I do not have good experience using AAS following aqua regia digestion (it is only partial digestion). I achieved better results using multi acid digestion (HNO3, HClO4, HF, HCl). To conclude, I was not satisfied with either of the digestion procedures mentioned. I achieved best results (note that bauxite may contain some refractory minerals) for major elements and some minor ones using X-ray fluorescence analysis following lithium borate fusion (fused discs).

It is depend on what metal and what matrices you are going to analysis, In generally if you have facility, microwave assisted digestion is good (with acid) or you can use hot plate digestion, sonication method etc..

There are various composition of different acids used for the digestion. We prefer to use use Nitric+Perchloric (4:1 ratio) for all type of sediment samples. Digest the sample in triplicate with different composition and check the recovery of all the metals of your interest.