MitoA is a ratiometric mass spectrometry probe that can be used for assessing changes in H2S within mitochondria in vivo. MitoA contains a triphenylphosphonium cation component that drives its accumulation in mitochondria where its aryl azide moiety selectively reacts with H2S to produce an amine product, MitoN. Quantifying the MitoN/MitoA ratio by LC-MS/MS reflects the mitochondrial matrix H2S concentration. In a mouse model of acute myocardial infarction with MitoA administered prior to ischemia, the MitoN/MitoA ratio is increased only in the region of ischemia.