Page 47 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

Page 31

Figure 2.9
Impact bead assembly.

efficiency. The use of organic solvents is known to do this. Organic solvents may affect the
nebulization because their different surface tension, density, viscosity or vapour pressure, all of which
enter into the relevant equations, probably results in smaller droplets which evaporate more quickly.
They also affect results because of their effect on the flame (they essentially act as a secondary fuel).

At this stage no atoms have been formed—only a mist. Hence the name nebulization, not atomization.
We will now discuss atomization.

The desolvation of the droplets is usually completed in the preheating zone. The mist of salt clotlets
then fuses and evaporates or sublimes. This is critically dependent on the size and number of the
particles, their composition and the flame mixture. As the absolute concentration of analyte in the flame
is very small (< 10 atm), the saturated vapour pressure may not be exceeded even at temperatures
-3
below the melting point.
Many of these vapours will break down spontaneously to atoms in the flame. Others, particularly
diatomic species such as metal monoxides (e.g. alkaline earth and rare earth oxides), are more
refractory. Monohydroxides which can form in the flame can also give problems. The high temperature
and enthalpy of the flame aid dissociation thermodynamically, as does a reducing environment. The
role of flame chemistry is also important. Atoms, both ground state and excited, may be produced by
radical reactions in the primary reaction zone. If we take the simplest flame (a hydrogen-oxygen flame),
some possible reactions are the following: