I am not surprised! Typically, the LOQ (depending on the element) is ~0.1-1 ppm or 0.1-1.0 ug/L. You either have a graphite furnace attacment to your AAS (thus GFAAS) or use 'cold vapor for Mercury analysis (thus CVAAS). Or use an ICP.

Your question is too general. The detection limit depends on on method, in flame AAS on oxidant (air/ nitrous oxide), or graphite furnace method. For the elements like Zn and Cd with graphite furnace method thous limits you mentioned can be easily reached. In fact, especially in case of Zn, the detection limit is so low that contamination even in laboratory environment can be a real risk.

Greetings, Pentti Minkkinen