FOURIER TRANSFORM INFRARED SPECTROSCOPY

Fourier Transform Infrared (FT-IR) Spectroscopy first developed by astronomers in the early 1950’s to study the infrared spectra or a distant star has now been developed into a very powerful technique for the detection of very weak signals from the environmental noise. It is a simple mathematical technique to resolve a complex wave into its frequency components. The conventional IR spectrometers are not of much use for the far IR region (20 – 400 cm-1) as the sources are weak and detectors insensitive. FTIR has made this energy limited region more accessible. It has made the middle infrared (400 – 4000 cm-1) also more useful.

FT – IR Absorption

Infrared spectroscopy is an important technique in organic chemistry. It is an easy way to identify the presence of certain functional groups in a molecule. Also, one can use the unique collection of absorption bands to identify of a pure compound or to detect the presence of specific.

Sample Handling Techniques

Recording of IR spectra of solid sample are more difficult because the particles reflect and scatters the incident radiation, and therefore, transmittance is always low. Three different techniques mull technique, pellet technique and solid films are employed commonly in recording the spectra.

Pellet Technique

The first step in this method is to grind the sample very finely with potassium bromide. The mixture is then pressed into transparent pellets with the help of suitable dies. This is then placed in the IR beam in a suitable holder. This method has the following advantages.

  • Absence of interfering band
  • Lower scattering losses
  • Higher resolution of spectra
  • Possibility of storage for future studies
  • Ease in examination
  • Better control of concentration and homogeneity of sample

Instrumentation

The main parts of the FT – IR spectrometer is shown in the figure 3. It consists of

  • Infrared Source
  • Detector
  • Amplifier
  • Recorder

A source provides radiation over the whole range of the IR spectrum. The monochromator disperses the light and selects a narrow wave number range. The energy transmitted is measured by a detector and converted into an electrical signal. This is then amplified and registered by a recorder.

Description of a FT – IR Spectrometer

A FT Infrared spectrometer consists of two parts

  • Optical System-An optical system which uses an interferometer.
  • Dedicated Computer-A dedicated computer which stores data performs computations on data and plots the spectra.

Explanation

    A schematic diagram of the essential components of a FT – IR spectrometer based on Michelson interferometer. It consists of two perpendicular mirrors; one of which is a stationary mirror and the other a movable mirror at a constant velocity. Between these two mirrors is a beam splitter set as 45˚ from the initial position of the movable mirror. A parallel beam of radiation from an infrared source is passed on to the mirrors through the beam splitter.

    The beam splitter reflects about half of the beam to the fixed mirror which reflects it back to the beam splitter. The returning beams are again split and mixed about half going back to source and half passing through the sample compartment.The composition of the beam splitter depends of the spectral region of interest. For example, in the mid infrared region (4000 – 400 cm-1), a beam splitter of germanium coated on KBr plate (substrate) is often used. Germanium reflects the radiation while KBr transmits most of the desirable radiation.

    The return beams from both the mirrors along the same path length as their incident path are recombined into a single beam at the beam splitter. The path length of one of the return beams is changed in order to create on over all phase difference to cause an interference pattern. The recombined radiation is then directed through the sample and focused on to the detector. The detector measures the amount of energy at discrete intervals mirror movement. The movable mirror can be moved in a range of say ±5 cm.

    The mirror velocities from 0.05 cm s-1 are used. Interferometer instruments needed detectors with response times short enough to detect and transmit rapid energy changes to the recorder. The detector used in conjunction with rapid scanning interferometers in the mid infrared region at room temperature is triglyine sulfate with KBr windows as pyroelectric bolometer. It has high response time. Other common detectors used such as thermocouples, bolometer and Golay detectors have short response time.

    The design of Michelson Interferometer is such as to make measurement in any infrared region by simply changing the beam splitter and detector. The simplicity of the FT infrared optical system is an added advantage. The infrared source in an FT spectrometer may be a water cooled globar (6000 – 50 cm-1) or a high pressure mercury lamp (700 – 10 cm-1).The entire interferometer is evacuated since water vapor strongly absorbs in the far infrared region.

Working of a FT – IR Spectrometer

    The interferogram contains information on intensity of each frequency in the spectrum. The sample absorption will show up as gaps in the frequency distribution. The interferograms are converted into the normal infrared by Fourier transformation. The measurement of the spectrum may be carried out as follows. The movable mirror is moved smoothly over a period of time say one second through a distance of 1 cm. The movement of the movable mirror should be known very precisely. This manipulation is carried out on a digital computer after digitizing the value of interferogram at regular intervals of mirror movement. This information is read directly into the memory of a computer of stored on a magnetic tape. The detector signal that is the interferogram, may be recorded every thousandth of a second during the mirror movement and each information stored in the computer.

    It is usual to refer the recording of the interferograms as ‘Scans’. Fourier transformation is performed on the stored data. The number of storage points chosen determines the resolution of the final spectrum. By increasing the storage capacity of the computer and the distance travelled by the mirror, the resolution may be increased. A spectrum recorded over the region 2200 to 200 cm-1, that is an interval of 2000 cm-1 would have maximum resolution of about 2.0 cm-1 for chosen 1000 locations since it would be split into 1000 points 2.0 cm-1 a part. Thus for low resolution spectrum over the mid infrared region as few as 1000 data points need be collected per scan. For high resolution spectra, this number should be increased by one or more orders of magnitude.

The spectrum obtained from transformed interferogram is a single beam spectrum of the sample. To obtain the transmittance spectrum of the sample, the interferogram with no sample in the beam is obtained and Fourier transformed to obtain the intensity spectrum of the reference. The sample is then inserted and another interferogram is obtained. Fourier transformation of it yields a second intensity spectrum. The transmittance spectrum is then obtained by radioing the single beam sample spectrum against the single beam reference spectrum.

Advantages

FT – IR techniques have made significant impact with regard to

  • Rapid Scanning
  • Signal to Noise Ratio
  • High Sensitivity
  • High Resolution

Applications

Applications of infrared spectroscopy are of varied types. The advent of FT – IR created renewed interest in the field of IR spectroscopy. It is one of the most widely used analytical tools available today. The rapidly increasing demand for routine analysis of a wide range of a wide range of compounds and the data handling capabilities have generated this interest.

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