Prism Instruments

Until a few years ago dispersive (grating or prism) instruments were used to acquire infrared spectra. These instruments were relatively slow and insensitive, which made recording spectral information on-the-fly frmi chromatograpbic instruments all but impossible except under conditions which severely compromised the performance of the chi tograph. This... 

Optical emission spectroscopy by grating instruments by prism instruments by Quantometers 200,000 10,000 60... 

The method just described is not usually applicable in the ultraviolet because ultraviolet lamps of known spectral distribution are not readily available at present. The spectral sensitivity caii be calculated directly if the values of B L and P, are known. The first of these is obtained from the dispersion curve of the monochromator the second is somewhat difficult to measure—for prism instruments over restricted wavelength regions above 250 m t it is often reasonably constant. The photomultiplier sensitivity, P can be determined by comparison with a thermopile or with the ferrioxalate actinometer.11 12 Direct calculation of S, is subject to inaccuracies due to the accumulation of errors in the measurement of the three separate quantities B L and P,. A more convenient... 

TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary energy of 25 keV, a pulse width of 10-50 ns, and a total integrated ion dose of about 10" ions/cm2. This is well below the generally accepted upper limit of 5 x 1012 ions/cm2 for static SIMS conditions in the analysis of organic materials [12], The mass resolution at mass 50 amu varied from M/AM= 1000 at 50 ns pulse width to about 2500 at 10 ns pulse width. 

Since the early 1950s, 1R spectroscopy has been a routine analytical tool for lignin chemists. In the past, spectra were recorded using the so-called dispersive technique, i.e., with grating-type or prism instruments. In the last decade, Fourier transform infrared (FT1R) spectrometers have become increasingly available for routine laboratory work. 

During the 1960s further improvements made infrared spectroscopy a very useful tool used worldwide in the analytical routine laboratory as well as in many fields of science. Grating spectrometers replaced the prism instruments due to their larger optical conductance (which is explained in Sec. 3 of this book). The even larger optical conductance of interferometers could be employed after computers became available in the laboratory and algorithms which made Fourier transformation of interferograms into spectra a routine. The computers which became a necessary component of the spectrometers made new powerful methods of evaluation possible, such as spectral subtraction and library search. 

The first dispersive infrared instruments employed prisms made of materials such as sodium chloride. The popularity of prism instruments fell away in the late 1960s when the improved technology of grating construction enabled cheap, good quality gratings to be manufactured. 

The experimental technique involves the use of a high-resolution instrument, one with better resolving power than the usual sodium chloride prism instrument. The solvent commonly used is carbon tetrachloride (sometimes carbon disulfide), both of which have practically no absorption in the 3300 cm region. Amino alcohols cannot be studied in these solvents, however, since they slowly react with the solvents. Tetra-chloroethylene is quite suitable for these compounds, since they are stable in it for extended periods at room temperature. 

The fiftieth edition of the Handbook of Chemistry and Physics (1969-1970) contains a spectra index of organic compounds. The index is a listing of the Sadtier Standard Spectra recorded with an infrared prism instrument. The index also contains the Sadtier infrared grating, ultraviolet, and NMR numbers of many compounds listed in the Table of Physical Constants of Organic Compounds of the Handbook. 

The Fellgett or multiplex advantage deals with the fact that a Fourier transform spectrometer records data from the entire spectral region throughout the experiment. This is quite different to the case with a dispersive spectrometer, as the grating or prism instrument only measures a narrow bandwidth at any time. The measurement bandwidth of the dispersive spectrometer is regulated by the instrument s exit slit. This difference has important effects on the acquisition of data. 

Two basic designs of prism spectrometer are commonly used for spectrochemical analytical purposes, the Cornu and the Littrow types. Many of these instruments are presently in use, although grating spectrometers are gradually replacing prism instruments. 

Linear interpolation can be used with grating spectrometers operated in the normal position. It also can be used to obtain approximate wavelengths with prism instruments if the wavelength difference between the two known spectral lines is small. For prism instruments interpolation is possible using the Hartmann formula ... 

Atomic fluorescence spectra are simple, with relatively few spectral lines therefore monochromators of extremely high resolution are not required. Either grating or prism instruments may be used. A monochromator... 

As a general rule, there is more to gain from the use of programmed scan in prism instruments than in grating instruments. The reason for this is the larger variation in resolution inherent in prism instruments. In grating instruments with linear frequency abscissa presentation, the variation in spectral slit width in frequency units is generally no more than 2 1. 

Figure 2-18. Resolution of a polystyrene film sample on an NaCl prism instrument.

Also, for the less versatile instruments the resolution for a particular wavelength may be quoted. Experience serves to indicate the implied resolution throughout the region. Such extrapolation is easier for a grating instrument because, it may be recalled, the resolution of grating instruments varies much less than that of prism instruments. In some cases a plot of spectral slit width versus wavelength or wavenumber has been published for the instrument in the scientific literature or in manufacturers brochures. This is also true of the more versatile instruments in reference to their standard survey conditions. 

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