The focus of this review is on the development of a variable-pressure hollow cathode discharge (HCD), a quasi-ambient, ion source for mass spectrometry that uses air as the carrier gas for practical applications. The designed HCD ion source (Fig. ) has been employed successfully in conjunction with a linear ion trap MS for explosive detection and quantification at trace to ultra-trace levels. The HCD ion has also been used to investigate gas phase ionmolecule reactions involving explosives and related compounds. The use of air as a carrier gas in the HCD ion source eliminates the need for reagent gas changes (e.g., He or Ar). An ambient ion source like this could enable the development of a miniature-MS system capable of quickly analyzing illicit substances in the real world. The HCDI-MS system's versatility in analyzing explosives in both negative and positive modes of operation at trace to ultra-trace levels is demonstrated in Scheme 1.