I write this article based on my personal experience as I had an opportunity to use both systems at start of my career. IR spectroscopy is a useful tool for characterization of a compound based on the presence of functional groups in the molecule. Such functional groups have characteristic absorption wavelengths in the IR region and the presence of such groups is easily identified by such characteristic absorptions.
FT-IR offers significant advantages in terms of speed and sensitivity which has resulted in dispersive instruments becoming obsolete. Those of you who have worked on earlier dispersive instruments will remember that a single spectral scan would take about 10 – 12 min and the graph would be generated on the printer peak by peak. FT-IR technique offered several benefits which contributed to its popularity.
High sample throughput – the biggest advantage is high sample throughput. In comparison to the time taken by dispersive instruments the complete spectrum is scanned and displayed in a matter of seconds.
Improvement in sensitivity – multiple spectra can be collected and co-added in given time, say 1 min. On averaging noise which is random in nature gets reduced significantly whereas signal increases and results in higher signal to noise ratio thereby increasing sensitivity. The increase in sensitivity can be as high as 100 fold
Increased energy throughput – higher sensitivity due to absence of energy limiting slits results in high energy throughputs in interferometer based FT-IR design.
Auto calibration of wavelength – use of He-Ne laser as an internal reference provides auto calibration of frequencies to better than 0.01 cm-1. This obviates the need for external calibration of wavelengths
Elimination of stray light – the Michelson interferometer modulates all frequencies simultaneously so there is no equivalent of stray light as in dispersive instruments
Low maintenance – the system consists of: one moving part only, that is, the moving mirror. On the other hand there are several moving parts in dispersive systems. This results in greater system reliability and less of maintenance due to reduced wear and tear
Constant resolution – resolution is uniform at all the wavelengths as there areno variable split programs which was a common feature in dispersive instruments
Freedom from spectral discontinuities – as there are no gratings or filter changes there are no discontinuities in the resulting spectrum
Positive Compound identification – the spectra can be overlaid and compared through software with in-house or customized libraries which helps in confirmation of compound identity
The above benefits make it possible to make use of a number of accessories for handling a range of samples such as liquids, solids and gases using available accessories which was not possible with earlier dispersive instruments. A subsequent article will explain the principle of operation of FTIR spectrometer for better appreciation of advantages of FT-IR systems over dispersive systems.