 The concentrations of metals in the acid solution can be measured by a number of analytical chemistry methods, including atomic emission spectrometry, atomic absorption spectrometry (AAS), or mass spectrometry.These are known as "simultaneously extracted metals" because they are the metals liberated from the sediment "while the volatilization of sulfide is occurring".Metals extracted from sediment by digestion, or with a different acid than HCl are "not" simultaneously extracted metals.