27 Optical Characterizations Part 1 : Fourier Transform Infrared Spectroscopy (FTIR)

Introduction

FTIR stands for Fourier transform infrared, the preferred method of infrared spectroscopy. When IR radiation is passed through a sample, some radiation is absorbed by the sample and some passes through (is transmitted). The resulting signal at the detector is a spectrum representing a molecular ‘fingerprint’ of the sample. The usefulness of infrared spectroscopy arises because different chemical structures (molecules) produce different spectral fingerprints.

FTIR spectroscopy (Fourier Transform Infrared Spectroscopy) is a technique that uses infrared light to observe properties of a solid, liquid, or gas. It is used in many different applications to measure the absorption, emission, and photo-conductivity of matter by shining a narrow beam of infrared light at the matter in various wavelengths and detecting how the matter responds to each wavelength. Once the data has been obtained, it is converted into digital information using a mathematical algorithm known as the “Fourier transform.”

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 some 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. Afterward, a computer takes all this data and works backward to infer what the absorption is at each wavelength.

How Does an FT-IR Spectrometer Work?

The Michelson Interferometer

An FT-IR is typically based on The Michelson Interferometer Experimental Setup; an example is shown in Figure 1. The interferometer consists of a beam splitter, a fixed mirror, and a mirror that translates back and forth, very precisely. The beam splitter is made of a special material that transmits half of the radiation striking it and reflects the other half. Radiation from the source strikes the beam splitter and separates into two beams. One beam is transmitted through the beam splitter to the fixed mirror and the second is reflected off the beam splitter to the moving mirror. The fixed and moving mirrors reflect the radiation back to the beamsplitter. Again, half of this reflected radiation is transmitted and half is reflected at the beam splitter, resulting in one beam passing to the detector and the second back to the source.

What are OPD and ZPD?

Optical Path Difference (OPD) is the optical path difference between the beams travelling through the two arms of an interferometer. OPD is equal to the product of the physical distance travelled by the moving mirror (multiplied by 2, 4, or other multiplier which is a function of the number of reflecting elements used) and n, the index of refraction of the medium filling the interferometer arms (air, Nitrogen for purged systems, etc.). The raw FT-IR data consists of a number of (signal, OPD) pairs of values.

FT-IR has a natural reference point when the moving and fixed mirrors are the same distance from the beam splitter. This condition is called zero path difference or ZPD. The moving mirror displacement, Δ, is measured from the ZPD. In Figure 2 the beam reflected from the moving mirror travels 2Δ further than the beam reflected from the fixed mirror. The relationship between optical path difference, and mirror displacement, Δ, is: OPD = 2Δn

Figure 2: Schematic representation of waves and their phases, input, output, and the two arms of the interferometer as the scan goes from zero path difference condition to OPD=λ . (a) OPD=0 case. (b) λ/4 OPD case. (c) λ/2 OPD case. (d) 3λ/4 OPD case. (e) 1λ OPD case.

The Interferogram

Interferogram is the name of the signal format acquired by an FT-IR spectrometer. It is usually significantly more complex looking than a single sinusoid, which would be expected if only a single wavelength of light was present. Figure 3 shows the beam path of a two wavelength source; Figure 4 is the interferogram of a broadband light source. The centerburst, the big spike in the center of Figure 4 is a telltale signature of a broadband source. Its origin lies in the fact that all wavelengths are in-phase at the ZPD. Therefore, their contributions are all at maximum and a very strong signal is produced by the system’s detector.

As the optical path difference, OPD, grows, different wavelengths produce peak readings at different positions and, for a broadband signal, they never again reach their peaks at the same time. Thus, as you move away from centerburst, the interferogram becomes a complex looking oscillatory signal with decreasing amplitude.

The X-axis of the interferogram represents the optical path difference. Each individual spectral component contributes to this signal a single sinusoid with a frequency inversely proportional to its wavelength. This leads us to the definition of the unit of spectral measurement. The wavenumber (cm-1), denoted as n. A wavenumber represents the number of full waves of a particular wavelength per cm of length (typically in vacuum; index of refraction n=1). The advantage of defining the spectrum in wavenumbers is that they are directly related to energy levels. A spectral feature at 4,000 cm-1 spectral location represents a transition between two molecular levels separated by twice the energy of a transition with spectral signature at 2,000 cm^-1.

FTIR sampling introduction

There are four major sampling techniques in FTIR. Each technique has strengths and weaknesses which motivate their use for specific samples :

1.      Transmission : In transmittance the infrared radiation is passed through a sample and the transmitted radiation is measured. The spectra obtained will be representative of the whole of the volume sampled and ‘localised’ (eg. surface) properties can quite easily be lost in the ‘bulk’ properties depending on the size and nature of the sample. It is only useful for thin (<10 µm) samples or when looking at weak bands, such as overtones, in thicker samples. Often sample preparation, such as the manufacture of KBr discs or Nujol mull preparation, is necessary which can be time consuming and difficult to reproduce.

2.      Attenuated Total Reflection (ATR) : An attenuated total reflection accessory operates by measuring the changes that occur in a totally internally reflected infrared beam when the beam comes into contact with a sample (indicated in Figure 5). An infrared beam is directed onto an optically dense crystal with a high refractive index at a certain angle. This internal reflectance creates an evanescent wave that extends beyond the surface of the crystal into the sample held in contact with the crystal. It can be easier to think of this evanescent wave as a bubble of infrared that sits on the surface of the crystal. This evanescent wave protrudes only a few microns (0.5 µ – 5 µ) beyond the crystal surface and into the sample. Consequently, there must be good contact between the sample and the crystal surface. In regions of the infrared spectrum where the sample absorbs energy, the evanescent wave will be attenuated or altered. The attenuated energy from each evanescent wave is passed back to the IR beam, which then exits the opposite end of the crystal and is passed to the detector in the IR spectrometer. The system then generates an infrared spectrum.

For the technique to be successful, the following two requirements must be met:

a)      The sample must be in direct contact with the ATR crystal, because the evanescent wave or bubble only extends beyond the crystal 0.5 µ – 5 µ.

b)     The refractive index of the crystal must be significantly greater than that of the sample or else internal reflectance will not occur – the light will be transmitted rather than internally reflected in the crystal. Typically, ATR crystals have refractive index values between 2.38 and 4.01 at 2000 cm-1. It is safe to assume that the majority of solids and liquids have much lower refractive indices.

Advantages : ATR is an IR sampling technique that provides excellent quality data in conjunction with the best possible reproducibility of any IR sampling technique. It has revolutionized IR solid and liquid sampling through:

•  Faster sampling

•  Improving sample-to-sample reproducibility

•  Minimizing user to user spectral variation Most importantly, the improved spectral acquisition and reproducibility associated with this technique leads to better quality database building for more precise material verification and identification. ATR is clearly an extremely robust and reliable technique for quantitative studies involving liquids.

3.      Specular Reflection : The incident radiation focused onto the sample may be directly reflected by the sample surface, giving rise to specular reflection, and it may also undergo multiple reflections at the sample, resulting in diffuse reflection. In external reflectance techniques, the radiation reflected from a surface is evaluated (Figure 6).

 

Fig. 6. Illustration of external reflection.

Specular reflection is defined as light reflected from a smooth surface (such as a mirror, any irregularities in the surface are small compared to λ) at a definite angle. The incident radiation focused onto the sample may be directly reflected by the sample surface, giving rise to specular reflection, and it may also undergo multiple reflections at the sample, resulting in diffuse reflection. In external reflectance techniques, the radiation reflected from a surface is evaluated.

Specular reflectance techniques basically involve a mirror-like reflection from the sample surface that occurs when the reflection angle equals the angle of incident radiation. It is used for samples that are reflective (smooth surface) or attached to a reflective backing. Thus, specular techniques provide a reflectance measurement for reflective materials, and a reflection–absorption (transflectance) measurement for the surface films deposited on, or pressed against reflective surfaces (Figure 7).

In absorption-reflection measurement, one fraction of the radiation is reflected on the upper interface and contributes towards the spectrum via specular reflection. Another part of the radiation penetrates the surface film and is reflected by the reflective surface, thus, the light passes through the surface layer twice-to and from the reflective surface, leading to increase the intensity of the reflectance spectrum as compared to the normal transmission. The effective path length depends on the angle of incidence, therefore, for thin films, a grazing angle of incidence as high as 80Ő- 85Ő from normal incidence should be used, and for thick films an angle close to normal incidence is applied. The most common applications of this technique are evaluation of surfaces such as: coating, thin films, contaminated metal surface.

4. Diffuse Reflectance : is produced by rough surfaces that tend to reflect light in all directions. There are far more occurrences of diffuse reflection than specular reflection in our everyday environment. In diffuse reflectance spectroscopy, the electromagnetic radiation reflected by roughened surfaces is collected and analyzed. When this technique is applied in (FT) IR region, it is termed as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It is also www.intechopen.com Reflectance IR Spectroscopy 241 known “Kubelka–Munk “reflection,because they developed a theory on the radiation transport in scattering media. Light incident onto a solid sample may be partly reflected regularly (specular reflection) by the sample surface, partly scattered diffusely, and partly penetrates into the sample. The latter part may be absorbed within the particles or be diffracted at grain boundaries, giving rise to diffusely scattered light in all directions. Diffuse reflectance spectroscopy associated with the reflected lights which are produced by diffuse scattering (Figure 8). Since regular reflection distorts the DRS spectra, thus, the regular reflection component should be eliminated in diffuse reflectance measurement. The DRIFTS accessory is designed to eliminate the specularly reflected radiation.

Application of diffuse reflectance spectroscopy : Diffuse reflectance technique is used for powders and solid samples having rough surface such as paper, cloth. In diffuse reflectance technique, particles size, homogeneity, and packing density of powdered samples play important role on the quality of spectrum. A sample with smaller particle size having narrow size distribution is preferred. Thus, in order to obtain a qualified spectrum, the sample should be ground into smaller size. In this method,the sample can be analyzed either directly in bulk form or as dispersions in IR transparent matrices such as KBr and KCl. Sometimes, a thin film of KBr powder placed on the sample surface to improve the quality of the spectrum. Dilution of analyte in a nonabsorbing matrix increases the proportion of diffuse reflectance in the reflected light. Typically, the solid sample is diluted homogeneously to 5 to 10% by weight in KBr. The spectra of diluted samples are similar to those obtained from pellets when plotted in units such as log 1/R (R is the reflectance) or Kubelka- Munk units.

REFERENCES

1. Griffiths, P.; de Hasseth, J. A. (18 May 2007). Fourier Transform Infrared Spectrometry (2nd ed.). Wiley-Blackwell.
2. “The Infracord double-beam spectrophotometer”. Clinical Science. 16 (2). 1957.
3. Peter R. Griffiths; James A. De Haseth (2007). Fourier Transform Infrared Spectrometry (2nd ed.). John Wiley & Sons.
4. Brault, James W. (1996). “New Approach to high-precision Fourier transform spectrometer design”. Applied Optics. 35 (16): 2891–2896.
5. Connes, J.; Connes, P. (1966). “Near-Infrared Planetary Spectra by Fourier Spectroscopy. I. Instruments and Results”. Journal of the Optical Society of America. 56 (7): 896–910.
6. Smith, D.R.; Morgan, R.L.; Loewenstein, E.V. (1968). “Comparison of the Radiance of Far-Infrared Sources”. J. Opt. Soc. Am. 58 (3): 433–434.
7. Griffiths, P.R.; Holmes, C (2002). Handbook of Vibrational Spectroscopy, Vol 1. Chichester: John Wiley and Sons.
8. Chamberain, J.; Gibbs, J.E.; Gebbie, H.E. (1969). “The determination of refractive index spectra by fourier spectrometry”. Infrared Physics. 9 (4): 189–209.