Burner graphite furnace

Figure 14.16—Elements determined by AAS or FES. Most elements can be determined by atomic-absorption or flame emission using one of the available atomisation modes (burner, graphite furnace or hydride formation). Sensitivity varies enormously from one element to another. The representation above shows the elements in their periodic classification in order to show the wide use of these methods. Some of the lighter elements, C, N, O, F, etc. in the figure can be determined using a high temperature thermal source a plasma torch, in association with a spcctropholometric device (ICP-AbS) or a mass spectrometer (1CP-MS).

Figure 13.18 Elements measured by AAS and FES. Most elements can be measured by atomic absorption or flame emission by using one of the available modes of atomization (burner, graphite furnace or device for hydride formation). The sensitivity varies from several ppb (Cu, Cd, Cr) to several ppm (the lanthanides). The elements of the table (in white) for which the atomic number is not shown are not measurable by atomic absorption. However, the hybrid apparatus AAS/OES containing plasmas as a thermal source, has more recently pushed back the limits of this method of elemental analysis.

Instruments that have burners and require nebulisation of dilute aqueous sample solutions generally have low background noise in the signal. With graphite furnaces, incomplete atomisation of the solid sample at elevated temperatures can produce interfering absorptions. This matrix effect does not exist in an isolated state and thus cannot be eliminated by comparison with a reference beam. This is notably the case for solutions containing particles in suspension, ions that cannot be readily reduced and organic molecules, all of which create a constant absorbance in the interval covered by the monochromator. 

Several types of atomization cell are available flame, graphite furnace, hydride generation and cold vapour. Flame is the most common. In the premixed laminar flame, the fuel and oxidant gases are mixed before they enter the burner (the ignition site) in an expansion chamber. The more commonly used flame in FAAS is the air-acetylene flame (temperature, 2500 K), while the nitrous oxide-acetylene flame (temperature, 3150K) is used for refractory elements, e.g. Al. Both are formed in a slot burner positioned in the light path of the HCL (Fig. 27.4). 

Instrument detection limits (IDLs) for most metals by FIAA are in the low-ppm realm in contrast to graphite furnace AA (GFAA). The conventional premixed chamber-type nebulizer burner is common. The sample is drawn up through the capillary by the decreased pressure created by the expanding oxidant gas at the end of the capillary, and a spray of fine droplets is formed. The droplets are turbulently mixed with additional oxidant and fuel and pass into the burner head and the flame. Large droplets deposit and pass down the drain 85-90% of the sample is discarded in this way. Figures 10-15 in Ref. 2 (pp. 216-218) provides a good schematic of the laminar flow burner. 

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

Muffle Furnaces or Retorts of Graphite or Silicon Carbide. The metal is fed into the furnace either batchwise as a solid or continuously as a liquid. The heat of vaporization is supplied by heating the outside of the retort with a burner. The nonvolatile residues (iron and lead in the case of dross from smelting) accumulate in the retort and must be removed at intervals. This is facilitated by tipping the retorts. 

This process is strongly exothermic (flame temperature > 2000°C) and is especially used when particularly pure hydrogen chloride (hydrochloric acid) is required e.g. in the food sector. It places considerable demands on the construction materials of the plant, particularly that of the burner for which quartz or graphite is preferred. The synthesis furnace and the adjacent cooler can be constructed of steel when dry chlorine and dry hydrogen are used. 

The rotary furnace has been used in non-ferrous melting for many years. In this application traditional oil-air burners can provide the relatively low melting temperatures. The development of oxygen-air burners has enabled the introduction of cast iron production, using a higher relative amount of steel scrap and applying graphite for carburisation. 

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