Flame cells

Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

Note The number of cells in a quarter of a section of each of 10 plerocercoids was counted. The percentages of flame cells associated with the calcareous corpuscles were determined from separate sections. The plerocercoids were treated for 5h with colchicine at 38°C. 

Fig. 10 Illustration of a microburner developed by Masel et al. (A) cross section of alumina burner (B) microburner operation and (C) flame cells observed from top of microreactor. (View this art in color at www. dekker.com.)...

Sensitized Fluorescence. In this type of fluorescence, an atom emits radiation after collisional activation by a foreign atom that was excited previously by absorbing resonance radiation, but which has not yet been deactivated again. An example is the sensitized fluorescence of thallium atoms in a gas mixture containing a high pressure of mercury vapor and a low pressure of thallium vapor. When irradiated at the 253.65-nm mercury line, the thallium atoms emit at 377.57 and 535.05 nm. This type of fluorescence requires a higher concentration of foreign atoms than can be obtained in flame cells, but presumably it could be observed in nonflame cells. 

The simplest arrangement to obtain atomic absorption data dispenses with the chopper, uses a hollow cathode powered by a dc source, and a dc amplifier. Such a unit will receive a dc signal from the flame cell due to spectral emission lines. Accurate readings of the absorption signal thus depend on obtaining a steady emission state in the flame as well as a steady absorption state. 

The neutral atom distribution is quite variable in the flames of both premixed and total-consumption burners. Therefore the maximum absorption signal from a flame cell will only be obtained if the optical path traverses... 

It should be obvious that no flame will meet all the above requirements in practice, therefore, an attempt is made to approach this ideal situation as closely as possible. Present practice is to adapt flame cells in use for flame emission and atomic absorption for use in atomic fluorescence spectroscopy. 

Flame background or a continuum is a third type of spectral interference. It also can be controlled by use of an ac detection system. Random noise in the flame cell, since it contains a large number of ac components, cannot be entirely eliminated by use of an ac electronic system but the magnitude of this effect can be reduced substantially by use of an ac system, preferably the tuned (lock-in) type of system. 

The procedure to obtain the fluorescence intensity for the desired element after the preliminary adjustments have been made depends in part on the type of equipment in use. If an ac system with a chopper or electrically modulated source is in use and is properly adjusted, any dc signal component of the flame cell can be ignored since the amplifier will not respond to the dc signal. This includes any continuum and any thermally excited spectral lines within the band pass of the monochromator. 

Flame atomic absorption was until recently the most widely used techniques for trace metal analysis, reflecting its ease of use and relative freedom from interferences. Although now superceded in many laboratories by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, flame atomic absorption spectrometry still is a very valid option for many applications. The sample, usually in solution, is sprayed into the flame following the generation of an aerosol by means of a nebulizer. The theory of atomic absorption spectrometry (AAS) and details of the basic instrumentation required are described in a previous article. This article briefly reviews the nature of the flames employed in AAS, the specific requirements of the instrumentation for use with flame AAS, and the atomization processes that take place within the flame. An overview is given of possible interferences and various modifications that may provide some practical advantage over conventional flame cells. Finally, a number of application notes for common matrices are given. 

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