How can mass spectrometry be used to determine concentration?
As indicated in the Wikipedia page, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is the technique of choice to quantify metal ions in a sample. In summary, this technique is based on the atomization and ionization of the sample metal atoms in a plasma torch, and the ions are then separated based on their mass, usually with a quadrupole mass filter, and then directly quantified on the detector. External calibration with standard solutions of known metal concentration or isotopic dilution then affords access to absolute concentrations.