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The Fourier transform IR (FTIR) spectrometer works on an entirely different principle, and involves much simpler instrumentation but far more complicated data processing. In addition the FTIR spectrometer can scan a sample far more rapidly than the dispersive instrument. The basic difference is that the dispersion instrument scans the sample one wavelength at a time, whereas the FTIR spectrometer examines the sample using all the wavelengths coincidentally. A diagram of the basic system is shown in figure 10.
Collimated light from a broadband infrared source passes into the optical system and impinges on a Beam Splitter that comprises a very thin film of germanium. Approximately 50% of the light passes through the film and is reflected back along its path by a fixed mirror, where half of the light intensity (25% of the original light intensity) is reflected by the same Beam Splitter, through the sample cell, to the infrared sensor. The other 50% fraction of the incident light is reflected at right angles to its incident path onto a moving mirror. Light from the moving mirror returns along its original path and half of the light intensity is transmitted through the Beam Splitter, through the sample cell, to the infrared sensor. As a result, 25% of the incident collimated light from the source reaches the sensor from the fixed mirror and 25% from the movable mirror. Now the path length of the two light beams striking the sensor will be different so there will be destructive and constructive interference. Actually, the system constitutes a form of the Michelson interferometer.
As the movable mirror traverses it’s programmed path and constructive and destructive interference takes place, a series of maxima and minima signals will be monitored by the sensor. It is also seen that the frequency of this waveform will be determined by the velocity of the moving mirror, which is experimentally controllable. This results in the interferometer actually taking a Fourier transform of the incoming signal. An example of an Interferogram obtained from the FTIR is shown in figure 11.
The resolution of the Interferogram does not appear to be very good, and an inverse Fourier transform of the numerical data by the computer might not be expected to give a conventional IR Spectrum with satisfactory resolution. Actually, one scan from an FTIR instrument (taking about a second) can give an conventional IR Spectrum with a resolution equivalent to that obtained by a dispersive instrument when scanned over a period of 10 to 15 minutes. In addition, the resolution of the Interferogram can be further improved by taking replicate scans. The scans are added together and the sum is processed. This, procedure significantly increases the signal to noise by a factor equivalent to the square root of the number of scans. For example, 16 accumulated scans would increase the resolution by a factor of four. This, of course, is achieved at the expense of time but as the scans are fast this is not a significant disadvantage.
The accumulation of spectra also increases the sensitivity of the device, which is useful if there is limited sample. Again, as a single scan is complete in about a second, in order to double the sensitivity, the total scanning time would be increased to only four seconds.
About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.