Output Devices

 

The electrical signal from the transducer is suitably amplified or processed before it is
sent to the recorder to give an output. In dispersive IR instruments, as discussed later,
an optical null system is employed to achieve the equal intensities of the sample and Some bolometers are prepared from semiconductors and are called thermistors. The pyroelectric material undergo electric polarisation when an electric field is applied across it. The polarization persists even on removing the field and the degree of polarisation depends on the temperature.
On the other hand in FT-IR instruments, the interferogram obtained in the time domain is mathematically processed by the computer to generate its Fourier transform i.e., the signal in frequency domain. This is then further processed to get the recorder output.
We are sure that having read about different components of IR instruments and their importance, you are equipped to learn about the IR spectrometers. It would be worthwhile to make an assessment of the understanding.
Rotating prisms and diffraction gratings as monochromators which disperse the   radiations from the source falling on it and can be used to allow the radiations of different wavelengths to come out of the exit slit. These are then passed through the sample and the interaction is monitored. In dispersive IR spectrometers (discussed in subsection 3.4.1) we use similar components, the only difference being that the monochromator is placed after the sample.
In FT-IR instruments , used quite extensively now a days, the radiations are not dispersed before or after passing through the sample. Instead, the response of the sample to all the wavelengths in the range is measured simultaneously and the signal so obtained is modulated and collected in time domain. This is then Fourier transformed to get the desirable frequency domain signal. The modulation of the signal is achieved by  Michelson interferometer. It modulates the frequency of the IR radiation into the audio frequency range.
A monochromator is an optical device that transmits a selectable Infrared region of the electromagnetic spectrum spreads from the wavelengths of 0.8 to 1000 μm or wave numbers from roughly 12,500 to 10 cm–1. It is bound by the red end of the visible region at lower wavelength end and the microwave region at higher wavelength end. It is further divided into three regions called near IR, mid IR and far IR regions (Fig. 3.10). The far infrared region (200 cm-_1 to 10 cm_1) is useful for molecules containing heavy atoms such as inorganic compounds and the near IR region (12000 cm_1 to 4000 cm_1) concerns routine quantitative determinations of the simple molecules like water, CO2 etc. of industrial and agricultural importance. It is the mid infrared region (4,000 cm_1 to 200 cm_1) that is of interest for general chemical analysis purposes.
Since it is the mid IR region, which is commonly employed we would like to confine ourselves with the instrumentation of mid IR spectrometers only. Further, we do not intend to take up the detailed working and the construction of an IR spectrometer. We
would be contented if you get an impression of the basic principles that lie behind an IR spectrometer. There are three types of IR instruments. These are,

• Dispersive infrared spectrometers
• Fourier-transform infrared spectrometers
• Non-dispersive infrared spectrometers