Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) is different from Flame AAS mainly in how the sample is heated and measured.
In Flame AAS, a flame made from gases like air and acetylene is used to turn the sample into atoms. It needs more sample and works best for detecting elements present in higher amounts. It is fast and good for routine analysis, but not very sensitive for very small quantities.
In GFAAS, instead of a flame, a small graphite tube (furnace) is used. The sample is placed inside this furnace and heated in steps — first to dry, then to remove impurities, and finally to form atoms. It needs only a tiny amount of sample and can detect very low concentrations. GFAAS is much more sensitive than Flame AAS but takes more time and is more expensive to run.
In short, Flame AAS is quicker and cheaper, while GFAAS is slower but more sensitive and accurate.