[beam splitter]]. Ideally 50% of the light is reflected towards the fixed mirror and 50% is transmitted towards the moving mirror. Light is reflected from the two mirrors back to the beam splitter and (ideally) 50% of the original light passes into the sample compartment. There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the retardation. An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of the retardation. The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength. This results in a maximum at zero retardation, when there is [[constructive interference]] at all wavelengths, followed by series of "wiggles". The position of zero retardation is determined accurately by finding the point of maximum intensity in the interferogram. When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample.

Shawndouglas: Recreated article due to necessary redirect.

2011-10-27T20:56:20Z

Recreated article due to necessary redirect.

New page

{{Redirect-acronym|FTIR|[[Frustrated total internal reflection]]}}
'''Fourier transform infrared spectroscopy''' ('''FTIR''')<ref name=Griffiths>{{cite book|last1=Griffiths|first1=P.|last2=de Hasseth|first2=J.A.|title=Fourier Transform Infrared Spectrometry |edition=2nd|date=18 May 2007|publisher=Wiley-Blackwell|isbn=0471194042|url=http://books.google.com/?id=C_c0GVe8MX0C&printsec=frontcover}}</ref> is a technique which is used to obtain an [[infrared]] spectrum of [[Absorption (electromagnetic radiation)|absorption]], emission, [[photoconductivity]] or [[Raman scattering]] of a solid, liquid or gas. An FTIR spectrometer simultaneously collects spectral data in a wide spectral range. This confers a significant advantage over a [[Dispersion (optics)|dispersive]] spectrometer which measures intensity over a narrow range of wavelengths at a time. FTIR has made dispersive infrared spectrometers all but obsolete (except sometimes in the near infrared), opening up new applications of [[infrared spectroscopy]].

The term ''Fourier transform infrared spectroscopy'' originates from the fact that a [[Fourier transform]] (a mathematical algorithm) is required to convert the raw data into the actual spectrum. For other uses of this kind of technique, see [[Fourier transform spectroscopy]].

==Conceptual introduction==
[[File:FTIR-interferogram.svg|thumb|An FTIR interferogram. The central peak is at the ZPD position ("Zero Path Difference" or zero retardation) where the maximum amount of light passes through the [[Michelson interferometer|interferometer]] to the detector.]]
The goal of any [[absorption spectroscopy]] (FTIR, [[Ultraviolet-visible spectroscopy|ultraviolet-visible ("UV-Vis") spectroscopy]], etc.) is to measure how well a sample absorbs light at each wavelength. The most straightforward way to do this, the "dispersive spectroscopy" technique, is to shine a [[monochromatic]] light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength. (This is how [[Ultraviolet-visible spectroscopy|UV-Vis spectrometers]] work, for example.)

Fourier transform spectroscopy is a less intuitive way to obtain the same information. Rather than shining a ''monochromatic'' beam of light at the sample, this technique shines a beam containing many different frequencies of light at once, and measures how much of that beam is absorbed by the sample. Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process is repeated many times. Afterwards, a computer takes all these data and works backwards to infer what the absorption is at each wavelength.

The beam described above is generated by starting with a [[broadband]] light source—one containing the full spectrum of wavelengths to be measured. The light shines into a certain configuration of mirrors, called a [[Michelson interferometer]], that allows some wavelengths to pass through but blocks others (due to [[wave interference]]). The beam is modified for each new data point by moving one of the mirrors; this changes the set of wavelengths that pass through.

As mentioned, computer processing is required to turn the raw data (light absorption for each mirror position) into the desired result (light absorption for each wavelength). The processing required turns out to be a common algorithm called the [[Fourier transform]] (hence the name, "Fourier transform spectroscopy"). The raw data is sometimes called an "interferogram".

==Developmental background==
The first low-cost [[spectrophotometer]] capable of recording an [[infrared spectroscopy|infrared spectrum]] was the [[Perkin-Elmer]] Infracord produced in 1957.<ref>{{cite journal|year=1957|title=The Infracord double-beam spectrophotometer|journal=Clinical Science|volume=16|issue=2}}</ref> This instrument covered the wavelength range from 2.5 μm to 15 μm ([[wavenumber]] range 4000 cm−1 to 660 cm−1). The lower wavelength limit was chosen to encompass the highest known vibration frequency due to a fundamental [[molecular vibration]]. The upper limit was imposed by the fact that the [[Dispersion (optics)|dispersing element]] was a [[Dispersive prism|prism]] made from a single crystal of rock-salt ([[sodium chloride]]) which becomes opaque at wavelengths longer than about 15 μm; this spectral region became known as the rock-salt region. Later instruments used [[potassium bromide]] prisms to extend the range to 25 μm (400 cm−1) and [[caesium iodide]] 50 μm (200 cm−1). The region beyond 50 μm (200 cm−1) became known as the far-infrared region; at very long wavelengths it merges into the [[microwave]] region. Measurements in the far infrared needed the development of accurately ruled [[diffraction grating]]s to replace the prisms as dispersing elements since salt crystals are opaque in this region. More sensitive detectors than the [[bolometer]] were required because of the low energy of the radiation. One such was the [[Golay detector]]. An additional issue is the need to exclude atmospheric [[water vapour]] because water vapour has in intense pure [[rotational spectrum]] in this region. Far-infrared spectrophotometers were cumbersome, slow and expensive. The advantages of the [[Michelson interferometer]] were well-known, but considerable technical difficulties had to be overcome before a commercial instrument could be built. Also an electronic computer was needed to perform the required Fourier transform and this only became practicable with the advent of [[mini-computer]]s, such as the [[PDP-8]] which became available in 1965.

==Michelson interferometer==

[[File:FTIR Interferometer.png|thumb|374 px|Schematic diagram of a Michelson interferometer, configured for FTIR]]
In a Michelson interferometer adapted for FTIR, light from the [[polychromatic]] infrared source, approximately a [[black-body]] radiator, is [[collimated]] and directed to a [[beam splitter]]. Ideally 50% of the light is reflected towards the fixed mirror and 50% is transmitted towards the moving mirror. Light is reflected from the two mirrors back to the beam splitter and (ideally) 50% of the original light passes into the sample compartment. There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the retardation. An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of the retardation. The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength. This results in a maximum at zero retardation, when there is [[constructive interference]] at all wavelengths, followed by series of "wiggles". The position of zero retardation is determined accurately by finding the point of maximum intensity in the interferogram. When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample.

There are two principal advantages for a FT spectrometer compared to a scanning (dispersive) spectrometer.<ref>{{cite book|last1=Banwell|first1=C.N.|last2=McCash|first2=E.M.|title=Fundamentals of Molecular Spectroscopy|edition=4th|year=1994|publisher=McGraw-Hill|isbn=0-07-707976-0}}</ref><ref name=white>{{cite book|url=http://books.google.com/?id=t2VSNnFoO3wC&pg=PA7|title=Chromatography/Fourier transform infrared spectroscopy and its applications|author=Robert White|publisher=Marcel Dekker|year=1990|isbn=0824781910}}</ref>
#The multiplex or [[Fellgett's advantage]]. This arises from the fact that information from all wavelengths is collected simultaneously. It results in a higher [[Signal-to-noise ratio]] for a given scan-time or a shorter scan-time for a given resolution.
#The throughput or Jacquinot's advantage. This results from the fact that, in a dispersive instrument, the [[monochromator]] has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source..

Other minor advantages include less sensitivity to stray light,<ref name=white/> and "Connes' advantage" (better wavelength accuracy),<ref name=white/> while a disadvantage is that FTIR cannot use the advanced electronic filtering techniques that often makes its signal-to-noise ratio inferior to that of dispersive measurements.<ref name=white/>

===Resolution===

The interferogram belongs in the length domain. [[Fourier transform]] (FT) inverts the dimension, so the FT of the interferogram belongs in the reciprocal length domain, that is the [[wavenumber]] domain. The [[spectral resolution]] in wavenumbers per cm is equal to the reciprocal of the maximum retardation in cm. Thus a 4 cm−1 resolution will be obtained if the maximum retardation is 0.25 cm; this is typical of the cheaper FTIR instruments. Much higher resolution can be obtained by increasing the maximum retardation. This is not easy as the moving mirror must travel in a near-perfect straight line. The use of [[Corner reflector|corner-cube]] mirrors in place of the flat mirrors is helpful as an outgoing ray from a corner-cube mirror is parallel to the incoming ray, regardless of the orientation of the mirror about axes perpendicular to the axis of the light beam. Connes measured in 1966 the temperature of the atmosphere of [[Venus]] by recording the [[Rovibrational coupling|vibration-rotation spectrum]] of Venusian CO2 at 0.1 cm−1 resolution.<ref>{{cite journal|last=Connes|first=J.|coauthors=Connes, P.|year=1966|title=Near-Infrared Planetary Spectra by Fourier Spectroscopy. I. Instruments and Results|journal=Journal of the Optical Society of America|volume=56|issue=7|pages=896–910|doi=10.1364/JOSA.56.000896}}</ref> [[Albert Abraham Michelson|Michelson]] himself attempted to resolve the hydrogen [[H-alpha|Hα emission band]] in the spectrum of a [[hydrogen]] atom into its two components by using his interferometer.<ref name=Griffiths/> p25 A spectrometer with 0.001 cm−1 resolution is now available commercially from [[Bruker]]. The throughput advantage is important for high-resolution FTIR as the monochromator in a dispersive instrument with the same resolution would have very narrow [[monochromator#Czerny-Turner monochromator|entrance and exit slits]].

===Beam splitter===
The beam-splitter can not be made of a common glass, as it is opaque to infrared radiation of wavelengths longer than about 2.5 μm. A thin film, usually of a plastic material, is used instead. However, as any material has a limited range of optical transmittance, several beam-splitters are used interchangeably to cover a wide spectral range.

===Fourier transform===
The interferogram in practice consists of a set of intensities measured for discrete values of retardation. The difference between successive retardation values is constant. Thus, a [[discrete Fourier transform]] is needed. The [[fast Fourier transform]] (FFT) algorithm is used.

==Far-infrared FTIR==
The first FTIR spectrometers were developed for far-infrared range. The reason for this has to do with the mechanical tolerance needed for good optical performance, which is related to the wavelength of the light being used. For the relatively long wavelengths of the far infrared (~10 μm), tolerances are adequate, whereas for the rock-salt region tolerances have to be better than 1 μm. A typical instrument was the cube interferometer developed at the [[National Physical Laboratory (United Kingdom)|NPL]]<ref>{{cite journal|last=Chamberain|first=J.|coauthors=Gibbs,J.E.; Gebbie, H.E.|year=1969|title=The determination of refractive index spectra by fourier spectrometry|journal=Infrared Physics|volume=9|issue=4|pages=189–209|doi=10.1016/0020-0891(69)90023-2|bibcode = 1969InfPh...9..185C }}</ref> and marketed by [[Grubb Parsons]]. It used a stepper motor to drive the moving mirror, recording the detector response after each step was completed.

==Mid-infrared FTIR==
With the advent of cheap [[microcomputer]]s it became possible to have a computer dedicated to controlling the spectrometer, collecting the data, doing the Fourier transform and presenting the spectrum. This provided the impetus for the development of FTIR spectrometers for the rock-salt region. The problems of manufacturing ultra-high precision optical and mechanical components had to be solved. A wide range of instruments is now available commercially. Although instrument design has become more sophisticated, the basic principles remain the same. Nowadays, the moving mirror of the interferometer moves at a constant velocity, and sampling of the interferogram is triggered by finding zero-crossings in the fringes of a secondary interferometer lit by a [[helium-neon laser]]. This confers high wavenumber accuracy on the resulting infrared spectrum and avoids wavenumber [[calibration]] errors.

==Near-infrared FTIR==
{{Main|Near-infrared spectroscopy}}
The near-infrared region spans the wavelength range between the rock-salt region and the start of the [[visible spectrum|visible]] region at about 750 nm. [[Overtones]] of fundamental vibrations can be observed in this region. It is used mainly in industrial applications such as [[process control]] and [[Chemical imaging]].

==Applications==
FTIR can be used in all applications where a dispersive spectrometer was used in the past (see [[#External links|external links]]). In addition, the multiplex and throughput advantages have opened up new areas of application. These include:
*GC-IR (gas chromatography-infrared spectrometry). A [[gas chromatograph]] can be used to separate the components of a mixture. The fractions containing single components are directed into an FTIR spectrometer, to provide the infrared spectrum of the sample. This technique is complementary to GC-MS ([[gas chromatography-mass spectrometry]]). The GC-IR method is particularly useful for identifying [[isomer]]s, which by their nature have identical masses. The key to the successful use of GC-IR is that the interferogram can be captured in a very short time, typically less than 1 second. FTIR has also been applied to the analysis of [[liquid chromatography]] fractions.<ref name=white/>
*TG-IR (thermogravimetry-infrared spectrometry) IR spectra of the gases evolved during thermal decomposition are obtained as a function of temperature.<ref>{{cite book|last1=Nishikida|first1=K.|last2=Nishio|first2=E.|last3=Hannah|first3=R.W.|title=Selected applications of FT-IR techniques|year=1995|url=http://books.google.com/?id=Bjj7wSEP2lsC&pg=PA240|page=240|publisher=Gordon and Breach|isbn=2884490736}}</ref>
*Micro-samples. Tiny samples, such as in forensic analysis, can be examined with the aid of an infrared [[microscope]] in the sample chamber. An image of the surface can be obtained by scanning.<ref>{{cite journal|last=Beauchaine|first=J.P.|coauthors=Peterman, J.W.; Rosenthal,R.J.|year=1988|title=Applications of FT-IR/microscopy in forensic analysis|journal=Microchimica Acta|volume=94|issue=1-6|pages=133–138|doi=10.1007/BF01205855}}</ref> Another example is the use of FTIR to characterize artistic materials in old-master paintings.<ref>{{cite journal|last=Prati|first=S.|coauthors=Joseph, E.; Sciutto, G.; Mazzeo, R.|year=2010|title=New Advances in the Application of FTIR Microscopy and Spectroscopy for the Characterization of Artistic Materials|journal=Acc. Chem. Res.|pmid=20476733|volume=43|issue=6|pages=792–801|doi=10.1021/ar900274f}}</ref>
*Emission spectra. Instead of recording the spectrum of light transmitted through the sample, FTIR spectrometer can be used to acquire spectrum of light emitted by the sample. Such emission could be induced by various processes, and the most common ones are [[luminescence]] and [[Raman scattering]]. Little modification is required to an absorption FTIR spectrometer to record emission spectra and therefore many commercial FTIR spectrometers combine both absorption and emission/Raman modes.<ref>{{cite book|url=http://books.google.com/?id=QBoTvW_h1FQC&pg=PA263|page=263|title=Luminescence spectroscopy of minerals and materials|author=Michael Gaft, Renata Reisfeld, Gérard Panczer|publisher=Springer|year= 2005|isbn=3540219188}}</ref>
*Photocurrent spectra. This mode uses a standard, absorption FTIR spectrometer. The studied sample is placed instead of the FTIR detector, and its photocurrent, induced by the spectrometer's broadband source, is used to record the interferrogram, which is then converted into the photoconductivity spectrum of the sample.<ref>{{cite book|url=http://books.google.com/?id=SvVYBK6YAxAC&pg=PA189|page=189|title=Thin film solar cells: fabrication, characterization and applications|author=Jef Poortmans, Vladimir Arkhipov|publisher=John Wiley and Sons|year= 2006|isbn=0470091266}}</ref>

==References==
{{Reflist|2}}

==External links==
*[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC441765/pdf/jcinvest00449-0191.pdf Infracord spectrometer] photograph
*The Grubb-Parsons-NPL cube interferometer [http://books.google.co.uk/books?id=XTolQEkzSR0C&pg=PA93&lpg=PA93&dq=grubb+parsons+cube&source=bl&ots=l0Sm8klxHG&sig=SzKdjgTB8YPyitRgAAlwH2zIOlo&hl=en&ei=w6lRTM-BNYaD4QaC48ygAw&sa=X&oi=book_result&ct=result&resnum=9&ved=0CDgQ6AEwCA#v=onepage&q=grubb%20parsons%20cube&f=false Spectroscopy, part 2 by Dudley Williams, page 81]
*[http://las.perkinelmer.com/Catalog/TechLibDetails.htm?expand=Application&20Notes&ObjectId=FTIR+%26+FTNIR+Spectrometers&CategoryID=FTIR+%26+FTNIR+Spectrometers&type=CATEGORY FTIR application notes] from [[Perkin Elmer]]
*[http://www.varianinc.com/cgi-bin/nav?applications/ftir&cid=LNHOMKLPFQ FTIR application notes] from [[Varian, Inc.|Varian]]
*[http://www.selectscience.net/infrared-+-ftir/application-notes Infrared / FTIR Application Notes] Recent publications.
*[http://www.piketech.com/technical/application-pdfs/SemiconductorApplOverview.pdf Semiconductor applications] FTIR Sampling Techniques Overview.
*[http://infrared.als.lbl.gov/content/web-links/58-irwindows infrared materials] Properties of many salt crystals and useful links.

[[Category:Spectroscopy]]