chemical ionisation mass spectrometry atmospheric pressure chemical ionisation mass spectrometry APCI MS CHMO:0000473 Mass spectrometry where the sample is ionised by interaction (electron or proton transfer) with reagent (N2, O2, H2O) gaseous ions at atmospheric pressure. Reagent ions are produced by electron ionisation with electrons from a radioactive beta-source or a corona discharge. atmospheric pressure chemical ionisation mass spectroscopy atmospheric pressure chemical ionization mass spectroscopy atmospheric pressure chemical ionization mass spectrometry APCI-MS