Spectrograph triple

Ultrapure (triple distilled) mercury is commonly tested by evaporation or spectrographic analysis. In the former, a composite sample is evaporated and the residue weighed. In spectrographic analysis, a sample is dissolved and evaporated, the residue mixed with graphite [7782-42-5] and the emission spectmm determined with a spectrograph. 

A detachable monochromator (19) developed by Spex Industries, was another approach in minimizing stray light. It is a modified Czerny-Turner spectrograph which can be coupled to the exit slit of a double monochromator and function as a variable bandpass, variable frequency filter. This accessory, while providing the versatility of a triple monochromator, does not add much mechanical and optical complexity and can be removed when not wanted. 

Materials. The UFe used in this work was a portion of a larger batch originally obtained from Oak Ridge National Laboratory. Almost two-thirds of the original batch had been distilled away in previous experimental work, presumably contributing to the purification of the UFe from low boiling impurities e.g., HF, CF4, F2). Emission-spectrographic analysis of the material indicated that the predominant impurities were P, at a concentration of <400 p.p.m., and As, B, Cs, Pd, Re, Sb, Sn, and Th, each present at concentrations of <100 p.p.m. Two determinations of the triple point of a sample of the UFe yielded values of 64.1 °C. and 64.2°C. The best literature value 19) for this is 64.05°C. 

The single grating instrument depicted in Figure 8.3 is very common in optical spectroscopy (1), but its application to Raman is relatively recent because of inadequate stray light rejection. The introduction of effective laser rejection filters (Section 8.2.5) removed this limitation and single spectrographs became attractive. They are simpler and more efficient than double or triple systems and usually much more compact. Variations of the classical Czerny-Turner... 

Figure 8.12. Schematic of a triple spectrograph, consisting of a subtractive dispersion double monochromator preceding a single spectrograph. Similar to Spex Triplemate" or Dilor X-Y. ...

Figure 8.14. Spectrum of solid sulfur obtained with a single spectrograph and holographic laser rejection filter (upper) or a triple spectrograph (lower).

Typical performance tests of one of these systems were published in 1985 [28] and 1987 [29]. Comparisons were made with a double 1-m spectrograph in use at the time. Spectra recorded on a triple with a PMT had slightly degraded spectral resolution (relative to a scanning double), but the multichannel advantage implicit in the technique was clearly... 

The technology of these filters will certainly be covered more completely in a later chapter in this book, so details will not be covered here. However, it is impossible to overemphasize the importance of this innovation. The double subtractive premonochromator that functioned as a laser filter in triple systems became necessary only in cases when it was required to acquire signals closer to the laser line than the holographic filter will allow (effectively closer than 75-100 cm ), or when the laser excitation was tuned. Consequently, elimination of the filter stage immediately decreased the instrument size. It also became clear that the focal length of spectrographs did not need to be 0.5 m or larger, especially when the spectra were to be excited with a red laser. 

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