 Although some compounds can be ionized directly by Penning ionization, the most common positive-ion formation mechanism for DART involves ionization of atmospheric water.Although the exact ion formation mechanism is not clear, water can be ionized directly by Penning ionization.Another proposal is that water is ionized by the same mechanism that has been proposed for atmospheric pressure chemical ionization

Ionized water can undergo further ion-molecule reactions to form protonated water clusters([(H2O)nH]+).