FTIR and Its Methods of Measurement SHIMADZU CORPORATION

Principle and Structure of FTIR

Structure of Dispersive Infrared Spectrophotometer Shimadzu IR-470

Structure of FTIR

Fourier Transform of Various Functions

Interference Phenomenon

Relationship between the Interferogram and the Spectrum

Multiplex Advantage

Aperture Advantage

Connes AdvantageWavenumber accuracy of spectrum is determined by Laser (He-Ne)Spectrum of high wavenumber accuracy is obtained

Methods of Measurement

Attenuated Total Reflection (ATR) Spectroscopy

Schematic representation of ATR(b)

n1

n2

Sample

Prism

Infrared beam

Sample

Prism

dp

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Wavelength dependency of dpNote ; the refractive index of 1.5, which is the refractive index of common organic compounds in the mid-infrared region, is used to the calculation as that of sample.

KRS-5 and ZnSe (n=2.4)

Ge (n=4.0)

Incident angle

Incident angle

Wavelength

(Wavenumber)

45

60

30

45

60

2.5m (4000 cm-1)

0.50m

0.28m

0.30m

0.17m

0.13m

3.3m (3000 cm-1)

0.66m

0.37m

0.40m

0.22m

0.17m

5.0m (2000 cm-1)

1.01m

0.56m

0.60m

0.33m

0.25m

6.7m (1500 cm-1)

1.35m

0.74m

0.81m

0.44m

0.34m

10m (1000 cm-1)

2.01m

1.11m

1.20m

0.66m

0.51m

25m (400 cm-1)

5.03m

2.78m

3.01m

1.66m

1.27m

ATR correctionATR Correction

Infrared Spectrum of Polyvinyl Alcohol dissolved in Water

Specular Reflection Spectroscopy

Specular Reflection from thin Film

_946920507.doc

Metal plate

Film

a

Infrared beam

0.120m

a

Specular Reflection from glossy flat Plastic

_946920523.doc

Infrared beam

b

a

Sample

SRM-8000DetectorInterferometerSample

Kramers-Kronig ConversionKramers-Kronig conversion

Diffuse Reflectance Spectroscopy

Reflection in Powder Sample

Kubelka-Munk ConversionKubelka-Munk Conversion

DRS-8000

Procedure for IR Spectral Measurement of Fraction

Dual Channel GPC Chromatograms of Polyvinyl Chloride UV(254nm)RIABC

Infrared Spectra of Peak B and CSebacic Acid Dialkyl Ester Phthalic Acid Dialkyl Ester(B)(C)

Reflection Absorption Spectroscopy (RAS)

Schematic Diagram of Reflection Absorption Spectroscopy

Principle of Reflection Absorption Spectroscopy

RAS-8000DetectorInterferometerSampleMaskShield Plate

Infrared Spectra of LB film of Lead Stearate

Infrared Microspectroscopy

Optical Diagram

Direct ratio photometric system is adopted, by which reference signals and sample signals are measured independently. And then the ratio between the 2 signals are calculated. Light beams which are emitted from a light source pass through the target sample or the reference sample, and the transmitted light beams are subjected to chopping by a chopper with 2 different frequencies. The reference light and sample light are combined by a beam combiner, and the combined light is introduced into a monochromator. The light is the separated into monochromatic light by diffraction gratings and filters. The light from monochromator converted by detector into electrical signals, and then separated into reference signals and sample signals by a band-pass filter. The transmittance or absorbance ( = infrared spectrum) of the sample is calculated with both signals.

The Fourier transform infrared spectrophotometer (FTIR) does not use a dispersing element. Instead, an interferometer is used in the FTIR to generate an interference light. In the FTIR, spectrum signal is not directly obtained from the detector, but an interference waveform called an interferogram is obtained. This waveform is obtained with the reciprocating back-and-forth movement of the movable mirror of the interferometer. In order to obtain the spectrum from an interferogram, Fourier transform, a mathematical calculation, is used. The calculation is performed by computer at high speed. If a given waveform is represented by a function f(x), the Fourier transformed function F(u) of f(x) is determined as follows. F(u) = f(x) e-i2ux dx Where, f(x) is the original function, and F(u) is the image function or an expression of the frequency domain.

- A beam of radiation emitted from a light source is split by the beam splitter into 2 different paths. The split beams are reflected on the fixed or movable mirrors, and getting together again at the beam splitter. Here, the 2 waves of beams interfere with each other, generating variation in intensity with the movement of the mirror. Above figure explains the interference phenomenon for only a beam with wavelength 2.5m. In the case the beam (A) reflected by the fixed mirror is in phase with the beam reflected by the movable mirror, both beams intensify each other, presenting a greater intensity of light as is shown by (B). If the shift of the movable mirror is 1/4 of a wavelength (0.625m), I.e. x (optical path difference) = 1/2 of wavelength, offsetting their intensity. On the other hand, at the position of x (optical path difference) = 0, the beam of 5m which is shown is as strong as the 2.5m beam, becoming weakest at the position of x = 21.25 = 2.5m. This change of intensity against the change of optical difference differs from that of the 2.5m beam. Since the actual interferogram represents the simultaneous interference of beams of all wavelength, it presents a waveform which shows the strongest intensity at x = 0, with symmetrical gradual attenuation on both side of 0. For example, lets consider about lights of 3 wavenumbers (or wavelengths), 1,2 and 3 in the incident white light getting into the interferometer. As the mirror in the interferometer moves, these beams make waveform of different cycles. The interferogram is made up of the 3 beams add together, representing a symmetric waveform in which the beam intensity is strong at the position where the path difference is 0 (x = 0). Actually, the signal at the detector in the FTIR is an interferogram in which the interference waveforms of all wavenumbers such as1,2 and 3 are added. The wave heights A(1),A(2) and A(3) of the interference signals of the beams1,2 and 3 correspond to the intensities of the respective wavenumbers. A(1),A(2) and A(3) can be calculated by the Fourier transform of interferogram. In general, any waveform is composed of sinusoidal function of different cycles. Therefore, the intensity of each sinusoidal function can be determined by breaking down the waveform into respective sinusoidal functions. This method is called Fourier transform. The intensities of light beams of respective wavenumbers determined by the Fourier transform are plotted continuously, then the spectrum can be obtained as shown in above figure. If the intensity of the beam of the wavenumber2 attenuates as sample absorption and becomes A(2), the intensity of the beam at2 becomes smaller as shown in above figure. The spectral element (m) of a spectrum measured from wavenumber1 to 2 with the resolution of can be calculated by (1-2 )/. A dispersive IR examines only one spectral element of spectrum at a time. If T is the time required to examine spectral elements between1 to 2, the time spent for observing a spectral element will be T/m. The multiplex advantage of FTIR is the concurrent observation of all the infrared spectral elements (m) over the entire wavenumber range (1 to 2) . Therefore, the time spent for observing a spectral element in FTIR is T, not T/m. As the signal-to-noise (S/N) ratio improves proportionally to the square root of the observation time of each spectral element, S/N ratio of FTIR is superior to that of dispersive IR by a factor of m1/2.

The aperture advantage is a consequent of larger optical throughput of the interferometer in FTIR relative to that of the dispersive IR spectrometer. The optical throughput, ED, for a dispersive IR spectrometer is given in Eq.(a), ED = AB (a)where A is the area of entrance slit and B is the incident solid angle of the beam. In the case of a FTIR, A is defined as the area of aperture size. Since the FTIR does not use the slit used in the dispersive IR spectrometer, the optical throughput for the FTIR is about 2 order than that of the dispersive IR spectrometer. Another difference between FTIR and the dispersive IR spectrophotometer is the use of the He-Ne laser in the FTIR. This offers better wavenumber accuracy for the FTIR spectrum than that of dispersive IR. The ATR technique is used to measure an infrared spectrum of sample which is in contact with an optically denser and transparent medium. As shown above figure, the infrared beam passes through the optically denser crystal (prism) made of KRS-5, ZnSe or Ge and is reflected at the interface between the sample and the prism. If the sample is infrared active, the propagating beam becomes attenuated and gives rise to attenuated total reflection. The penetration depth of a infrared beam at a given wavelength is given by Eq. (b),where is the wavelength of a infrared ray, is the incident angle, and n1 and n2 are the refractive index of the prism and the sample respectively.

Above table shows the wavelength dependency of penetration depth for KRS-5, ZnSe and Ge prism. The penetration depth of a infrared beam at 45- incident angle for KRS-5 and ZnSe which is often used as the incident angle is about 1/5 of the wavelength and that of Ge is less than 1/10 of the wavelength. The higher incident angle and refractive index of a prism leads to the shallower penetration depth. Therefore, it is used to study the thin coating on a sample surface.

The infrared rays of longer wavelength penetrate deeper into the sample than that of the shorter wavelength. Accordingly, the peak intensity of an infrared spectrum obtained by an ATR attachment is strongly dependent on the wavelength. The variation of peak intensity due to the mentioned wavelength dependency can be easily corrected by the ATR correction through a computerized data processing. Above figure shows the infrared spectrum before and after ATR correction. After the ATR correction, the intensity of peaks are enhanced in the higher wavenumber region and weakened in the lower wavenumber region conversely. The infrared spectrum after ATR correction closely resembles to the transmission spectrum in terms of its peak intensities. Infrared Spectrum of Polyvinylalcohol Dissolved in Water (A) and (B) are infrared spectra of water and polyvinylalcohol dissolved in water measured with a horizontal ATR attachment. (C) is the subtraction, (B)-(A). The peaks of polyvinylalcohol hidden in the absorption of water can be clearly observed by obtaining the spectral subtraction.

This technique detects the infrared beam reflected from a flat sample at the same angle as the incident angle, which is useful in the measurement of an infrared spectrum of a thin coating (more than 1 m) on a metal plate, and a glossy flat plastic. The figure shows the specular reflection of an infrared beam obtained by a thin film on a metal plate. In this case, both the infrared beam reflected on the surface of film (a) and that which transmits the film and is reflected by the metal (b) are detected. However, by means of the relation of a