2.3 Specialized Sample Introduction Techniques and Analysis
2.3.5 Glow-Discharge Atomization for Solid Samples
The glow-discharge device is a highly specialized sample introduction system since it is mostly used for electrically conductive samples, such as a piece of metal or semiconductors. Like the GFAA unit it is an attachment that replaces the burner head. A sample is placed in a low-pressure argon chamber where a potential is placed between the container and the sample. Excited argon atoms sputter atoms from the surface of the sample, similar to the operation of the hollow cathode lamps. Gaseous phase metal atoms rise into the path of the source radiation and absorb their characteristic wavelength. The largest advantage of the glow discharge technique is the direct analysis of solid samples.
Fluorescence: The above discussions have mostly focused on absorption and emission processes and instruments. Recent advances in atomic fluorescence spectrometry make this technique possible for a few elements (mercury, arsenic, selenium, tellurium, antimony and bismuth). Again, fluorescence occurs when an electron is excited to a higher electronic state and decays by resonance, direct line, or stepwise fluorescence (refer to Section 1.2.2). Instrument components are similar to those discussed above and in the next chapter, but the key difference is that the source lamp is located at a 90° angle with respect to the detector in order to prevent the source radiation from being measured by the detector. Lamps used to excite electrons include hollow cathode lamps, electrode-less discharge lamps, lasers, plasmas, and xenon arc lamps. Atomizers include flames, plasmas, electrothermal atomizers, and glow discharge chambers; thus samples can be introduced as cold vapors, liquids, hydrides, and solids. Sub-parts per billion detection limits are obtainable from these instruments.
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