ICP-MS
/ EYE-see-pee em-ess /
ICP-MS uses the same fiery plasma as ICP-OES, but instead of reading the light the atoms give off, it weighs them one by one. The plasma not only atomizes the sample but knocks an electron off each atom, turning it into an ion; those ions are then sorted by mass and literally counted, which is about as sensitive as elemental analysis gets.
More precisely, the plasma ionises the elements; the ions are pulled out of the plasma, focused into a beam, and sent to a mass analyzer that separates them by mass-to-charge ratio. A detector counts ions at each mass, so the instrument reports both which elements are present and how much, and can even tell apart different isotopes of the same element.
It matters because ICP-MS reaches parts-per-trillion levels — astonishingly low — and measures dozens of elements and isotopes fast, making it the gold standard for ultratrace metal analysis in water, blood, and food. The honest caveats are high cost and isobaric interferences, where ions of the same nominal mass (an element overlapping a molecular ion) muddy a reading.
To certify drinking water, an ICP-MS measures arsenic, lead, and cadmium down to parts per trillion in one run, and can even separate lead's isotopes to trace the metal's geographic origin.
Same plasma, but ions are weighed and counted — sensitivity down to parts per trillion.
ICP-MS and ICP-OES share the same plasma source but differ in what reads it: a mass spectrometer (counting ions by mass) versus an optical spectrometer (reading emitted light). The ionisation that hurts AAS is exactly what ICP-MS depends on.