Analytical Instrumentation: Fourier Transform Infrared (FTIR) Spectroscopy

Nikhilbinoy.CAssistant ProfessorICE DepartmentAnalytical Instrumentation:Fourier Transform Infrared (FTIR) Spectroscopy

IntroductionInstead of viewing each component frequency sequentially as in a dispersive IR spectrometer, all frequencies are examined simultaneously in FTIR spectroscopy.Superior speed and sensitivity.Have been applied to many areas that are very difficult or nearly impossible to analyse by dispersive instruments.

Each component is viewed sequentially in IR spectrophotometer

All wavelength components are viewed simultaneously in FTIR spectroscopy

Instrumentation

Radiation SourcesSame types of radiation sources are used for both dispersive and Fourier Transform Spectrometer.The source is more often water cooled in FTIR instruments to provide better power and stability.

InterferometerMonochromator is replaced by an interferometer.Divides radiant beams, generates an optical path difference between the beams, then recombines them in order to produce repetitive interference.These interference signals after passing through a sample contain IR spectral information.Most commonly used interferometer is Michelson Interferometer.

Michelson InterferometerHave three active components.Moving mirror.Fixed mirror.Beam splitter.The two mirrors are perpendicular to each other.The beam splitter is a semi reflecting device and is often made by deposing a thin film of Germanium onto flat KBr substrate.

Michelson InterferometerIt is a complex summation of superimposed sinusoidal waves, each waves corresponding to a single frequency. When this IR beam is directed through the sample, the amplitude of a set of waves are reduced by absorption.Mathematical tool FT (Fourier Transform) converts the interferogram to final IR spectrum, which is the familiar frequency domain spectrum.

Zero Position Displacement (ZPD)

If Monochromatic light is passed

If Polychromatic light is passed

DetectorThe response time of detector used for dispersive instruments are too slow for the rapid scan.The two most popular detectors for a FTIR areDeuterated Triglycine Sulfate (DTGS)Mercury Cadmium Telluride (MCT)

Detector:Deuterated Triglycine Sulfate (DTGS)Is a pyroelectric detector that derives rapid responses because it measures the changes in temperature rather than the value of temperature.Operates at room temperature.

Detector:Mercury Cadmium Telluride (MCT)Is a photon (or quantum) detector that depends on the quantum nature of radiation.Faster and more sensitive than DTGS.MCT detectors must be maintained at liquid nitrogen temperature (77K).

DetectorThe detector signal is sampled at small, precise intervals during the mirror scan.The sampling rate is controlled by an internal, independent reference.A modulated monochromatic beam from a Helium-Neon laser focused on a separate detector.

Simple Spectrometer Layout

AdvantagesIt is a non destructive technique.It provides a precise measurement which requires no external calibration.Fast response.It can increase sensitivity.It is mechanically simple.

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