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IN 504 Analytical InstrumentsModule 2

1 Presented by; Anju Sunny CUSATReference Text: R S Khandpur Handbook of Analytical Instrumentation

OverviewIntroductionDifference between UV-VIS and IR SpectrometerVarious regions of the IR range of the spectrumBasic components of IR spectrometer

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Infrared (IR) Sources

The Nernst Glower: The Nernst glower is composed of rare earth oxides formed into a cylinder having a diameter of 1 to 2 mm and a length of 20 mm. Platinum leads are sealed to the ends of the cylinder to permit electrical connection to what amounts to a resistive heating element. A current is passed through the device, heat into a temperature between 1200 K and 2200 K to result IR emission. The Globar Source: A Globar is a silicon carbide rod, usually about 50 mm in length and 5 mm in diameter. It also is electrically heated (1300 to 1500 K). Spectral energies of the Globar and the Nernst glower are comparable except in the region below 5m, where the Globar provides a significantly greater output.

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Infrared (IR) Sources Incandescent Wire Source ( Nichrome wire): A source of somewhat lower intensity but longer life than the Globar or Nernst glower is a tightly wound spiral of nichrome wire heated to about 1100 K by an electrical current.The Mercury Arc: For the far-infrared region of the spectrum (> 50 m), none of the thermal sources just described provides sufficient radiant power for convenient detection. Here, a high-pressure mercury arc is used. This device consists of a quartz-jacketed tube containing mercury vapor at a pressure greater than one atmosphere. Passage of electricity through the vapor forms an internal plasma source that provides continuum radiation in the far-infrared region.

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Infrared (IR) Sources The Tungsten Filament Lamp: An ordinary tungsten filament lamp is a convenient source for the near-infrared region of 4000 to 12,800 cm-1.The Carbon Dioxide Laser Source: A tunable carbon dioxide laser is used as an infrared source for monitoring the concentrations of certain atmospheric pollutants and for determining absorbing species in aqueous solutions. A carbon dioxide laser produces a band of radiation in the 900 to 1100 cm-1 range.

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Optical Components used in SpectroscopyEntrance Slit: Purpose is to provide rectangular optical image.Collimating Mirror or lens: Purpose is to produce parallel beams of radiation, it overcomes diffraction.Prism or Grating: Disperses radiation into its component wavelengths.Focusing Mirror or lens: Reforms image from slit onto focal plane.Exit Slit: Isolates Spectral Band.

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Optical Components used in Spectroscopy7

Diffraction Grating MonochromatorPrism type Monochromator

IR DetectorsThermal radiant energy Electrical energy8DetectorsThermal DetectorsQuantum DetectorsEg: Thermocouple Thermopile Bolometer Pneumatic detector (Golay) Pyroelectric detector

Eg: Intrinsic type - Solid state Photo Detectors Extrinsic type - Photoconductive cell

FeaturesThermal DetectorsQuantum DetectorsResponsitivity with little dependence on wavelength.Operation at room temperature.Slow response and low detectivity.Responsitivity is wavelength dependent.Cooling is normally used.Fast response and high detectivity.

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Quantum DetectorsSolid state Photo DetectorsPhotoconductive cellPrinciple:- Photo electric effect

Materials:- Cadmium-Mercury- Telluride(CMT) or Indium Antimonide (InSb)

Sensitivity:- 2-6 (InSb) 10-12 (CMT)Principle:- Electrical resistors, which decrease in resistance in relation to the intensity of light striking there surface.Materials:- Semiconductors like Lead sulphide or Lead telluride

Sensitivity:- 3.5 (Lead sulphide) 6 (Lead telluride)

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Thermal Detectors1) ThermocouplePrinciple:- In these detectors, the signal originates from a potential difference caused by heating a junction of the unlike metals by the infrared beam, while the other junction is kept at constant temperature.

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Thermal Detectors

2) ThermopilesPrinciple:- It is possible to increase the output voltage by connecting several thermocouples in series. This arrangement is referred to as Thermopiles.12

Thermal Detectors3) BolometerPrinciple:- It provides an electrical signal as a result of the variation in resistance of a conductor with temperature.

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Thermal Detectors 4) Pneumatic detector (Golay Detector)Principle:- It measures the intensity of IR radiation by the expansion of a gas filled in its chamber, upon heating.14

Light sourcePhotocell

Thermal Detectors 5) Pyroelectric detector Principle:- Pyroelectric effectIn a pyroelectric detector which is composed of a pyroelectric material, a change in temperature due to the application of IR radiation creates a change in polarization . Such a crystal will create an accumulation of charge and this charge is collected by electrodes on the crystal.ie, by connecting 2 electrodes to the crystal, the pyroelectric detector can act as a capacitor and the resulting voltage = charge / crystal capacitance, V= Q/C.The detector will also ignore the effects of background radiation.Pyroelectric detectors are commonly used in FTIR spectrometers.15

Thermal Detectors 5) Pyroelectric detector Commonly used crystal material for pyroelectric detector is Tri Glycine Sulphate (TGS).

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Thermal Detectors 5) Pyroelectric detector The very small electrical charges are generally converted within the detector housing to convenient signal voltages by use of extremely low noise and low leakage Field Effect Transistors (FET).17

FTIR Spectrometer(Fourier Transform Spectrometer)

Fourier TransformsFourier transform defines a relationship between a signal in time domain and its representation in frequency domain. Being a transform, no information is created or lost in the process, so the original signal can be recovered from the Fourier transform and vice versa. The Fourier transform of a signal is a continuous complex valued signal capable of representing real valued or complex valued continuous time signals.

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What is a FTIR Spectrometer?FTIR (Fourier Transform InfraRed) spectrometer obtains an infrared spectra by first collecting an interferogram of a sample signal using an interferometer, then performs a Fourier Transform on the interferogram to obtain the spectrum. An interferometer is an instrument that uses the technique of superimposing (interfering) two or more waves, to detect differences between them. The FTIR spectrometer uses a Michelson interferometer.

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21FT-IR SpectrometerPrinciple: Michelsons Interferometer

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Basic Optical Components of the Interferometer (Principle)

Schematic diagram of a Michelson interferometer for FTIR

Monochromatic radiation entering the interferometer is split into 2 beams having two different path lengths by Beam splitter. When beams A and B recombine, an interference pattern is produced.A detector measures the intensity variations of the exit beam as a function of path difference.When two beams are in phase at beam splitter, maximum intensity will reach detector.

Interferometer

When two beams are out of phase intensity will be minimum.When mirror M2 moved uniformly, Detector output will be a sine wave.Amplitude of signal will depend on the intensity of incoming radiation. Frequency is determined byTranslation velocity of M2Wavelength of incoming radiation

Interferometer

FTIR Spectrometer - Block Diagram

Components of FTIRIR Source (Glowbar)InterferometerSample cellDetector (Pyroelectric Detector)ComputerRecorder or Plotter

How to Perform Fourier Transform?27Computer controls scan system and carry out math transformation, ie performs Fourier transform. The interferogram in practice consists of a set of intensities measured for discrete values of path length differences (retardation). Adiscrete Fourier transform (Fast Fourier transform(FFT) algorithm) is used to get the spectrum.

Very high resolution (< 0.1 cm 1 )Resolution governed by distance movable mirror travels

Very high sensitivity (nanogram quantity)can be coupled with GC analysis (> measure IR spectra in gas-phase)

High S/N ratios - high throughputFew optics, no slits mean high intensity of light

Rapid (