![]() ![]() 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. Increasing the current too much can also result in a shorter lifetime of the lamp, as well as melting the metal on the cathode thus ruining the lamp.\)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). The process of self adsorption results in weaker intensities. These unexcited atoms can interfere with good output data because they are able to absorb the radiation that is emitted from the excited electrons returning to their ground state. These higher currents allow for more intense data output but at the same time can produce a number of unexcited atoms in the cloud. When a high voltage is applied, higher currents arise. How well the HCL works and how long it lasts is dependent on the care of operation. The material of the window is specifically selected in order to get the best transmission of spectral lines for the cathode element. Contamination of the cathode compromises the purity of the metal of interest and the data obtained for that metal. It is important that the cathode be stored under a vacuum to avoid any kind of contamination. The cathode is made of the pure metal of interest or a mixture of metals containing the metal of interest. ![]()
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