Spectrophotometry - IR Spectroscopy - Instrumentation

In the earlier post, we have tried to understand the basic concepts and a little theory about the infra-red (IR) spectrophotometer. The basic instrumentation of IR spectrometer consists of the components which will be explained here briefly. 

Sources: The IR spectrometer consists of a source of infrared light, emitting radiation throughout the whole frequency range of the instrument. An inert solid is electrically heated to a temperature in the range of 1500-2000K. This heated material will then emit IR radiation. Following are some of the sources:

The Nerst Glower: It is a cylinder of rare earth oxides. Platinum wires are sealed to the ends and a current is passed through the cylinder and can reach temperatures of around 2200K.

The Globar source: It is a silicon carbide rod which is electrically heated to around 1500K. The spectral output is comparable with the Nerst glower, except at short wavelengths (less than 5mm) where it’s output becomes larger.

The Incandescent wire source: This is a tightly wound coil of nichrome wire, which is electrically heated to 1100K. It produces a lower intensity of radiation than the above mentioned Nerst or Globar sources, but it has a longer working life.

Light from these sources is split into two beams of equal intensity. One beam is allowed to pass through the sample while other is allowed to behave as reference beam. Now, you might be thinking that why there is a need of double beam? So, the function of such a double beam operation is to measure the difference in the intensities between the two beams at each wavelength. 

Chopper: The two beams are reflected to a chopper which is rotating at a speed of 10 rotations per second.  This chopper makes the reference and the sample beam to fall on the monochromator grating alternately.

Monochromator grating: The grating also rotates, though slowly. This rotation sends individual frequencies to the detector. 

Detector:At the wavelength where the sample has absorbed, the detector will receive a weak beam from the sample while the reference beam will retain full intensity. This leads to a pulsating or alternating current to flow from detector to amplifier. On the other hand, at the frequencies where the sample doesn’t absorb, both the beams will have equal intensities and the current flowing from the detector to the amplifier will be direct and not alternating. The amplifier is designed to amplify only the alternating current. 
There are three different types of detectors.

Thermocouples: They consist of a pair of junctions of different metals. The potential difference (i.e.; the voltage) between the junction changes according to the difference in temperature between the junctions.

Pyroelectric detectors: They are made from a single crystalline wafer of a pyroelectric material (eg; triglycerine sulphate). The properties of a pyroelectric material are such that when an electric field is applied across it, electric polarisation occurs. In a pyroelectric material, when the field is removed, the polarisation persists.  This degree of polarisation is temperature dependent.

Photoelectric detectors: They comprise a film of semiconducting material deposited on a glass surface, sealed in an evacuated envelope (such as mercury cadmium telluride detector).

The above mentioned description is that of a “dispersive infra-red spectrometer”. Most of the modern IR absorption instruments use Fourier transform techniques with Michelson interferometer (about which we will not discuss here) which is referred to as Fourier Transform Infra-red Spectroscopy or FTIR Spectroscopy.

In the next post, we will discuss about the various sampling techniques for IR spectroscopy.