Flame fluorescence spectra

Figure 13. Fluorescence spectrum for S02 and SO in a H2-02 N2 (3 1 5) flame with 1 % HsS added to the unburnt gas. Laser excitation at 266.5 nm.

The latter permits spectral scanning of the entire fluorescence spectrum using boxcar averaging. In Figure 15 is shown a comparison of the normal CH flame emission from an oxy-acetylene slot torch and the laser induced fluorescence spectrum. The CH was... 

Figure 15. Comparison of CH flame emission and laser-excited fluorescence spectrum in an oxy-acetylene slot torch...

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

The laser atomic fluorescence excitation and emission spectra of sodium in an air-acetylene flame are shown below. In the excitation spectrum, the laser (bandwidth = 0.03 nm) was scanned through various wavelengths while the detector monochromator (bandwidth = 1.6 nm) was held fixed near 589 nm. In the emission spectrum, the laser was fixed at 589.0 nm, and the detector monochromator wavelength was varied. Explain why the emission spectrum gives one broad band, whereas the excitation spectrum gives two sharp lines. How can the excitation linewidths be much narrower than the detector monochromator bandwidth ... 

Fluorescence excitation and emission spectra of the two sodium D lines in an air-acetylene flame, (a) In the excitation spectrum, the laser was scanned, (to) In the emission spectrum, the monochromator was scanned. The monochromator slit width was the same for both spectra. [From s. J. Weeks, H. Haraguchl, and J. D. Wlnefordner, Improvement of Detection Limits in Laser-Excited Atomic Fluorescence Flame Spectrometry," Anal. Chem. 1976t 50,360.]... 

Figure 19. The laser-induced fluorescence excitation spectrum of the Ct swan band system in an acetylene-air flame (21)...

Figure 22. The fluorescence excitation spectrum of the MgO B A Jn transition under conditions described in Figure 21. Rotational analysis of the spectrum demonstrates that the A 2n metastable is thermalized relative to the bulk flame temperature.

From Figures 6, 18, and 20 we see that relative fluorescence measurements for OH, SH, S2, and SO along with the method for data reduction leads to reasonable agreement with the equilibrium expectations. In Figures 19 and 21 there is a somewhat wider spread of the data about the equilibrium expectation. This is probably caused by the use of non-optimal measuring conditions and data reduction for S02 which has a very complex spectrum at flame temperatures. We are expecting a Nd-Yag laser shortly which will operate deeper into the UV than our present flash lamp pumped dye laser and will permit a more extensive characterization of SO2 fluorescence in the flame environment. 

In AFS there is no need to isolate a single wavelength in the fluorescence emission spectrum from nearby, less-intense emission wavelengths, since all lines contribute to the fluorescence signal. Therefore quite large spectral bandpasses are often employed in flame AFS, especially when a low-background flame is being used. Indeed, as seen in Chapter 2, section 14, non-dispersive, filter-based systems may sometimes be employed.7,8... 

---