Page 39 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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adjusting the burner up and down with respect to the light beam, a region of optimum absorbance can
be found.
This non-uniformity in the distribution of the atoms in the flame arises because the flame has a distinct
structure. Figure 2.4 shows the structure of a typical premixed flame. Premixed gases are heated in the
preheating zone, where their temperature is raised exponentially until it reaches the ignition
temperature. Surrounding the preheating zone is the primary reaction zone, where the most energetic
reactions take place.
-5
-4
The primary reaction zone is a hollow cone-like zone, only 10 -10 m thick. The actual shape of the
cone is determined largely by the velocity distribution of the gas mixture leaving the burner. While the
velocity of the gases at the burner walls is virtually zero, it reaches a maximum in the centre. The
rounding at the top is caused, in part, by thermal expansion of the gases, which also produces a back-
pressure which distorts the base
Figure 2.4
A premixed (or laminar) flame.
