# Fourier Transform Infra-red spectrophotometers - Instrumental Methods of Analysis B. Pharma 7th Semester

Fourier Transform Infra-red spectrophotometers

Objectives

After this session students will be able to

Explain the construction and working of Fourier Transform Infra-red spectrophotometers

Outline the advantages of FTIR over dispersive type of instruments

Fourier Transform IR

v  Fourier Transform Infrared (FT-IR) spectrometry was developed in order to overcome the limitations encountered with dispersive instruments mainly the slow scanning process.

v  A solution 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.

FTIR systems

1. Mechanical operation

Encode (modulate) the spectral information using a Michelson Interferometer.

1. Mathematical operation

Computer processing of encoded information to produces the spectrum (Decoding).

Optical Diagram Michelson Interferometer

Interference is superimposing of waves

Relationship between light source spectrum and the interferogram (signal output from interferometer)

Note that the time domain signal, even after modulation, contains the same information as in the frequency domain.

Michelson Interferometer (Mechanical operation)

v  Most interferometers employ a beamsplitter which takes the incoming infrared beam and divides it into two optical beams. One beam reflects on a flat mirror which is fixed in place. The other beam reflects on a flat mirror which is on a mechanism which allows this mirror to move a very short distance (typically a few millimeters) away from the beamsplitter.

v  Because 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 which makes up the signal has information about every infrared frequency which comes from the source.

Fourier transform (Mathematical Operation)

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. A means of decodingthe 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.

FT-IR summary

Background Spectrum

Ã˜  A background spectrum (with no sample in the beam) must be collected for all IR measurements. This can be compared to the measurement with the sample in the beam to determine the “percent transmittance.” A single background measurement can be used for many sample measurements because this spectrum is characteristic of the instrument itself and its environment.

Ã˜  The strong background absorption from water and carbon dioxide in the atmosphere can be reduced by purging the optical bench with an inert gas or with dry carbon dioxide – scrubbed air .

Schematic illustration of FTIR system

1- Fellgett's (multiplex) Advantage

• Fast: All frequencies of the source reach the detector simultaneously (all of the energy is on the detector all of the time), instead of analyzing a sequence of small wavebands available from the monochromator in dispersing IR instruments.

Get data for the entire spectrum in one second or less.

Improve signal to noise ratio (S/N ratio):

Fast scans enable recording and averaging many scans.

Why is there a laser in FT-instruments?

Interferogram is not recoded continuously, but sampled at discrete intervals to give different data points. The closer the spacing between data points, the greater the wavenumber range of the spectrum.

Monochromatic visible He-Ne laser beam is passed along with the polychromatic IR light to provide a single wavelength interferogram that oscillates much more quickly than anything in the IR (shorter wavelength).

The laser is used as an internal clock to trigger data point’s acquisition events.

IR data points might be taken at every zero point of the laser interferogram.

FTIR instruments employ a He-Ne laser as an internal wavelength calibration standard. These instruments are self-calibrating and never need to be calibrated by the user.

The precise reproduction of wavenumber positions from one spectrum to the next will increase the resolution of the spectrum, and make it easy to differentiate between adjacent peaks too close to each other (high resolving power).

Frequency accuracy makes signal averaging highly precise and thus adds further improvement in S/N ratio.

3- Jacquinot (throughput) Advantage

Few optical elements and no slits (greater throughput of radiation)

The detector receives up to 50% of the energy of original light source (much larger than the dispersion spectrometer)

This will enhance the sensitivity of measurement and causes further improvement in the S/N ratio.

4- No stray light

Because the FT experiment modulates the source radiation and then detects only modulated radiation, there is essentially no stray light problems as there are with scanning instruments.

Any stray light that reaches the detector is not incorporated into the spectrum since it is unmodulated. Thus there is no possibility of errors occurring during measurement (accurate quantitative analysis).

Summary of FT-IR Advantages

Speed Because all of the frequencies are measured simultaneously.

Sensitivity is dramatically improved with FT-IR ; detectors are much more sensitive, the optical throughput is much higher, higher signal to noise ratio.

Mechanical Simplicity The moving mirror in the interferometer is the only continuously moving part in the instrument. Thus, there is very little possibility of mechanical breakdown.

Internally Calibrated These instruments employ a He-Ne laser as an internal wavelength calibration standard .These instruments are self-calibrating and never need to be calibrated by the user.

Analytical information obtained using IR techniques

I)            Qualitative

a)      Structural Elucidation through interpretation of functional group region (4000- 1300 cm-1), fingerprint region (1300- 910 cm-1), aromatic region (910- 650 cm-1).

b)  Compound Identification to find a reference IR spectrum that matches that of the unknown compound.

c)  IR mostly used for rapid qualitative but not quantitative analysis.

II ) Quantitative

A = a b c

• The intensity of an absorption band is linearly proportional to the concentration of analyte of interest at a certain frequency.
•  Quantification parameters include peak height, peak area; integration of band area should be done carefully to ensure maximum accuracy, near IR region is better suited for quantitation.

Applications of Infrared Analysis

Ã˜  Analysis of petroleum hydrocarbons, oil and grease content (detection of Freons).

Ã˜  Determination of air contaminants.

Ã˜  Determination of protein, starch, oil, lipids and cellulose in agricultural products.

Ã˜  Far- Infrared region is particularly useful for inorganic studies (crystals and semiconducting materials).

General Applications of Infrared Analysis

• Pharmaceutical research.
• Forensic investigations.
• Polymer analysis.
• Lubricant formulation and fuel additives.
• Foods research.
• Quality assurance and control.
• Environmental and water quality analysis methods.
• Biochemical and biomedical research.
• Coatings and surfactants.

Summary

Michelson interferometer is an important component in FTIR instrument

The spectrum obtained is an interferogram

By using a mathematical algorithm, Fourier transform, interferogram is converted into dispersive IR spectrum

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