Shows a cross-section through the flame, looking down the source radiation’s optical path.
Fuels and Oxidants Used for Flame Combustion fuel 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 10.4.1 Absorbance versus height profiles for Ag and Cr in flame atomic absorption spectroscopy.įlame.
CALCIUM HOLLOW CATHODE LAMP FREE
For a metal, such as Ag, which is difficult to oxidize, the concentration of free atoms increases steadily with height. , for a metal this is easy to oxidize, 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 an 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 change the height within the flame from which absorbance is monitored. Horizontal adjustments ensure 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 is directly proportional to pathlength, a long pathlength provides greater sensitivity. Although the unit shown here is from an instrument dating to the 1970s, the basic components of a modern flame AA spectrometer are the same.Ī provides a long optical pathlength and a stable flame. 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. The flame’s thermal energy then volatilizes the particles, producing a vapor that consists of molecular species, ionic species, and free atoms. The aerosol mist is swept through the spray chamber by the combustion gases-compressed air and acetylene in this case-to the burner head where the flame’s thermal energy desolvates the aerosol mist to a dry aerosol of small, solid particulates. When the sample exits the nebulizer it strikes a glass impact bead, which converts it into a fine aerosol mist within the spray chamber. In the unit shown here, the aqueous sample is drawn into the assembly by passing a high-pressure stream of compressed air past the end of a capillary tube immersed in the sample. Shows a typical flame atomization assembly with close-up views of several key components. On the other hand, if our interest is biologically available metals, we might extract the sample under milder conditions using, for example, a dilute solution of HCl or CH 3COOH at room temperature. This destroys the sediment’s matrix and brings everything into solution. If we need to know the total amount of metal in the sediment, then we might try a microwave digestion using a mixture of concentrated acids, such as HNO 3, HCl, and HF. What reagent we choose to use to bring an analyte into solution depends on our research goals. For this reason, only the introduction of solution samples is considered in this chapter. When analyzing a lake sediment for Cu, Zn, and Fe, for example, we bring the analytes into solution as Cu 2 +, Zn 2 +, and Fe 3 + by extracting them with a suitable reagent. If the sample is a solid, then we must bring the analyte into solution before the analysis. In most cases the analyte is in solution form. There is, however, an important additional need in atomic absorption spectroscopy: we first must covert the analyte into free atoms. \)Ītomic absorption spectrophotometers use the same single-beam or double-beam optics described earlier for molecular absorption spectrophotometers (see Figure 10.3.2 and Figure 10.3.3).
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