Oxidation ratio is a metric which is calculated over the normalization beads that are spiked into every mass cytometry sample. It is defined as the median of mass 156 divided by mass 140 over all bead channels. The metric measures the amount of oxidation that happened during acquisition: Ce140 is the main bead channel, and 16 is the mass of a single oxygen atom. High oxidation could indicate issues with the instrument and means that other masses could be affected. For example, if CD3 is on mass 146 and CD19 is on mass 164, oxidation could lead to spurious CD3+ CD19+ cells.