AAS literally requires atomic species. In other words, it requires the element that is being analysed to be present within the flame as single atoms and not bound to other atoms / molecules / ions. So, for example, if I wanted to analyse a sample for iron content, I would want the iron to be present in an aqueous solution as (say) [Fe(H2O)6]2+. When aspirated into the flame, the water molecules would evaporate and an unbound iron(II) ion released. Some metals can be present as coordination complexes with ligands that can be oxidised and removed in the flame - for example, a species like [M(NH2CH2CH2NH2)3]2+ will release M2+ ions as the ligands are oxidised to CO2, H2O, and an oxidise of nitrogen.
If the element to be analysed is present in a form that does not become atomised in the flame, then quantity detected will be decreased, as will the accuracy of the results. Calcium samples in the presence of phosphate, for example, forms clusters that remain bound in the flame. In such circumstances, suitable ligands are added to the sample to compete with the phosphate for the calcium. This increases the amount of calcium detected but the accuracy will still be reduced as some clusters will remain bound and thus go unmeasured.
AAS also requires calibration with a series of standards and if the concentration of any of these standards is inaccurate then the accuracy of the result for the unknown sample will similarly be affected.
