Fourier Transform Infrared Spectrometry

Introduction
FTIR stands for Fourier Transform Infrared, the preferred
method infrared spectroscopy. In infrared spectroscopy IR radiation is passed through a sample.
Some of the infrared radiation is absorbed by the sample and some of it is passed through (transmitted). The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint no two unique molecular structures produce the same infrared spectrum. This makes infrared spectroscopy useful for several.
Types of analysis
FTIR provides information like
Ø It can identify unknown materials.
Ø It can determine the quality or consistency of a sample.
Ø It can determine the amount of components in a mixture.
Why Infrared Spectroscopy ?
Infrared spectroscopy has been a workhorse technique for materials analysis in the
laboratory for over seventy years. An infrared spectrum represents a fingerprint of a sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material. Because each different material is unique combination of atoms, no two compounds produce the exact same infrared spectrum. Therefore, infrared spectroscopy can result in a positive identification (qualitative analysis ) of every different king of material. In addition, the size of the peaks in the spectrum is a direct indication of the amount of material preset. With modern software algorithms, infrared is an excellent tool for quantitative analysis.
OlderTechnology
«The original infrared instruments were of the dispersive type. These instruments
separated the individual frequencies of energy emitted from the infrared source. This was accomplished by the use of a prism or grating. An infrared prism works exactly the same as a visible prism which separates visible light into its colors (frequencies). A grating is more modern dispersive element which better separates the frequencies of infrared energy. The detector measures the amount of energy at each frequency which has passed through the sample. This results in a spectrum which is a plot of intensity vs frequency.Fourier transform infrared spectroscopy is preferred over dispersive of filter methods of infrared spectral analysis for several reasons; It is a non destructive technique.
«It provides a precise measurement method which requires no external
calibration

Applications
FTIR can be used in all applications where a dispersive spectrometer was used in the past. 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 A gas chromatograph can be used to separate the components of a mixture. The fractions containing single components are divided into an FTIR spectrometer  to provide the infrared spectrum of the sample. This technique is complementary to GCMS (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.

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.

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.

«It can increase sensitivity one second scan can be co added together to ration out random noise
«It has greater optical thoughput
«It is mechanically simple with only one moving part
Why FTIR ?
Fourier Transform Infrared (FTIR) spectrometry was developed in order to overcome the limitation encountered with dispersive instruments. The main difficulty was the slow scanning process. A method for measuring all of the infrared frequencies simultaneously, rather than individually, was needed.Asolution was developed which employed a very simple optical device called an interferometer. The interferometer produces a unique type of signal which has all of the infrared frequencies “encoded” into it. The signal can be measured very quickly, usually on the order of one second or so. Thus, the time element per sample is reduced to a matter of a few seconds rather than several minutes.Most interferometer employ a beamsplitter which takes the incoming infrared beam and divides it into two optical beams. One beam reflects off of a flat mirror which is fixed in place. The other beam reflects off of a flat mirror which is on a mechanism which allows this mirror to move a very short distance away from the beamsplitter. The two beams reflect off of their respective mirrors and are recombined when they meet back at the beamsplitter. Because the path that one beam travels is a fixed length and the other is constantly changing as its mirror moves, the signal which exits the interferometer is the result of these two beams “interfering” with each other. The resulting signal is called an interferogram which has the unique property that every data point ( a function of the moving mirror position) which makes up the signal has information about every infrared frequency which comes from the source.This means that as the interferogram is measured, all frequencies are being measured simultaneously. Thus, the use of the
interferometer results in extremely fast measurements.
Because the analyst requires a frequency spectrum (a plot of the intensity at each individual frequency) in order to make an identification, the measured interferogram signal can not be interpreted directly. Amean of decoding the individual frequencies is required. This can be accomplished via a well known mathematical technique called the Fourier transformation. This transformation is performed by the computer which then presents the user with the desired spectral information for analysis.

The sample Analysis Process
The normal instrumental process is as follows:
1. The source: Infrared energy is emitted from a glowing black –body source. This beam passes through an aperture which controls the amount of energy presented to the sample.
2.The Interferometer: The beam enters the interferometer where the spectral encoding takes place. The resulting interferogram signal then exits the interferometer.
3.The sample: The beam enters the sample compartment where it is transmitted through or reflected off of the surface of the sample, depending on the type of analysis being accomplished. This is where specific frequencies of energy, which are
uniquely characteristic of the sample, are absorbed.
4.The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal.
5.The Computer: The measured signal is digitized and sent to the computer where the Fourier transformation takes place. The final infrared spectrum is then presented to the user for interpretation and any further manipulation.

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 wave number accuracy on the resulting infrared spectrum and avoids wave number calibration errors.

Near-infrared FTIR
The near-infrared region spans the wavelength range between the rock-salt region and the start of the visible region at about 750 nm. of fundamental vibrations can be observed in this region. It is used mainly in industrial applications such as process control andchemical imaging.