The basic chemical units of which all matter is composed, not breakable into simpler substances by ordinary chemical means. The atoms of each element share the same number of protons and electrons, while differing neutron counts create isotopes of different atomic weight. They underpin food nutrition, mineral analysis and contaminant testing. Essential elements — iron, zinc, iodine, selenium — underpin nutrition labelling and fortification programmes, while toxic elements such as lead, cadmium, mercury and arsenic are monitored against strict maximum levels. ICP-MS now quantifies dozens of elements simultaneously in a single food digest.