Sharp line source:

When the bandwidth of the primary radiation is low with respect to the profile of the analyte absorption, the given amount of analyte would absorb more radiation. Thus, the radiation sources having low widths of the emitted analyte lines are preferred. A continuous radiation sources on the other hand have low radiant densities and do not provide sufficiently high sensitivity.

Alkali metals have low ionisation energies and even at the low temperatures obtained in the flame a sufficient amount of the sample may be in the excited or ionised state. That means that the concentration of the analyte atoms in the ground state will not be a true representation of the analyte concentration. As the AAS method depends on the radiation absorption by the analyte in ground state it is not ideal for such determinations. Flame photometry would be the method of choice for such determinations.