Burner heads absorption

Flame Sources Atomization and excitation in flame atomic emission is accomplished using the same nebulization and spray chamber assembly used in atomic absorption (see Figure 10.38). The burner head consists of single or multiple slots or a Meker-style burner. Older atomic emission instruments often used a total consumption burner in which the sample is drawn through a capillary tube and injected directly into the flame. 

Atomic absorption spectrometer with air-acetylene burner head. Pressurized acetylene cylinder. Air compressor. 

If mercury ions in solution are reduced to the free element, and a current of air or inert gas is passed through the solution, mercury vapour, which is monatomic, will be swept out of the solution into the gas phase. This provides a very sensitive basis for the determination of this toxic element.1 The apparatus required is illustrated in Figure 1. The flame is replaced by a glass tube atom cell with silica end windows in atomic absorption. Usually, for convenience, the atom cell is clamped to the top of a conventional AAS burner head. If atomic fluorescence is... 

Procedure Use an atomic absorption spectrophotometer equipped with a 4-in., single-slot burner head. Set the instrument to previously determined optimum conditions for organic solvent aspiration (3 to 5 mL/min) and at a wavelength of 283.3 nm Use an air-acetylene flame adjusted for maximum lead absorption with a fuel-lean flame. Aspirate the blanks, the Standard Solutions, and the Sample Solution, flushing with water and then with Aqueous Butyl Acetate between... 

Procedure Use a Perkin-Elmer 403 atomic absorption spectrophotometer equipped with a deuterium arc background corrector, a digital readout device, and a burner head capable of handling 20% solids content. Blank the instrument with water following the manufacturer s operating instructions. Aspirate a portion of the Standard Preparation, and record the absorbance as As then aspirate a portion of the Sample Preparation, and record the absorbance as Av. Calculate the lead content, in milligrams per kilogram, of the sample taken by the formula... 

Procedure Concomitantly determine the absorbances of the Sample Blank, the Diluted Standard Lead Solutions, and the Sample Preparation at the lead emission line of 283.3 nm, using a slit-width of 0.7 nm. Use a suitable atomic absorption spectrophotometer equipped with a lead electrodeless discharge lamp (EDL), an air-acetylene flame, and a 4-in. burner head. Use water as the blank. 

The technique of flame atomic absorption spectrophotometry accomplishes this by aspirating the sample solution into a burner chamber, where it is mixed with a fuel gas and an oxidant gas. The mixture is then burned in a specially designed burner head (Fig. 2). The light beam is directed lengthway down the burner, and the absorption of the analyte atoms in the flame is measured. The most commonly used gas mixtures are air with acetylene and nitrous oxide with acetylene. Experimental conditions are well-defined in the literature, and cookbook conditions are available from most instrument manufacturers. In addition, many instruments are computer-controlled, and typical conditions are available directly on the operating screen. 

In the atomic absorption procedure 5-10 mg of organoantimony compound is dissolved in 5 ml of butan-2-one, acetone, dimethoxyethane or THF depending on the solubility of the organoantimony compound. Concentrated hydrochloric acid and ethanol are added and the mixture diluted with water. The solution is aspirated into a fuel-rich air-hydrogen or air-acetylene flame and the absorbance measured at 217.6 nm in the flame just above the burner head. The calibration graph is slightly curved over the range... 

Atomic absorption instrumentation. Perkin-Elmer model 403 or equivalent instrument, equipped with deuterium background corrector, arsenic hollow cathode lamp, triple-slot burner head and a strip chart recorder. 

Heated Chamber Burner. One approach, which is now being offered commercially, is a burner in which the mixing chamber is heated to a temperature between 300 and 500°C. by infrared radiation. After introduction, the sample is converted into a vapor. It then passes into a cooling chamber, where the steam is condensed and allowed to flow out of a drain tube. In the ideal case, only the solid components of the sample are passed into the burner head. The heated chamber system overcomes the previously noted factor that standard premix burners are only 5% to 10% eflScient. By being able to use all of the sample that had been introduced, the heated chamber burner can produce ten to twenty times higher absorption for a given concentration. 

If the burner head Is rotated to reduce sensitivity, we find that the limiting noise Is no longer flame transmission flicker, but source shot noise, since the absorption path has been reduced by a factor of 20. Although the sensitivity Is decreased by a factor of 20, the detection limit Is decreased by only a factor of 10, since the flame transmission noise Is no longer limiting. Thus, referring back to a statement made earlier, sensitivity, or more correctly characteristic concentration [18] cannot be used as an accurate measure of detection limit in AAS. Unlike the case of SBR in emission, because of the complexity of noises In atomic absorption, a general and simple relationship cannot be derived to relate characteristic concentration and detection limit. 

Flame atomisation In flame atomisation, the sample solution is introduced into the flame with a particularly designed nebuHser (Fig. 12.6). The function of the nebuHser is to disrupt the continuous sample stream into a mist of fine droplets of typically 5—20 pm diameter which are swept into the mixing chamber. The aerosol is then mixed with the fuel gas and the oxidant gas before reaching the burner head. As a number of physical and chemical reactions, e. g., vaporisation, dissociation, reduction, or oxidation, may occur, it becomes evident that precise control of the operating conditions of flame atomisation is required to obtain stable and sensitive signals. Optimisation of flame atomic absorption measurements has thus the double role of maximising the elemenf s response while minimising the undesired side-reactions. 

The burner head is constructed in the form of a long narrow slit ( laminar flow burner ). The long slit through which the flame expands (typically 10 or 5 cm) increases the absorption path length and thus also the sensitivity. 

Amos and Willis suggested another combination of fuel and oxidant, acetylene and nitrous oxide, as another approach to the analysis of refractory elements. The combination of acetylene and nitrous oxide produces a high-temperature flame (2950°C) with little free oxygen to react with the metal elements. This flame is now very successfully used in atomic absorption spectroscopy and permits satisfactory atomic absorption analysis for many refractory elements. Use of nitrous oxide and acetylene requires a burner head that will withstand the temperatures produced by the flame. A common burner head for this combination of fuel and oxidant is 5 cm long and 0.05 cm wide. 

The most common fuel-oxidant combination is acetylene and air, which produces a flame temperature of 2 400-2 700 K (Table 20-1). When a hotter flame is required for refractory elements (those with high boiling points), acetylene and nitrous oxide is usually the mixture of choice. The height above the burner head at which maximum atomic absorption or emission is observed depends on the element being measured, as well as flow rates of sample, fuel, and oxidant. These parameters can be optimized for a given analysis. 

Set up the atomic absorption spectrophotometer for the air/acetylene flame analysis of cadmium according to the SOP (5.8.) or the manufacturer s operational instructions. For the source lamp, use the cadmium hollow cathode or electrodeless discharge lamp operated at the manufacturer s recommended rating for continuous operation. Allow the lamp to warm up 10 to 20 min or until the energy output stabilizes. Optimize conditions such as lamp position, burner head alignment, fuel and oxidant flow rates, etc. See the SOP or specific instrument manuals for details. Instrumental parameters for the Perkin-Elmer Model 603 used in the validation of this method are given in Attachment 1. 

Fig. 82. Burner-nebulizer assembly for flame atomic absorption spectrometry, a Burner head with mixing chamber b nebulizer c impactor bead d impact surfaces e nebulizer socket. (Courtesy of Bodenseewerk PerkinElmer, Oberlingen.)...

Simplex optimisation of the overall response of a simultaneous multi-element flame atomic absorption spectrometer (air to fuel ratio, slit width, height above the burner head, and four hollow cathode lamp currents). Cu, Fe, Mn and Zn were measured. 

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