Instrumentation of IR spectroscopy

IN THIS ARTICLE I DISCUSS ABOUT THE INSTURMENTATION OF IR SPECTROSCOPY

         The instrument that determines the absorption spectrum for a compound is called an infrared spectrometer or, more precisely, a spectrophotometer. Two types of infrared spectrometers are in common use in the organic laboratory:

                                                    dispersive and Fourier transform (FT) instruments.

           Both of these types of instruments provide spectra of compounds in the common range of 4000 to 400 cm−1. A double beam infrared spectrophotometer consists of the same basic component as the UV/VIS spectrophotermeter and operates in a similiar manner. IR spectrometer consist of following parts.

1. Source
2. Sample compartments
3. Detector 
4. Recorder

Source

          An inert solid is electrically heated to a temperature in the range 1500-2000 K. The heated material will then emit infra red radiation.. The common source of infrared radiation is either Nernst or Globar. The Nernst glower is a cylinder (1-2 mm diameter, approximately 20 mm long) of rare earth oxides. Nernst is composed of a mixture of oxides of zirconium, thorium, yttrium, which is formed into a hollow rod.  Glowbar is bonded  silicon  carbide rod , when heatinfg electrically to 1500 celcious, these rod emits infrared radiation.

Detectors

                There are three catagories of detector;

• Thermal
• Pyroelectric
• Photoconducting

Thermal 

          Thermocouples (10-6K); bolometers (thermistors) bolometers (thermistors)

 Photoconducting 

        Semiconductor layer semiconductor layer exposed to IR exposed to IR – change in resistance is change in resistance is proportional to radiation. Lead sulfite, proportional to radiation. Lead sulfite, cadmium tellurite, indium antimonide.

Pyroelectric

           Temperature dependent capacitors - Dielectric material, pyroelectric material such as triglycine sulfate between electrodes - One electrode is expose to IR radiation - Change in temperature change charge distribution in material and current is measured.

Monochromators 

• Prism and slit 
• Prism and filter 
• Grating and slit 
•  Grating and filter 
• This type of equipment is called - Dispersive spectrometer or Dispersive spectrometer or spectrophotometer spectrophotometer.

           A beam of IR radiation is passed through the sample and its intensity is constantly campared  with intensity od IR radiation of a reference beam as the wavelength of the radiation is varried. The spectrophotometer  record the results as a specimen showing absorbance or transmittence vers wavwlength or frequenvy.