FT-IR Instrument

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i)Dispersive spectrometers or ii)Fourier transform spectrometers.

Most commercial instruments separate and measure IR radiation using

Dispersive SpectrometersDispersive spectrometers, introduced in the mid-1940s and widely used since, provided the robust instrumentation required for the extensive application of this technique. This consists of three basic components: radiation source, monochromator, and detector

The common radiation source for the IR spectrometer is an inert solid, heated electrically to 1000 to 1800 °C. Three popular types of sources are Nernst glower (constructed of rare-earth oxides), Globar (constructed of silicon carbide), and Nichrome coil. They all produce continuous radiations, but with different radiation energy profiles.

Radiation source

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Monochromator

•The monochromator is a device used to disperse a broad spectrum of radiation and provide a continuous calibrated series of electromagnetic energy bands of determinable wavelength or frequency range.

•Prisms or gratings are the dispersive components used in conjunction with variable-slit mechanisms, mirrors, and filters.

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DetectorsMost detectors used in dispersive IR spectrometers can be categorized into two classes:

•Thermal detectors and •Photon detectors.

Thermal detectors include thermocouples, thermistors, and pneumatic devices (Golay detectors).

They measure the heating effect produced by infrared radiation. A variety ofphysical property changes are quantitatively determined: expansion of a nonabsorbing gas (Golay detector), electrical resistance (thermistor), and voltage at junction of dissimilar metals (thermocouple).

Photon detectors rely on the interaction of IR radiation and a semiconductor material. Nonconducting electrons are excited to a conducting state. Thus, a small current or voltage can be generated. Thermal detectors provide a linear response over a wide range of frequencies but exhibit slower response timesand lower sensitivities than photon detectors.

General Concepts Interferometry

• Optical Interfrometry is an optical measurement technique that provides extreme precise measurements of distance, displacement or shape and surface of objects.

• It exploits the phenomenon of light waves interference .

Where under certain conditions a pattern of dark and light bars called interference fringes can be produced. Fringes can be analyzed to present accurate measurements in the range of nanometer.

• The recent developments in laser, fiber optics and digital processing techniques have supported optical interferometry .

• Applications ranging from the measurement of a molecule size to the diameters of stars.

Interference • Interference is a light

phenomenon .It can be seen in everyday life. e.g.. colures of oil film floating on water.

• In electromagnetic waves , interference between two or more waves is just an addition or superposition process. It results in a new wave pattern .

Superposition of two waves

• When two waves with an equal amplitudes are superposed the output wave depends on the phase between the input waves.

Y = y1 + y2

Where: y1=A1 sin (wt + θ1 ) y2=A2 sin (wt + θ2)

• Since the energy in the light wave is intensity I ,which is proportional to the sum of square amplitudes A^2

where: A=A1^2+A2^2+2A1A2 cos (θ1 – θ2)

If A1=A2=A then:

A=2A^2+2A^2 cos (θ1 – θ2)

If y1&y2 in phase ,cos(0)=1 hence,

Y = 4A^2 ,it gives a bright fringe.

If y1&y2 out of phase by (π) ,cos (π)=-1 hence, Y = 0 ,it gives a dark fringe

Optical Path Length [OPL]

• When light beam travels in space from one point to another, the path length is the geometric length d multiplied by n (the air refractive index) which is one:

OPL = d

• Light beam travels in different mediums will have different optical path, depending on the refractive index (n)of the medium or mediums.

OPL = n d

Components

• Source• Michelson Interferometer• Sample• Detector

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Fourier transform spectrometers• Superior speed and sensitivity• Instead of viewing each component frequency sequentially, as in a Dispersive IR spectrometer, all frequencies are examined simultaneously in Fourier transform infrared (FTIR) spectroscopy.

Michelson Interferometer

Sources• Black body radiators• Inert solids resistively heated to 1500-2200 K • Max radiation between 5000-5900 cm-1 (2-1.7

m), falls off to about 1 % max at 670 cm-1 (15 m)

• Nernst Glower – cylinder made of rear earth elements

• Globar- SiC rod• CO2 laser• Hg arc (Far IR), Tungsten filament (Near IR)

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Michelson interferometer• Configuration:

Michelson interferometer consists of a coherent light source, a beam splitter (BS), a reference

mirror ,a movable mirror and a screen .

• Applications:

There are many measurements that Michelson interferometer can be used for, absolute distance measurements, optical testing and measure gases refractive index.

• Work method: The BS divides the incident beam into two parts one

travel to the reference mirror and the other to the movable mirror .both parts are reflected back to BS recombined to form the interference fringes on the screen.

Sample

• Sample holder must be transparent to IR- salts• Liquids

– Salt Plates– Neat, 1 drop– Samples dissolved in volatile solvents- 0.1-10%

• Solids– KBr pellets– Mulling (warm)(dispersions)

• Quantitative analysis-sealed cell with NaCl/NaBr/KBr windows

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Detector

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The two most popular detectors for a FTIR spectrometer are • deuterated triglycine sulfate (DTGS) and •mercury cadmium telluride (MCT).

The response times of many detectors (for example, thermocouple and thermistor) used in dispersive IR instruments are too slow for the rapid scan times (1 sec or less) of the interferometer. The DTGS detector is a pyroelectric detector that delivers rapid responses Because it measures the changes in temperature rather than the value of temperature. The MCT detector Is a photon (or quantum) detector that depends on the quantum nature of radiation and also exhibits very Fast responses. Whereas DTGS detectors operate at room temperature, MCT detectors must be maintained at liquid nitrogen temperature (77 °K) to be effective. In general, the MCT detector is faster and more sensitive than the DTGS detector.

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FTIR Advantages• Better speed and sensitivity (Felgett advantage). A complete spectrum can be obtained during a single scan of the moving mirror, while the detector observes all frequencies simultaneously.

• Increased optical throughput (Jaquinot advantage). Energy-wasting slits are not required in the interferometer because dispersion or filtering is not needed.

• Internal laser reference (Connes advantage). The use of a helium neon laser as the internal reference in many FTIR systems provides an automatic calibration in an accuracy of better than 0.01 Cm –1 . This eliminates the need for external calibrations.

• Simpler mechanical design. There is only one moving part, the moving mirror, resulting in less wear and better reliability.

•Elimination of stray light and emission contributions. The interferometer in FTIR modulates all the frequencies. The unmodulated stray light and sample emissions (if any) are not detected.

•Powerful data station. Modern FTIR spectrometers are usually equipped with a powerful, com- puterized data system. It can perform a wide variety of data processing tasks such as Fourier transformation, interactive spectral subtraction, baseline correction, smoothing, integration, and library searching.