aqua regia is a 3:1 mixture of hydrochloric acid and nitric acid. It is a strong oxidizing mixture that dissolves many base metal sulfide minerals as well as gold and platinum. However, it is a partial leach that leaves an undigested silicate and alumina residue as well as refractory minerals such as garnet and spinel. It is possible to vary the sample size from less than 1g up to 20 to 50g depending on the target elements and their expected concentration. Sub-ppb detection limits are possible and base metal detection limits of low ppb are common.
Its digestion can result in the partial loss of volatile arsenic and antimony. Insoluble barium and lead sulfates can precipitate when sulfide is oxidized to sulfate. This is common since its a strong oxidizing agent.
Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.