FTIR

• フーリエ変換赤外分光法

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Fourier transform infrared spectroscopy

同

フーリエ変換赤外分光光度計

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/01/02 13:22:39」(JST)

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赤外分光法（せきがいぶんこうほう、infrared spectroscopy、 略称IR）とは、測定対象の物質に赤外線を照射し、透過（あるいは反射）光を分光することでスペクトルを得て、対象物の特性を知る方法のことをいう。対象物の分子構造や状態を知るために使用される。

塩化水素の赤外吸収スペクトル。振動準位・回転準位の遷移に由来する構造が明確に現れている。

概要

物質は、赤外線を照射すると、それを構成している分子が光のエネルギーを吸収し、量子化された振動あるいは回転の状態が変化する。したがって、ある物質を透過（あるいはある物質で反射）させた赤外線は、照射した赤外線よりも、分子の運動の状態遷移に使われたエネルギー分だけ弱いものとなっている。この差を検出することで、分子に吸収されたエネルギー、言い換えれば対象分子の振動・回転の励起に必要なエネルギーが求まる。

分子の振動・回転の励起に必要なエネルギーは、分子の化学構造によって異なる。したがって、照射した赤外線の波数を横軸に、吸光度を縦軸にとることで得られる赤外吸収スペクトルは、分子に固有の形を示す。これにより、対象とする物質がどのような構造であるかを知ることができ、特に有機化合物の構造決定によく使われている。

また、同じ分子であっても、温度や周囲の状況（自由に動いているか、何かの表面に吸着しているか、など）によって、赤外スペクトルは微妙に変化する。これより、物質の表面構造などについても知ることができる。

赤外分光法は、他の分光法に比べて感度が高いため、気体や微量の試料を対象とすることの多い物理化学の研究においてもよく使用されている。特に小さな分子の振動・回転スペクトルは非常に細かい構造まで観測できるため、理論化学によって得られた結果に実験的な裏付けを与えるものとしても利用されている。

理論

赤外線の吸収は、分子振動に伴って双極子モーメントが変化する場合に生じる。一方、ラマン効果は分子の振動により分極率が変化する場合に観測される。

一酸化炭素 (CO) や塩化水素 (HCl) などの振動は、赤外分光法でもラマン分光法でも観測される。一方、水素分子 (H₂) や窒素分子 (N₂) などの等核二原子分子では、振動が起こっても双極子モーメントは変化しないため、赤外吸収は示さない（分極率は変化するため、ラマン散乱は観測される）。

振動遷移の理論についての詳細は振動準位に記述があります。

有機化学での利用

ポリスチレンフィルムの赤外線吸収スペクトル。3100–3000 cm⁻¹ の吸収帯はベンゼン環の C-H 伸縮振動、3000–2800 cm⁻¹ の吸収帯はメチレンの C-H 伸縮振動に帰属される。

赤外線吸収スペクトルは、比較的簡単な装置で測定できるため、古くから化学物質の同定に用いられてきた。

赤外線の吸収される波長は、分子の官能基（金属錯体の場合は配位子）にだいたい固有なので、測定対象分子に含まれる官能基が分かる。特に特性基としてヒドロキシ基 (O-H)、カルボニル基 (C=O) あるいは ニトロ基 (NO₂) などは特徴ある強い吸収を示すので、ニトロ化合物、ケトン、アルデヒド、カルボン酸、カルボン酸誘導体、アルコール、フェノール類の定性は容易である。

特に 1300～650 cm⁻¹ の領域（指紋領域）には細かい吸収が多数みられ、そのパターンは物質に固有のものとなる。したがって、この領域の吸収を既知試料やスペクトルデータベースと照合することで、その物質が何かを同定することが可能である。

吸収バンド

赤外分光法は構造を調べるために用いられる。それぞれの官能基は特徴的な吸収強度・吸収エネルギー（波数）を持っている。バンドのエネルギーは以下に示す相関表に要約されている。

装置構成

現在よく用いられている赤外分光装置は、フーリエ変換型赤外分光 (FT-IR) のものである。この装置は、主に光源、試料設置部、分光部、および検出器からなる。ここでは、その構成の概要を示す。なお、FT-IR以外に回折格子を用いた分散型赤外分光光度計（モノクロメーターの原理を用いた分光光度計）もある。

光源

主な光源としては、12500～3800cm^-1の領域はタングステン・ヨウ素ランプが、7800～240cm^-1の領域では高輝度セラミック光源が用いられる。

試料部

試料の調製法には、測定対象に応じて以下の方法が用いられる。

透過測定

• ヌジョール法 - 測定物質を赤外線を透過する溶媒に溶かし、岩塩板で挟む。溶媒は多くの場合、流動パラフィンが用いられる。

• 液膜法 - 測定物質が液体である場合に、測定物質を塩化ナトリウム (NaCl) や臭化カリウム (KBr) 等の、赤外線を透過する窓板で挟む。

• 錠剤法 - 臭化カリウム (KBr) の粉末に測定物質を均一に混ぜ、プレスして錠剤に成型する。

窓板を左右に挟んだ筒型等の、特殊なセルを用いた場合、気体（ガス）を透過測定する事も可能である。

反射測定

• 反射吸収 (Reflection absorption, RA) 法 - 金属表面上の薄膜や分子吸着種の赤外スペクトルを、高感度に測定できる方法。通常、赤外線のp-偏光を大きな入射角 (grazing angle) で入射し、その反射光を測定する。試料とバックグラウンドの反射率の比から吸光度を求めた反射吸光度 (reflection‐absorbance) を縦軸とした表示をする。

他分野で利用される際、非金属表面での反射測定も RA法と呼ばれることがあるが、厳密に言うと誤りである。非金属表面での反射測定は、外部反射法と呼ばれる。

• 外部反射法 - 非金属表面上の薄膜や分子吸着種の赤外スペクトルを測定する反射測定法。s偏光および p偏光のいずれも利用できる。入射角に応じて、吸収スペクトルの形状や強度、さらには符号まで変化する、複雑なスペクトルを与える。特に符号は分子吸着種の官能基の配向を反映しているため、利用価値が大きい。しかし、一般に非金属上での反射率は非常に低く、MCT検知器を用いても S/Nの良い測定は難しいことが多い。

• 減衰全反射 (attenuated total reflection, ATR) 法 - 内部反射法に分類される。試料を屈折率の大きい媒質結晶に密着させ、入射角を臨界角より大きくとり、試料とATR結晶間で全反射が起きるように設定する。全反射が生じるとき、界面で光は試料側に少しだけもぐりこんで反射されてくる（エバネッセント波）。試料に吸収のある領域では、吸収の強さに応じて反射光のエネルギーが減少する。この反射光を測定することによりスペクトルが得られる。

反射回数は1回の単反射のものから7-21回程度の多重反射型まで選べる。単反射の場合は、半筒型プリズムによる入射角を変えた測定も可能だが、多重反射型の場合は固定入射角の台形（または平行四辺形）プリズムを用いる。全反射条件を守るため、プリズムと試料の屈折率をあらかじめ調べ、臨界角より十分大きな入射角に設定する必要がある。

分光部

FT-IR の分光部は、分光素子（プリズムや回折格子）の代わりに、主としてマイケルソン干渉計が用いられる。この干渉計は一枚のハーフミラーと二枚の反射鏡（固定鏡と移動鏡）より構成される。

干渉計に入射した光は、ハーフミラーによって反射光と透過光に分割される。一方の光は固定鏡で反射され、もう一方は移動鏡で反射されて、再びハーフミラーに戻り、合成されて検出器へと進行する。

ハーフミラーから2枚の反射鏡までの光路が等しい場合は、光の干渉は生じないため、強度は最大となる。一方、移動鏡が動いて光路に差が生じた場合、2つの反射光間で干渉が生じ、光の強度に変化が生じる。

簡単のため、光が単色（波長λ）とすると、光路差が波長の整数倍 (nλ) のとき干渉によって強めあい、光の強度は極大となる。一方、光路差が nλ + λ/2 となるとき、光の強度はゼロとなる。移動鏡を連続的に動かすと、検出器で観測される光の強度はサインカーブを描く。

実際の測定では光は連続光であるから、観測される光の強度は各波長の描くサインカーブの重ね合わせとなり、干渉パターン（インターフェログラム）は波束の形を示す。この干渉パターンを高速フーリエ変換 (FFT) することによって、各周波数成分を横軸としたスペクトルに変換できる。

この分光計では、FFT演算に堪えうる正確な干渉図形の測定を必要とするため、移動鏡の位置を精密に測定することが不可欠である。このため、He-Neレーザーを分光器内部に備え、赤外光のみならずレーザー光線の干渉図形も同時に測定されるように設計されている。この結果、波数ドメインのスペクトルに変換した後も、正確な横軸が再現性良く得られ、積算測定を理想的に行うことができる。すなわち、S/N比を大きく改善することができる。

検出部

FT-IR の検出器には、主として半導体型のテルル化カドミウム水銀（HgCdTe、通称MCT）検出器または焦電型の硫酸トリグリシン（Triglycine sulfate、通称 TGS（あるいは水素イオンを重水素化した DTGS））検出器が用いられる。MCT は暗い赤外光(5000～650c m⁻¹)を高感度に検出するのに適しており、液体窒素温度で動作する。一方、TGS は室温で動作し、明るい赤外光を大きなダイナミックレンジで測定(7800～350 cm⁻¹) するのに適している。このため、透過率や反射率の高い試料を測定するには TGS が向いており、逆に外部反射法や多重反射型ATR の測定には MCT が適していることが多い。

また近赤外光にはInGaAsやPbSeなどの検出器が対応しており、12500～3800 cm⁻¹ を検出する。

関連項目

• 熱赤外分光法
• 近赤外線分光法

脚注

参考文献

• 厚生労働省 「第15改正日本薬局方」、『厚生労働省告示』第285号、平成18年。http://jpdb.nihs.go.jp/jp15/。2009年7月5日閲覧。 
• 『表面赤外およびラマン分光』 末高洽編、アイピーシー、1990年。
• 『FT-IRの基礎と実際』 田隅三生編、東京化学同人、東京都文京区千石3-36-7、1994年、第二版。
• 「物質の構造I」『実験化学講座 9』 日本化学会編、丸善、東京都港区海岸一丁目9番18号 国際浜松町ビル、2005年。

wiki en

[Wiki en表示]

"FTIR" redirects here. FTIR may also refer to Frustrated total internal reflection.

Fourier transform infrared spectroscopy (FTIR) is a technique which is used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high spectral resolution data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer which measures intensity over a narrow range of wavelengths at a time.

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

An example of an FTIR spectrometer with an attenuated total reflectance (ATR) attachment.

Conceptual introduction

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 interferometer to the detector.

The goal of any absorption spectroscopy (FTIR, 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 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 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 Michelson interferometer—a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference. Different wavelengths are modulated at different rates, so that at each moment, the beam coming out of the interferometer has a different spectrum.

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 spectrum was the Perkin-Elmer Infracord produced in 1957. 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 dispersing element was a 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 gratings 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 an 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-computers, such as the PDP-8 which became available in 1965. Digilab pioneered the world's first commercial FTIR spectrometer (Model FTS-14) in 1969 (Digilab FTIRs are now a part of Agilent technologies's molecular product line after it acquired spectroscopy business from Varian).

Michelson interferometer

Main article: Michelson interferometer

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 refracted 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 some fraction 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 or optical path difference, OPD. 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.

Commercial spectrometers use Michelson interferometers with a variety of scanning mechanisms to generate the path difference. Common to all these arrangements is the need to ensure that the two beams recombine exactly as the system scans. The simplest systems have a plane mirror that moves linearly to vary the path of one beam. In this arrangement the moving mirror must not tilt or wobble as this would affect how the beams overlap as they recombine. Some systems incorporate a compensating mechanism that automatically adjusts the orientation of one mirror to maintain the alignment. Arrangements that avoid this problem include using cube corner reflectors instead of plane mirrors as these have the property of returning any incident beam in a parallel direction regardless of orientation. Systems where the path difference is generated by a rotary movement have proved very successful. One common system incorporates a pair of parallel mirrors in one beam that can be rotated to vary the path without displacing the returning beam. Another is the double pendulum design where the path in one arm of the interferometer increases as the path in the other decreases.

Interferometer schematics where the path difference is generated by a rotary motion.

A quite different approach involves moving a wedge of an IR-transparent material such as KBr into one of the beams. Increasing the thickness of KBr in the beam increases the optical path because the refractive index is higher than that of air. One limitation of this approach is that the variation of refractive index over the wavelength range limits the accuracy of the wavelength calibration

Measuring and processing the interferogram

The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required. In practice the scan can be on either side of zero resulting in a double-sided interferogram. Mechanical design limitations may mean that for the highest resolution the scan runs to the maximum OPD on one side of zero only.

The interferogram is converted to a spectrum by Fourier transformation. This requires it to be stored in digital form as a series of values at equal intervals of the path difference between the two beams. To measure the path difference a laser beam is sent through the interferometer, generating a sinusoidal signal where the separation between successive maxima is equal to the wavelength. This can trigger an analog-to digital converter to measure the IR signal each time the laser signal passes through zero. Alternatively the laser and IR signals can be measured synchronously at smaller intervals with the IR signal at points corresponding to the laser signal zero crossing being determined by interpolation. This approach allows the use of analog-to-digital converters that are more accurate and precise than converters that can be triggered, resulting in lower noise.

Values of the interferogram at times corresponding to zero crossings of the laser signal are found by interpolation.

The result of Fourier transformation is a spectrum of the signal at a series of discrete wavelengths. The range of wavelengths that can be used in the calculation is limited by the separation of the data points in the interferogram. The shortest wavelength that can be recognized is twice the separation between these data points. For example with one point per wavelength of a helium-neon reference laser at 0.633μm (15800cm-1) the shortest wavelength would be 1.266μm (7900cm-1). Because of aliasing any energy at shorter wavelengths would be interpreted as coming from longer wavelengths and so has to be minimized optically or electronically. The spectral resolution, i.e. the separation between wavelengths that can be distinguished, is determined by the maximum OPD. The wavelengths used in calculating the FT are such that an exact number of wavelengths fit into the length of the interferogram from zero to the maximum OPD as this makes their contributions orthogonal. This results in a spectrum with points separated by equal frequency intervals.

For a maximum path difference d adjacent wavelengths λ₁ and λ₂ will have n and (n+1) cycles respectively in the interferogram. The corresponding frequencies are ν₁ and ν₂

                   d = nλ1 and d = (n+1)λ2
                   λ1 = d/n and λ2 =d/(n+1)
                   ν1 = 1/λ1 and ν2 = 1/λ2 
                   ν1 = n/d and ν2 =(n+1)/d
                   ν2 - ν1 = 1/d

The separation is the inverse of the maximum OPD . For example a maximum OPD of 2cm results in a separation of 0.5cm-1. This is the spectral resolution in the sense that the value at one point is independent of the values at adjacent points. Most instruments can be operated at different resolutions by choosing different OPD’s. The best resolution of instruments for routine analyses is typically around 0.5cm-1 while spectrometers have been built with resolutions as high as 0.001cm-1, corresponding to a maximum OPD of 10m. The point the interferogram corresponding to zero path difference has to be identified, commonly by assuming it is where the maximum signal occurs. The centerburst is not always symmetrical in real world spectrometers so a phase correction may have to be calculated. The interferogram signal decays as the path difference increases, the rate of decay being inversely related to the width of features in the spectrum. If the OPD is not large enough to allow the interferogram signal to decay to a negligible level there will be unwanted oscillations or sidelobes associated with the features in the resulting spectrum. To reduce these sidelobes the interferogram is usually multiplied by a function that approaches zero at the maximum OPD. This so-called apodization reduces the amplitude of any sidelobes and also the noise level at the expense some reduction in resolution.

For rapid calculation the number of points in the interferogram has to equal a power of two. A string of zeroes may be added to the measured interferogram to achieve this. More zeroes may be added in a process called zero filling to improve the appearance of the final spectrum although there is no improvement in resolution. Alternatively interpolation after the FT gives a similar result.

Advantages of Fourier Transform spectroscopy

There are two principal advantages for an FT spectrometer compared to a scanning (dispersive) spectrometer.

1. 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. For a spectrum with m resolution elements this increase is equal to the square root of m. Alternatively it allows a shorter scan-time for a given resolution. In practice multiple scans are often averaged, increasing the signal-to-noise ratio by the square root of the number of scans.
2. 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. Although no slits are needed FTIR spectrometers do require an aperture to restrict the convergence of the collimated beam in the interferometer. This is because convergent rays are modulated at different frequencies as the path difference is varied. Such an aperture is called a Jacquinot stop. For a given resolution and wavelength this circular aperture allows more light through than a slit, resulting in a higher signal-to-noise ratio.

Other minor advantages include less sensitivity to stray light because it would have little effect on the interferogram, while it would directly impinge on the detector in a dispersive instrument. Another one is the "Connes' advantage" (better wavelength accuracy), because every scan can be calibrated with a helium-neon laser which has a stable and accurately known wavelength. However, 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.

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-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 vibration-rotation spectrum of Venusian CO₂ at 0.1 cm⁻¹ resolution. Michelson himself attempted to resolve the hydrogen H_α emission band in the spectrum of a hydrogen atom into its two components by using his interferometer. ^p25 A spectrometer with 0.001 cm⁻¹ resolution is now available commercially. The throughput advantage is important for high-resolution FTIR as the monochromator in a dispersive instrument with the same resolution would have very narrow entrance and exit slits.

Theory

FTIR is a method of measuring an infrared absorption spectrum. For a discussion of why people measure infrared absorption spectra, i.e. why and how substances absorb infrared light, see the article: Infrared spectroscopy.

Components

IR sources

FTIR spectrometers are mostly used for measurements in the mid and near IR regions. For the mid-IR region, 2-25µm (5000-400cm-1), the most common source is a silicon carbide element heated to about 1200K. The output is similar to a blackbody. Shorter wavelengths of the near-IR, 1-2.5µm (10000 – 4000cm-1), require a higher temperature source, typically a tungsten-halogen lamp. The long wavelength output of these is limited to about 5µm (2000cm-1) by the absorption of the quartz envelope. For the far-IR, especially at wavelengths beyond 50µm (200cm-1) a mercury discharge lamp gives higher output than a thermal source.

Detectors

Mid-IR spectrometers commonly use pyroelectric detectors that respond to changes in temperature as the intensity of IR radiation falling on them varies. The sensitive elements in these detectors are either deuterated triglycine sulfate (DTGS) or lithium tantalate (LiTaO3). These detectors operate at ambient temperatures and provide adequate sensitivity for most routine applications. To achieve the best sensitivity the time for a scan is typically a few seconds. Cooled photoelectric detectors are employed for situations requiring higher sensitivity or faster response. Liquid nitrogen cooled mercury cadmium telluride (MCT) detectors are the most widely used in the mid-IR. With these detectors an interferogram can be measured in as little as 10 milliseconds. Uncooled indium gallium arsenide photodiodes or DTGS are the usual choices in near-IR systems. Very sensitive liquid-helium-cooled silicon or germanium bolometers are used in the far-IR where both sources and beamsplitters are inefficient.

Beam splitter

Simple interferometer with a beam-splitter and compensator plate

An ideal beam-splitter transmits and reflects 50% of the incident radiation. However, as any material has a limited range of optical transmittance, several beam-splitters may be used interchangeably to cover a wide spectral range. For the mid-IR region the beamsplitter is usually made of KBr with a germanium-based coating that makes it semi-reflective. KBr absorbs strongly at wavelengths beyond 25μm (400cm^-1) so CsI is sometimes used to extend the range to about 50μm (200cm^-1). ZnSe is an alternative where moisture vapor can be a problem but is limited to about 20μm (500cm^-1). CaF2 is the usual material for the near-IR, being both harder and less sensitive to moisture than KBr but cannot be used beyond about 8μm (1200cm^-1). In a simple Michelson interferometer one beam passes twice through the beamsplitter but the other passes through only once. To correct for this an additional compensator plate of equal thickness is incorporated. Far-IR beamsplitters are mostly based on polymer films and cover a limited wavelength 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 NPL 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 microcomputers 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 are 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. In modern FTIR systems the constant mirror velocity is not strictly required, as long as the laser fringes and the original interferogram are recorded simultaneously with higher sampling rate and then re-interpolated on a constant grid, as pioneered by James W. Brault. This confers very high wavenumber accuracy on the resulting infrared spectrum and avoids wavenumber calibration errors.

Near-infrared FTIR

Main article: Near-infrared spectroscopy

The near-infrared region spans the wavelength range between the rock-salt region and the start of the 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). 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 isomers, 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.
• TG-IR (thermogravimetry-infrared spectrometry) IR spectra of the gases evolved during thermal decomposition are obtained as a function of temperature.
• 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. Another example is the use of FTIR to characterize artistic materials in old-master paintings.

References

External links

• Infracord spectrometer photograph
• The Grubb-Parsons-NPL cube interferometer Spectroscopy, part 2 by Dudley Williams, page 81
• infrared materials Properties of many salt crystals and useful links.

English Journal

• Fluorescence-based CdTe nanosensor for sensitive detection of cytochrome C.

• Amin RM1, Elfeky SA2, Verwanger T3, Krammer B3.
• Biosensors & bioelectronics.Biosens Bioelectron.2017 Dec 15;98:415-420. doi: 10.1016/j.bios.2017.07.020. Epub 2017 Jul 8.
• PMID 28711028

• Composition of dissolved organic matter (DOM) from periodically submerged soils in the Three Gorges Reservoir areas as determined by elemental and optical analysis, infrared spectroscopy, pyrolysis-GC-MS and thermally assisted hydrolysis and methylation.

• Jiang T1, Kaal J2, Liang J3, Zhang Y4, Wei S3, Wang D3, Green NW5.
• The Science of the total environment.Sci Total Environ.2017 Dec 15;603-604:461-471. doi: 10.1016/j.scitotenv.2017.06.114. Epub 2017 Jun 23.
• PMID 28641186

• Application of FT-IR spectroscopy on breast cancer serum analysis.

• Elmi F1, Movaghar AF2, Elmi MM3, Alinezhad H4, Nikbakhsh N5.
• Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy.Spectrochim Acta A Mol Biomol Spectrosc.2017 Dec 5;187:87-91. doi: 10.1016/j.saa.2017.06.021. Epub 2017 Jun 20.
• PMID 28666157

Japanese Journal

• Analysis of Combustion Gas Properties According to Combustion of Wood Materials Based in Korea

• 九州大学大学院農学研究院紀要 63(2), 191-199, 2018-09-01
• NAID 120006504333

• Contribution of Catalytic Performance of CeO_2 in Nonthermal Plasma Chemical Reaction

• Evergreen 5(3), 34-37, 2018-09
• NAID 120006528244

• FTIRによる医薬品の確認試験とFTIR/EDXによる異物分析

• Pharm stage 18(5), 51-55, 2018-08
• NAID 40021659356

Related Links

• フーリエ変換赤外分光光度計（FT-IR） : 株式会社島津製作所

FT-IRとは，フーリエ変換赤外分光光度計（ Fourier Transform Infrared Spectroscopy，FTIR）のことで，主に有機化合物の構造推定（定性）を行う分析装置です。 赤外線を分子に照射すると，分子を構成している原子間の振動エネルギー ...

• FTIRの基礎(2) FTIRの原理 - 日本分光株式会社

FTIRのメリット 分散型に比べたFTIRのメリットとして、以下の4点が挙げられます。 ①多波長同時検出 FTIRは移動鏡を動かすだけでIRスペクトルが測定できます。 分散型のように、回折格子を動かして波長をスキャンする必要が無く ...

Related Pictures

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★リンクテーブル★

リンク元	「フーリエ変換赤外分光法」「Fourier transform infrared spectroscopy」

「フーリエ変換赤外分光法」

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英

Fourier transform infrared spectroscopy、FTIR

関

Fourier変換赤外分光法

「Fourier transform infrared spectroscopy」

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• フーリエ変換赤外分光法、Fourier変換赤外分光法

関

FTIR