Absorbance vs. Height Profiles in FAAS

Where within the flame one measures absorbance is critical as two competing processes affect the concentration of free atoms in the flame. The more time the analyte spends in the flame the greater the atomization efficiency; thus, the production of free atoms increases with height. On the other hand, a longer residence time allows more opportunity for the free atoms to combine with oxygen to form a molecular oxide, decreasing the concentration of free atoms. For an easily oxidized metal, such as Cr, the concentration of free atoms is greatest just above the burner head. For metals, such as Ag, which are difficult to oxidize, the concentration of free atoms increases steadily with height. Other atoms show concentration profiles that maximize at a characteristic height.