with the strongest ion signal, the intensities (digits at the top in each spectrum) of the observed ions rose with increased ion source pressure (ion intensity 1.70E6) (Figs. and). Because TNT molecules have a positive electron affinity, the electron capturing mechanism became more advantageous when the ion source pressure was low . In the negative ion mode, a lower ion source pressure is appropriate for detecting TNT with better sensitivity; however, RDX, PETN, and NG in the negative ion mode, and TATP and HMTD in the positive ion mode, displayed better sensitivity with low LOD under higher ion source pressures. The ion generation mechanism for explosive compounds employing the HCD ion source was extensively described in this review paper .