Dinitrogen oxide-acetylene flame

Dinitrogen oxide-acetylene flame (Amos and Willis)... 

V Elements determined by FAAS using a dinitrogen oxide-acetylene flame... 

Calibration graphs may have curvatures away from the concentration axis (Figure 28C) when, for example, sodium, potassium, and gold are determined in an air-acetylene flame, or barium and europium in a dinitrogen oxide-acetylene flame. This is due to ionization of the analyte. The ionization decreases with increasing concentration. This is also seen in Figure 41. 

The dinitrogen oxide-acetylene flame can be used for those elements which cannot be determined successfully with an air-acetylene flame. The temperature of the N20-acetylene flame is only a little lower than that of the air-acetylene flame. In addition, the air-acetylene flame is safer because of its smaller burning velocity (Table 5). A slightly rich N2O-acetylene flame consists of about 2 to 4 mm high blue-white primary reaction zone, above that about 5 to 50 mm high red reduction zone, and on the top a blue-violet secondary reaction zone where the fuel gas oxidizes. The dissociation of the sample takes place in the red reduction zone. 

For the hydrogen-diffusion air flame the same burners are used as for the air- or dinitrogen oxide-acetylene flames. An inert gas (argon or nitrogen) is mixed with hydrogen to improve the analytical characteristics of the flame. Then the sample is sprayed into the spray chamber with the help of the inert gas. Diffusion flames are employed for determination of easily atomizing elements, such as arsenic or selenium. 

Figure 43 The effect of potassium on the determination of calcium (5 mg I ), strontium (5.5.mgl ), and barium (30mgl f using dinitrogen oxide-acetylene flame. (Adapted from M. D. Amos and J. B. Willis, Spectrochim. Acta, 1966, 22, 1325)...

Hi) Methods Based on the Formation of Volatile Compounds. A specific method for the determination of fluoride is based on the formation of gaseous SiF4- The atomic absorption signal of silicon is measured at 251.6 nm using either a dinitrogen oxide-acetylene flame (detection limit 30/ug of F ) or a graphite furnace (detection limit 0.17 fig of F ). 

The air-propane flame was commonly used in the early days of AAS for those elements which can be easily atomized (alkali metals, cadmium, copper, lead, silver, and zinc). The air—propane flame is not widely used except in laboratories where the use of acetylene is forbidden. The dinitrogen oxide-propane flame is more advantageous for use in these cases because it possesses less chemical interferences and it allows the determination of some refractory oxide elements. 

The construction of the burner depends on the oxidant/fuel gas mixture. The flame propagation velocity should be smaller than the gas flow velocity through the burner slot. In the opposite case, the flame may flash back down the burner stem and into the spray chamber with possible disastrous results. The length of the slot in air-acetylene, air-propane, and multislot burners is 10 cm, and that of the dinitrogen oxide-acetylene burner is 5 cm. 

Most commonly used instruments use a flame (flame AAS (FAAS)) produced by combustion of an air/acetylene or dinitrogen oxide/acetylene mixture. The few interferences are easy to avoid, and the sensitivities that are reached are adequate for the metals of greatest interest to the food industry. Variants of this technique, such as the coupling of hydride generation (HG) systems (HG-AAS), increase its scope to higher-sensitivity determination of elements like selenium, arsenic, tin, and other elements that form hydrides. In a similar vein, the determination of mercury using the cold vapor technique should be highlighted. 

Mixtures of acetylene and dinitrogen oxide used to create flames in atomic absorption apparatus detonate in the presence of perchloric acid. 

Flames employed in AAS may be divided into two groups the combustion flames and diffusion flames. In fuel-oxidant mixtures, the temperature of the flame varies generally from 2000 to 3000 K. Air and dinitrogen oxide (N2O) are the most widely used oxidants, and acetylene, propane, and hydrogen are the most common fuel gases. In diffusion flames, the fuel is also the carrier gas and it burns on coming into contact with the outer diffusion air. The temperatures of the diffusion flames are lower than those of the combustion flames. The characteristics of some commonly used flames are given in Table 5. 

In AAS, the flame is only required to produce ground state atoms. Two types of flame are employed to achieve this the premixed combustion flame consisting of a fuel and oxidant gas, and the diffusion flame where the fuel is also the carrier gas that burns on contact with air. Premixed flames commonly employ either air or dinitrogen oxide as the oxidant, and either acetylene, propane, or hydrogen as the fuel gas. 

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