Cayman’s Hydrogen Peroxide Ratiometric MaxSpec Kit enables the measurement of mitochondrial H2O2 in cell culture and animal models. MitoB (3-hydroxybenzyl)triphenylphosphonium bromide or MitoBoronic Acid) is taken up specifically by polarized mitochondria and accumulates in the matrix. The selective reaction of the arylboronic acid moiety of MitoB with H2O2 forms the phenol product, MitoP (MitoPhenol). Therefore, the increase in the MitoP/MitoB ratio over time indicates the level of mitochondrial H2O2. The ratio of MitoP/MitoB is measured in sample extracts by LC-MS/MS using deuterated internal standards (IS) (MitoB-d15 and MitoP-d15) to correct for extraction and detection variations.