Page 48 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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where Q = energy. So far, this is most unflame-like, because energy has only been consumed!
Equilibrium is approached through third-body collisions, such as

where B may be N , 02 or analyte containing molecules, e.g. NaCl:
2

where the asterisk denotes an atom excited by 'chemiluminescence'.

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of
analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the
nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have
been formed they may be lost by compound formation and ionization. The latter is a particular problem
for elements on the left of the Periodic Table (e.g. Na fi Na + e : the ion has a noble gas configuration,
+
-
is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in
temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth
elements and also some others (e.g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus,
we observe some self-suppression of ionization at higher concentrations. For trace analysis, an
ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e.g.
caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e.g. of
sodium) by a simple, mass action effect:

Differing amounts of easily ionizable elements in real samples cause varying ionization suppression and
hence the possibility of interference (see Section 2.4.2).