As atoms approach the flame’s secondary combustion zone, the decrease in temperature allows for formation of stable molecular species.įigure 10.44 Profile of typical flame using a slot burner. The hottest part of the flame is typically 2–3 cm above the primary combustion zone.
Absorption spectra of hydrogen and boron free#
The interzonal region generally is rich in free atoms and provides the best location for measuring atomic absorption. The primary combustion zone is usually rich in gas combustion products that emit radiation, limiting is usefulness for atomic absorption. Table 10.9 Fuels and Oxidants Used for Flame Combustionįigure 10.44 shows a cross-section through the flame, looking down the source radiation’s optical path. Normally the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be necessary for easily oxidized analytes. Of these, the air–acetylene and the nitrous oxide–acetylene flames are the most popular. The flame’s temperature, which affects the efficiency of atomization, depends on the fuel–oxidant mixture, several examples of which are listed in Table 10.9. Other atoms show concentration profiles that maximize at a characteristic height.įigure 10.43 Absorbance versus height profiles for Ag and Cr in flame atomic absorption spectroscopy.įlame. For metals, such as Ag, which are difficult to oxidize, the concentration of free atoms increases steadily with height (Figure 10.43). For an easily oxidized metal, such as Cr, the concentration of free atoms is greatest just above the burner head. On the other hand, a longer residence time allows more opportunity for the free atoms to combine with oxygen to form a molecular oxide. The more time the analyte spends in the flame the greater the atomization efficiency thus, the production of free atoms increases with height. This is important because two competing processes affect the concentration of free atoms in the flame. Vertical adjustments adjust the height within the flame from which absorbance is monitored. Horizontal adjustments ensure that the flame is aligned with the instrument’s optical path. The burner is mounted on an adjustable stage that allows the entire assembly to move horizontally and vertically. A stable flame minimizes uncertainty due to fluctuations in the flame. Because absorbance increases linearly with the path length, a long path length provides greater sensitivity. The slot burner in Figure 10.42a provides a long optical pathlength and a stable flame. Although the unit shown here is from an older instrument, the basic components of a modern flame AA spectrometer are the same.īurner. \)Ĭompressed air is one of the two gases whose combustion produces the flame.įigure 10.42 Flame atomization assembly with expanded views of (a) the burner head showing the burner slot where the flame is located (b) the nebulizer’s impact bead and (c) the interior of the spray chamber.
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