Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 478 of 707

trophotometry. Optimum ranges, detection limits, and sensitivities of metals vary with the various available instruments. In direct-aspiration atomic absorption (DAAA) analysis, the flame (usually air- acetylene or nitrous oxide-acetylene) converts the sample aerosol into atomic vapor, which absorbs radiation from a light source. A light source from a hollow cathode lamp or an electrodeless discharge lamp is used, which emits a spectrum specific to the element being determined. The high cost of these lamps is a dis- advantage of the AA method. A detector measures the light intensity to give a quantitative determination. DAAA is similar to flame photometry in that a sample is aspirated into a flame and atomized. The difference between the two methods is that flame photome- try measures the amount of emitted light, whereas DAAA measures the amount of light absorbed by the atomized element in the flame. In DAAA, the number of atoms in the ground state is much greater than the number of atoms in any of the excited states of the spectroscopic methods. Consequently, DAAA is more effi- cient and has better detection limits than the spectroscopic methods. Spectral interferences occur when a wavelength of an element being ana- lyzed is close to that of an interfering element. The analysis will result in an erro- neously high measurement. To compensate for this interference, an alternate wave- length or smaller slit width is used. When the physical properties (e.g., viscosity) of a sample differ from those of the standard, matrix interferences occur. Absorption can be enhanced or sup- pressed. To overcome these interferences, matrix components in the sample and standard are matched or a release agent, such as EDTA or lanthanum, is added. Chemical interferences are the most common interferences encountered in AA analysis. They result from the nonabsorption of molecularly bound atoms in the flame. These interferences are minimized by using a nitrous oxide-acetylene flame instead of an air-acetylene flame to obtain the higher flame temperature needed to dissociate the molecule or by adding a specific substance (e.g., lan- thanum) to render the interferant harmless. Chemical interferences can also be overcome by extracting the element being determined or by extracting the inter- ferant from the sample. The sensitivity and detection limits in AA methods vary with the instrument used, the nature of the matrix, the type of element being analyzed, and the par- ticular AA technique chosen. It is best to use concentrations of standards and sam- ples within the optimum concentration range of the AA instrument. When DAAA provides inadequate sensitivity, other specialized AA methods, such as graphite furnace AA, cold vapor AA, or hydride AA, are used. In graphite furnace AA (GFAA), the flame that is used in DAAA is replaced with an electrically heated graphite furnace. A solution of the analyte is placed in a graphite tube in the furnace, evaporated to dryness, charred, and atomized. The metal atoms being analyzed are propelled into the path of the radiation beam by increasing the temperature of the furnace and causing the sample to be volatilized. Only very small amounts of sample are required for the analysis. GFAA is a very sensitive technique and permits very low detection limits. The increased sensitivity is due to the much greater occupancy time of the ground state atoms in the optical path as compared with DAAA. Increased sen- sitivity can also be obtained by using larger sample volumes or by using an argon-hydrogen purge gas mixture instead of nitrogen. Because of its extreme sen- sitivity, determining the optimum heating times, temperature, and matrix mod- ifiers is necessary to overcome possible interferences. Interferences may occur in GFAA analysis due to molecular absorption and 477

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