Infrared Spectroscopy

Instrumentation……

→IR radiation sources.→Monochromators.→Sample cells and sampling of substances.→Detectors.

• The usual optical materials, glass or quartz absorb strongly in the infrared region. Thus the apparatus used in infrared spectroscopy is different from that used in visible and ultraviolet regions. The main parts of Infrared Spectrometer are

• Infrared region is divided into 3 basic segments with the dividing points based on the instrumental capabilities.

• Instrumentation

Region of Electromagnetic Spectrum

Near-Infrared Mid-Infrared Far-Infrared

Wave no. cm-1 12500-4000 4000-200 200-10

Wavelength µm 0.8 – 2.5 2.5 – 50 50 – 1000

Radiation Source Tungsten Filament Lamp

Nernst Glower, Globar, or coil of Nichrome

wire,

High Pressure Mercury Arc Lamp

Optical System 1 or 2 Quartz Prisms or Prism-grating

double Monochromator

2-4 plane diffraction with either fore prism

monochromator or infrared filters

Double beam grating instruments for use to

700 µm, interferometric

spectrometers for use to 1000 µm

Detector Lead Sulfide Photoconductive

Thermopile, Thermistor or Pyroelectric

Golay or Pyroelectric

• In order to be suitable for spectroscopic studies a source must generate a beam of radiation with sufficient power for ready detection and measurement. In addition, its output should be stable. Should extend over desired wavelengths.

• Commonly used is the Tungsten filament lamp for near infrared region.

• A close wound nichrome coil can be raised to incandescence by resistive heating. A black oxide film is coated around the coil which gives acceptable emissivity.

• Can tolerate temperature up to 11000 C. Requires no water cooling and is on low maintenance with long service.

• RECOMMENDED WHERE RELIABILITY IS ESSENTIAL SUCH AS IN NONDISPERSIVE PROCESS ANALYZERS OR FILTER PHOTOMETERS.

• This source is less intense than other infrared sources.

•Radiation Sources

• A hotter, therefore brighter, source is the Nernst glower which has an operating

temperature as high as 15000C.

• These are constructed from a fused mixture of oxides of zirconium, yttrium and thorium,

molded in form of hollow rods 1 -3 mm in diameter and 2-5 cm in length.

• The radiation energy is twice that of Nichrome and Globar sources except in the near-

infrared region.

• Globar Source is a rod of silicon carbide 6-8 mm in diameter and 50 mm in length.

It possesses characteristics intermediate between heated wire coil and Nernst glower. It is

self starting and has an operating temperature of 13000C.

• In very far-infrared, beyond 50 µm (200 cm-1), black body type sources lose their

effectiveness since their radiation decreases with the fourth power of wavelength. Here,

high pressure Mercury Arc, with extra quartz jacket to reduce the thermal loss, give

intense radiation in this region. Output is similar to that from blackbody sources but

additional radiation is emitted from plasma which enhances the long-wavelength output.

•Visuals

• Tungsten Filament Lamps

•Visuals

•Nernst Glower

•Visuals

•Mercury Arc Lamps

• Prism Monochromator• Prism used as a dispersive device is constructed of

various metal halide salts which transmits Infrared.• Because of its high dispersion in the region of 4-15 µm, a

region which is of special importance for functional group studies, sodium chloride is probably the most common prism salt. Only we have to consider the mechanical, thermal instability and the water solubility.

• It is necessary to select a desired frequency from a radiation source and reject radiation of other frequencies. This can be achieved by the use of monochromators which are of TWO types

• A: - Prism Monochromator and• B: - Grating Monochromator.

•Monochromators

• Single-pass and Double-pass Monochromator: -

Exit Slit

Source

• Sample is kept at or near the focus of the beam, just before the entrance slit to the monochromator.

• Radiation from the source after passing through the sample and entrance slit, strikes off the Parabolic Littrow mirror which renders the radiation parallel and sends to Prism. Then it returns back to prism second time from the Littrow and focuses into the exit slit of the monochromator, which finally passes into the Detector.

• The double pass monochromator gives 4 passes of Radiation through prism and produces more resolution and then it finally passes on to the detector.

Parabolic Littrow Mirror.

• To achieve a higher dispersion, a prism monochromator is replaced by a grating monochromator.

• Grating is an assembly of series of parallel straight lines cut into a plane surface.

• Dispersion by a grating follows the law of diffraction.

• The mathematical equation:

• nλ = d(Sin i ± Sin θ)

• Where • n = order (generally a whole number)

• λ = wavelength of the radiation

• d = distance between the grooves

• i = angle of incident beam of IR radiation

• θ = angle of dispersion of light of a particular wavelength.

• Grating Monochromator

• Grating Monochromators possesses following advantages over prism monochromator

• 1. it can be made with materials like aluminium which are not attacked by moisture, while metal salt prisms are subject to etching from atmospheric moisture

• 2. it can be used over a considerable range of wavelengths.

• For different wavelengths, the angle of dispersion is different. At a grating, separation of light occurs because of wavelengths dispersed at different angles.

•Visuals

d

θ

i