Quantification and remote detection of nitro explosives by helium plasma ionization mass spectrometry (HePI‐MS) on a modified atmospheric pressure source designed for electrospray ionization

Helium Plasma Ionization (HePI) generates gaseous negative ions upon exposure of vapors emanating from organic nitro compounds. A simple adaptation converts any electrospray ionization source to a HePI source by passing helium through the sample delivery metal capillary held at a negative potential. Compared with the demands of other He‐requiring ambient pressure ionization sources, the consumption of helium by the HePI source is minimal (20–30 ml/min). Quantification experiments conducted by exposing solid deposits to a HePI source revealed that 1 ng of 2,4,6‐trinitrotoluene (TNT) on a filter paper (about 0.01 ng/mm²) could be detected by this method. When vapor emanating from a 1,3,5‐trinitroperhydro‐1,3,5‐triazine (RDX) sample was subjected to helium plasma ionization mass spectrometry (HePI‐MS), a peak was observed at m/z 268 for (RDX●NO₂)^?. This facile formation of NO₂^? adducts was noted without the need of any extra additives as dopants. Quantitative evaluations showed RDX detection by HePI‐MS to be linear over at least three orders of magnitude. TNT samples placed even 5 m away from the source were detected when the sample headspace vapor was swept by a stream of argon or nitrogen and delivered to the helium plasma ion source via a metal tube. Among the tubing materials investigated, stainless steel showed the best performance for sample delivery. A system with a copper tube, and air as the carrier gas, for example, failed to deliver any detectable amount of TNT to the source. In fact, passing over hot copper appears to be a practical way of removing TNT or other nitroaromatics from ambient air. Copyright © 2012 John Wiley & Sons, Ltd.