Page 44 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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indicative. The figures for the burning velocity and temperature show that different burners need to be
used with different flames. The air-propane flame is rarely used nowadays, as it is cool and offers
insufficient atomization energy. It is, however, easy to handle. The air-hydrogen flame finds special
use in atomic fluorescence because of the low fluorescence-quenching cross-section of hydrogen, often
further improved by diluting (and cooling) the flame with argon. The flame has also found use for
atomic absorption spectrometry for analytes that have their most sensitive line at a low wavelength.
This is because this flame has a lower background signal arising from molecular species.

The air-acetylene flame is the most widely used flame. It is stable, simple to operate and produces
sufficient atomization to enable good sensitivity and freedom from interferences for many elements. It
is not only necessary for the flame to atomize the analyte, but also to break down any refractory
compounds which may react with or physically entrap the analyte. Atomization, as we shall see, occurs
both because of the high enthalpy and temperature of the flame, and through chemical effects. Thus,
increasing the oxygen content of the flame above the approximately 20% normally present in air, while
raising the flame temperature, does not necessarily enhance atomization, because more refractory
oxides may be produced. Making the flame more fuel rich lowers the temperature but, by making the
flame more reducing, increases the atomization of the elements such as molybdenum and aluminium.

The nitrous oxide-acetylene flame is both hot and reducing. A characteristic red, interconal zone is
obtained under slightly fuel-rich conditions. This red feather is due to emission by the cyanogen
radical. This radical is a very efficient scavenger for oxygen, thus pulling equilibria such as

over to the right. This appears to be a vital addition to the high temperature which also promotes
dissociation. Amongst those elements which are best determined in nitrous oxide-acetylene are Al, B,
Ba, Be, Mo, Nb, Re, Sc, Si, Ta, Ti, V, W, Zr, the lanthanides and the actinides. The nitrous oxide-
acetylene flame must be operated more carefully than the air-acetylene flame. For safety reasons, an air-
acetylene flame is lit first, made very fuel rich and then switched to nitrous oxide by a two-way valve.
Many modern instruments will perform this automatically. The flame is shut down by the reverse
procedure. The nitrous oxide-acetylene flame can normally be run without any problems, provided that
it is never run fuel lean and carbon deposits not allowed to build up. Any deposits should be cleaned
away when the flame is extinguished.