The principal advantage of flame atomization is the reproducibility with which the sample is introduced into the spectrophotometer. The signal for flame microsampling is a transitory peak whose height or area is proportional to the amount of analyte that is injected.Īdvantages and Disadvantages of Flame Atomization. Dip sampling is usually accomplished with an automatic sampler. Flame microsampling is accomplished using a micropipet to place 50–250 μL of sample in a Teflon funnel connected to the nebulizer, or by dipping the nebulizer tubing into the sample for a short time. For example, continuously aspirating a sample that has a high concentration of dissolved solids-sea water, for example, comes to mind-may build-up a solid deposit on the burner head that obstructs the flame and that lowers the absorbance. Continuous aspiration is sample intensive, typically requiring from 2–5 mL of sample.įlame microsampling allows us to introduce a discrete sample of fixed volume, and is useful when we have a limited amount of sample or when the sample’s matrix is incompatible with the flame atomizer. Sample Introduction.The most common means for introducing samples into a flame atomizer is a continuous aspiration in which the sample flows through the burner while we monitor the absorbance.
The relative size of each zone depends on many factors, including the choice of fuel and oxidant, and their relative proportions. 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.
CALCIUM HOLLOW CATHODE LAMP 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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