Basic IR Spectroscopy

Intfrared spectroscopy deals with the infrared region of the Electromagnetic Spectrum. Only in this region is the energy intense enough to exite bonds in a molecule to absorb energy. Molecules absorb these frequencies of light because they correspond to frequencies of vibrations of bonds in the molecule.

Regions of the IR spectrum

4000 cm¹ – 1300 cm¹ Absorbance bands in this region can indicate what functional groups are present in an unknown compound.

1300 cm¹ – 400 cm¹ Absorbance bands in this region are unique to each molecule, like a fingerprint! Spectra are compared to known spectra.

The spectrophotometer detects this absorption and records it as a plot of transmission versus frequency.

Since each type of bond vibrates at a characteristic wavenumber , infrared spectroscopy has become a useful technique to identify functional groups in a molecule. The method involves the examination of the twisting, bending, rotating, vibrational motions of atoms and molecules.

Infra-red Instrumentation

Dispersive Infrared Spectrophotometer

This is a double beam scanning IR instrument that makes use of a diffraction grating in a monochromator to disperse the different wavelengths of light.

The key components are the light source, entrance slit, grating and detector. Radiation from the source is directed along both sample and reference and then into the diffraction grating. The grating disperses the light into spatially separated wavelengths which are selectively directed by a narrow slit towards the detector.

The wavelengths are measured one at a time, with the slit controlling the bandwidth. The grating and slit combination selects the wavelength being measured, and the detector produces a spectrum of a plot of transmittance against frequency.

Fourier Transform Spectrophotometer

The FTIR instrument relies upon interferences of various frequencies of light to produce a spectrum. It has a source, sample, two mirrors, a laser reference, and detector, but the assembly of components also include a beamsplitter and the two strategic mirrors that function as an interferometer.

The source energy strikes the beamsplitter and produces two beams of roughly the same intensity. One beam strikes the fixed mirror and returns to the beamsplitter. The other beam goes to the moving mirror. The motion of the moving mirror makes the total pathlength variable versus that taken by the fixed mirror beam. When these two beams meet up again at the beamsplitter, they recombine, and the difference in their path lengths create constructive and destructive interference, an interferogram.

The recombined beam passes through the sample. The sample absorbs all the wavelengths characteristic of the its spectrum and then subtracts specific wavelengths from the interferogram. The detector now reports variation in energy-versus-time for all wavelengths simultaneously. A laser beam is superimposed to provide a reference for the operation of the instrument.

A mathematical function known as a Fourier transform is used to convert the intensity-versus-time spectrum into an intensity-versus-frequency spectrum.

Components that make up the IR spectrophotometer

Radiation source	Monochromator	Detector	Sample
NERST glower Incandescent Lamp Mercury Arc Tungsten Lamp Globar Source Nichrome wire	Alkali Halides Prism Gratings Filters	Thermocouple Bolometer Thermister Golay cell Pyroelectric Photoconductor	Solid Liquid Gas

Radiation Sources

The radiation source is composed of an inert solid which is electrically heated to a temperature in the range 1500-2000 K. The heated material will then emit infra red radiation.

The Nernst glower

This is a cylinder (1-2 mm diameter, approximately 20 mm long) of rare earth oxides. Platinum wires are sealed to the ends, and a current passed through the cylinder. The Nernst glower can reach temperatures of 2200 K.

The Globar source

This is a silicon carbide rod (5mm diameter, 50mm long) which is electrically heated to about 1500 K. Water cooling of the electrical contacts is needed to prevent arcing. The spectral output is comparable with the Nernst glower, execept at short wavelengths (less than 5 m) where it's output becomes larger.

The incandescent wire source

This is a tightly wound coil of nichrome wire, electrically heated to 1100 K. It produces a lower intensity of radiation than the Nernst or Globar sources, but has a longer working life.

INFRARED DETECTORS

Detector Type	Measurement	Construction	Spectral Region cm^-1
Bolometer	Resistance	Pt : Ni	4000 - 400cm^-1
Thermocouple	Thermoelectric effect	Bi : Sb	4000 -400cm^-1
Golay	Thermal expansion	Pneumatic cell	4000 - 400cm^-1
Pyroelectric	Capicitance	Triglycerine SO4	Mid range
Photoconductor or Photodiode	Resistivity	PbS PbSe InSb HgCdTe	10000 - 2777cm^-1 6666 - 1724cm^-1 5000 - 1666cm^-1 5000 - 525cm^-1
Photoconductor or Photovoltaic or Photoelectric	Reaction photons	InGaAs InAs InSb	24287 - 5882cm^-1 10000 - 3226cm^-1 10000 - 1818cm^-1

Thermocouples

Consist of a pair of junctions of different metals; for example, two pieces of bismuth fused to either end of a piece of antimony. The potential difference (voltage) between the junctions changes according to the difference in temperature between the junctions

Golay cell

This consist of a glass-filled enclosure with a diaphragm that expands on IR radiation and gives a change in signal due to the amount of movement of the diaphragm.

Pyroelectric detectors

This is made from a single crystalline wafer of triglycerine sulphate sandwiched between two electrodes. When an electric field is applied and the field is removed, the polarization persists, it is temperature dependent and the semi-conductor acts as a capacitor. The incident infrared heating causes a change in the capacitance of the material.

The pyroelectric effect depends on the rate of change of the detector temperature and not on the temperature itself, which allows for faster response time.

Pyroelectric detectors are used in mid-infrared region, works at room temperature and applied in most FT-IR instruments..

Photoconductive Detectors

Photoconductive detectors are thin-film semi-conductor devices which undergo a change in conductivity when exposed to varying quantities of infrared radiation. This change results in an increase in the amount of current flowing through the device, and the output of the detector is either a change in detector current or a change in voltage developed across a load resistor.

The PbS and PbSe photoconductive detectors make use of the photoconductive effect that resistance is reduced when infrared radiation enters the detecting material.

The incoming light produces free electrons which can carry electrical current so that the electrical conductivity of the detector material changes as a function of the incident light and a change in detector current. Example is the PbS and PbSe photoconductive detectors.

Photovoltaic Detector

This detector contains a junction in a semi-conductor material between a region where the conductivity is due to electrons, and a region where the conductivity is due to holes called a p-n junction. A voltage is generated when IR radiation strikes the device and the output is a change in voltage. Example is the InGaAs fabricated detector.

The energy gap of the InGaAs pin photodiode will vary according to the composition ratio of the In and Ga in the detecting elements. Various IR detectors with different spectral response ranges can be manufactured by altering the composition ratio.

Photoelectric detectors

The mercury cadmium telluride detector comprise a film of semiconducting material deposited on a glass surface, sealed in an evacuated envelope. Absorption of IR promotes non-conducting valence electrons to a higher, conducting, state. The electrical resistance of the semiconductor decreases. These detectors have better response characteristics than pyroelectric detectors and are used in FT-IR instruments - particularly in combination with other instruments such as GC – FT-IR.

Sample and Sampling Techniques

Sample Type	Sample cell	Technique	Procedure
Solid	NaCl	Mull	Sample slurry with nujol Mulls Technique Sample is crushed in agate mortar and pestle and mixed and ground together with nujol to a thick paste. A spot is applied between two NaCl plates and mounted in path of IR beam of instrument and a spectrum is recorded.
Solid	KBr	Pressed pellet	Sample pressed in Die Dried sample film The sample is dissolved in a solvent which is non-aqueous, non-reacting, non IR absorbing in the range to be measured. A drop of this solution is placed on NaCl plate and the solvent evaporated to dryness leaving a thin sample film.
Solid	NaCl plates	Thin film	Sample dries on plate Pressed pellet One milligram sample to 100 milligram KBr is finely ground and compressed into a transparent disc using an evacuable die and hydraulic press. Another disc is similarly made with pure dry KBr which is used as reference.
Solid	ATR crystal	Evanescent wave	Direct into IR beam ATR Evanescent wave This is an ATR crystal accessory used with IR spectrophotometer which allows both solid and liquid samples to be analyzed in their natural state without further sample preparation. Liquid samples are simply poured over the crystal and solid samples are clamped onto it. IR lightis passed through it, an evanescent wave is formed which is collected and measured by the detector.
Liquid	NaCl Liquid cell	Neat sample	Fill Liquid cell 1. Liquid sample cell Cell consist of two high quality pressed NaCl windows sealed and separated by a thin Teflon gasket and wetted with mercury. Cell thickness for most liquids is usually 0.01 – 0.05mm.
Gas	NaCl or KBr gas cell	Gas expansion	Fill evacuated cell

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Instrumental infrared and Fourier Ttansform Spectroscopy