energies and the detection is relatively inefficient. This has precluded the full usage of the
power of the analysis using complex materials isolated from the environment.
The advent of fast computers has made possible a new type of infrared spectral analysis.
This has made it possible to utilize the far infrared portions of the spectrum, to follow
rapid reaction rates with changes in spectral intensity, and to utilize different types of
sample exposures such as the photoacoustic spectroscopy. The secret lies in the array
processor computers that can perform Fourier transformations so rapidly that interference spectroscopy can be possible.
The FT/IR has several advantages over conventional IR spectroscopy.
1. The Fellgett advantage results from the fact that the entire spectrum passes through
the sample during the entire analytical interval. The spectrum is generated by the
interference between one portion of a split beam that is “retarded” in that it is reflected
from a vibrating mirror. In this way the entire interferogram is allowed to impinge on the
sample throughout the entire analytical interval. Conventional spectrophotometers
create a beam that scans the sample with a series of wavelenghts. In these instruments the
signal from each wavelength interval occurs for only a small portion of the analytical
interval whereas the noise continues to be generated throughout the entire analytical
interval. In the FT/ IR both the signal and the noise occupy the entire analytical interval.
Under the conditions of continuous analysis by the entire spectrum, during the time a
signal of amplitude n is generated, the random noise of n
1/2 is also generated. This means
that quadrupling the number of scans doubles the signal to noise ratio. This is the Fellgett
advantage.
2. In the throughput or Jacquinot advantage the FT/IR utilizes the whole beam.
Conventional spectrophotometers focus a narrow slit of light on the diffracting engine to
create the analytical beam. Thus only a small portion of the light entering the monochromator is utilized in the analytical beam. The high throughput of the FT/IR means that a
smaller beam can be utilized and thus a smaller portion of the sample can be analyzed.
The beam of the Nicola 60 SX in our laboratory is 3 mm in diameter.
The combination of the Fellgett and Jacquinot advantages in the FT/IR gives an increase
in the signal to noise ratio in the mid-IR spectral range of 2.4 orders of magnitude
(Griffiths, 1975, 1983).
3. The signal generated by the FT/IR interferogram is actually produced in the time
domain. To transform the signal into the frequency space in which spectra are usually
perceived requires a Fourier transformation. Although this step prevented the utilization
of interference spectroscopy until the advent of modern computers, it is actually an
advantage. Since data described in Fourier space are in general expressed as successive
approximations in terms of sines and phase (cosines) which are mathematically well
behaved, simple mathematical manipulations on data in Fourier space can be utilized to
correct for baseline shifts, overlapping, apodization, etc. that would require complex
mathematical manipulations in frequency space if they were even possible.
4. For the interferograms to be transformed from Fourier to frequency domains the
signal is digitized. This digitization is achieved using equal intervals of optical path
difference using the sinusoidal interferogram from a laser beam focused on another part
of the mirrors. The interferogram is digitized once per wavelength of the laser interferogram at the zero crossing which gives an extraordinary spectral resolution. Our instrument provides a spectral resolution that is continuously variable between 100 cm
-1
and
0.25 cm - 1 . The best conventional IR spectrometers formerly gave resolutions of about
5 cm -1 . The extremely high resolution greatly increases the information content of the
213
power of the analysis using complex materials isolated from the environment.
The advent of fast computers has made possible a new type of infrared spectral analysis.
This has made it possible to utilize the far infrared portions of the spectrum, to follow
rapid reaction rates with changes in spectral intensity, and to utilize different types of
sample exposures such as the photoacoustic spectroscopy. The secret lies in the array
processor computers that can perform Fourier transformations so rapidly that interference spectroscopy can be possible.
The FT/IR has several advantages over conventional IR spectroscopy.
1. The Fellgett advantage results from the fact that the entire spectrum passes through
the sample during the entire analytical interval. The spectrum is generated by the
interference between one portion of a split beam that is “retarded” in that it is reflected
from a vibrating mirror. In this way the entire interferogram is allowed to impinge on the
sample throughout the entire analytical interval. Conventional spectrophotometers
create a beam that scans the sample with a series of wavelenghts. In these instruments the
signal from each wavelength interval occurs for only a small portion of the analytical
interval whereas the noise continues to be generated throughout the entire analytical
interval. In the FT/ IR both the signal and the noise occupy the entire analytical interval.
Under the conditions of continuous analysis by the entire spectrum, during the time a
signal of amplitude n is generated, the random noise of n
1/2 is also generated. This means
that quadrupling the number of scans doubles the signal to noise ratio. This is the Fellgett
advantage.
2. In the throughput or Jacquinot advantage the FT/IR utilizes the whole beam.
Conventional spectrophotometers focus a narrow slit of light on the diffracting engine to
create the analytical beam. Thus only a small portion of the light entering the monochromator is utilized in the analytical beam. The high throughput of the FT/IR means that a
smaller beam can be utilized and thus a smaller portion of the sample can be analyzed.
The beam of the Nicola 60 SX in our laboratory is 3 mm in diameter.
The combination of the Fellgett and Jacquinot advantages in the FT/IR gives an increase
in the signal to noise ratio in the mid-IR spectral range of 2.4 orders of magnitude
(Griffiths, 1975, 1983).
3. The signal generated by the FT/IR interferogram is actually produced in the time
domain. To transform the signal into the frequency space in which spectra are usually
perceived requires a Fourier transformation. Although this step prevented the utilization
of interference spectroscopy until the advent of modern computers, it is actually an
advantage. Since data described in Fourier space are in general expressed as successive
approximations in terms of sines and phase (cosines) which are mathematically well
behaved, simple mathematical manipulations on data in Fourier space can be utilized to
correct for baseline shifts, overlapping, apodization, etc. that would require complex
mathematical manipulations in frequency space if they were even possible.
4. For the interferograms to be transformed from Fourier to frequency domains the
signal is digitized. This digitization is achieved using equal intervals of optical path
difference using the sinusoidal interferogram from a laser beam focused on another part
of the mirrors. The interferogram is digitized once per wavelength of the laser interferogram at the zero crossing which gives an extraordinary spectral resolution. Our instrument provides a spectral resolution that is continuously variable between 100 cm
-1
and
0.25 cm - 1 . The best conventional IR spectrometers formerly gave resolutions of about
5 cm -1 . The extremely high resolution greatly increases the information content of the
213