Two infrared gas cells are provided with KBr windows, one each for HCl and DCl. Needless to say, the windows are sensitive to moisture so that they should not be touched, and the cells should be kept in a dry environment. These must be filled with each gas to an appropriate pressure to obtain the best infrared spectrum. (Why does the pressure matter?) Rapid scans of the Fourier transform IR spectrometer at various pressures will establish the optimum pressure; then record the spectrum slowly to obtain the best S/N ratio you can in the time permitted.
The reaction used is
D2SO4(l) + 2KCl(s) -> 2DCl(g) + K2SO4(s)
and the apparatus for the preparation of DCl and for filling the gas cell is shown in Fig. 3.
The procedure is as follows:
About 6 g of solid KCl is placed in the 125 ml flask and 3 ml of D2SO4 (from a small ampoule) placed in the dropping funnel. Stopcocks 1 and 2 remain closed, but 3, 4, 5 and 6 are opened, and the system pumped out through the stopcock 6. Check for leaks. If the system is tight, close stopcock 6 and add the D2SO4 dropwise, with constant stirring, until the pressure gauge reads the desired pressure. Record the pressure and close stopcocks 4 and 5 to the gas cell. [Note: do not add all of the liquid D2SO4 since the air could enter the system through stopcock 2]. Remove the gas cell and run the infrared spectrum to see whether the DCl pressure is satisfactory to obtain an optimum spectrum.
Two gas cells, 10 cm long with 32 x 3 mm KBr windows. Nicolet 5 PC Fourier transform infrared spectrometer, 2 cm-1 resolution.
The advantages of FTIR spectroscopy are (1) large energy throughput relative to the small slits and small apertures of older conventional monochromators. (2) obtaining data at all frequencies simultaneously (multiplex advantage) instead of scanning the frequency range. These advantages result in the ability to obtain an infrared spectrum in the range of 500 to 4,000 cm-1 with a resolution of 6 cm-1 in less than 1 second of instrument time, where a comparable spectrum by older scanning methods would require perhaps 10 minutes. In FTIR an interferogram is obtained which is then converted by a Fourier transform to intensity versus frequency, i.e., to an infrared spectrum as we usually view it. (It is understood that the interferogram contains all the information contained in the spectrum but in a different form.) The conversion of the interferogram via the Fourier transform is rapidly done by computer. The integral to be evaluated is
where S(
) is the spectral function and I(x) the interferogram function.
The interferogram may be obtained by the Michelson interferometer depicted in Fig. (4).
Radiation from a hot source (globar) falls on a beam splitter which allows half of the energy to pass to the moveable mirror and half to the fixed mirror. If no sample is present, the two reflected beams then recombine and interference of the two waves leads to the interferogram measured by the detector. The moveable mirror, moving constantly back and forth through a displacement x, leads to constructive and destructive interference of the two waves. A continuous broad-band source, containing many closely-spaced frequencies, will yield a strong pattern at x=0 where all waves are in phase, but the signals will decay rapidly as the mirror displacement changes and the various waves destructively interfer. A sample placed between the interferometer and the detector absorbs some of the frequencies present, thereby modifying the interferogram. We note here also that the larger the displacement that the mirror can achieve, the higher the resolution, i.e., the better the ability of the instrument to discriminate between closely lying absorptions. However, there are practical limits to the extensive motion of the mirror.
This has, of course, been only a brief introduction to the field of Fourier transform spectroscopy which has produced dramatic improvements in infrared, Raman, and particularly nuclear magnetic resonance (NMR) spectrometry. A few references for additional reading are listed below.
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© 1996 // Innovative Teaching Labs // 3.10.1996