Nitrous oxide-acetylene flame, operation

Figure 5 Effect of operating at optimal (top) and sub-optimal (bottom) fuel flow when repeatedly nebulizing 10 mg l 1 aluminium into a nitrous oxide-acetylene flame...

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and the mechanical choppers are contributing to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS - such as the emission noise caused by the nitrous oxide -acetylene flame in the determination of Ba and Ca due to the CN band emission [3] - are equally absent in HR-CS AAS for the same reasons, that is, the higher intensity of the primary radiation source, and the high resolution. 

Operating parameters and sensitivities for yttrium and certain rare earths in the nitrous oxide-acetylene flame... 

Since the hollow-cathode lamp spectra used in AAS are relatively simple, spectral bandwidths narrower than 0.1 nm are seldom if ever used. In atomic emission analysis, however, higher resolving power is often essential, particularly when the excitation source (e.g. the nitrous oxide—acetylene flame) is producing a complex spectrum. The instrument should, therefore, provide a wide range of slit settings and a convenient digital display of the wavelength in use for the operator. 

The fuel-rich nitrous oxide—acetylene flame provides for interference-free operation. Somewhat greater (ca. two-fold) sensitivity is observed at the 235.48nm line over absorption at the 286.33 nm line a narrow 0.2nm (or less) spectral bandpass must be used to avoid spectral interferences at the latter line. Measurements should be conducted at 286.33 nm when non-atomic absorption is encountered at 235.48nm. Ionization in the nitrous oxide—acetylene flame should be suppressed by the addition of 1000 jug ml-1 of potassium or cesium. 

Apparatus. Perkin-Elmer (PE) 360 and 272 atomic absorption spectrometers, with an air-acetylene or a nitrous oxide-acetylene flame, were employed for the analyses. Standard hollow cathode lamps were used as light sources. In all cases, the instruments were operated as recommended by the manufacturer gas pressures, gas flows, slits, wavelengths, and other controls were adjusted to the prescribed values. The readout was obtained directly using a 5-s integration in the concentration (PE 360) or absorbance (PE 272) mode. 

The oldest spectroscopic radiation sources operate at low temperature (Section 21.5.3.1), but have good spatial and temporal stability. They readily take up wet aerosols produced by pneumatic nebulization. Flame atomic emission spectrometry [162] is still a most sensitive technique for determination of the alkali metals, e.g., for serum analysis. With the aid of hot flames such as the nitrous oxide - acetylene flame, a number of elements can be effectively excited, but cannot be determined at low concentration. Interference arising from the formation of stable compounds is high. 

The design of the burner employed in AAS depends on the oxidant-fuel gas mixture. Slot burners are now used almost exclusively, the length of the slot being 10 cm for use with air-acetylene and multishot burners, and 5 cm for nitrous oxide-acetylene burners, reflecting the higher burning velocity of the flame as shown in Table 1. The slot width and the conductivity of the metal used for the construction of the burner are also important in terms of stability of operation and prevention of clogging. The burner should also be constructed or coated in an inert material to avoid corrosion. 

Data for the several flame methods assume an acetylene-nitrous oxide flame residing on a 5- or 10-cm slot burner. The sample is nebulized into a spray chamber placed immediately ahead of the burner. Detection limits are quite dependent on instrument and operating variables, particularly the detector, the fuel and oxidant gases, the slit width, and the method used for background correction and data smoothing. 

Other uses of oxyacetylene flames in mill operations are in building up or hardfacing metal, lancing (piercing a hole in a metal mass), and a variety of metal cleaning procedures. A minor but interesting fuel use of acetylene is in flame spectrophotometry where oxygen and nitrous oxide are used as oxidants in procedures for a wide variety of the elements. 

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. 

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