Compounds containing nitro and nitro-groups have a positive electron affinity; for example, the electron affinity of NO3 is 3.92 eV , while that of TNT is only 2.50 eV . applied ab initio calculations to determine the values of the electron affinities for RDX, HMX, TNT, and PETN, and they revealed that these explosive chemicals have positive electron affinities . Except for TNT in the negative mode of operation, none of these explosive compounds formed the molecular ion [M] -• . The findings imply that RDX, PETN, and NG prefer adduct/cluster ion production with reagent ions, such as NO2 -/NO3 -, over electron capture in the HCD ion source. Other groups that employed plasma-based ionization methods found similar results . The NOx is one of the major products in plasma-excited air . Because of their higher electron affinity than the relevant explosives, the nitro-species (NOx) generated in plasma-excited air preferentially capture electrons, resulting in a decrease in the signal strength of the [TNT] -• ion with pressure .