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

Abstract

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/mm(2)) 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●NO(2))(-). This facile formation of NO(2)(-) 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.

Transformation of the gas‐phase favored O‐protomer of p‐aminobenzoic acid to its unfavored N‐protomer by ion activation in the presence of water vapor: An ion‐mobility mass spectrometry study

“…Spectra were acquired on a Synapt G2 HDMS instrument (Waters, Milford, MA, USA) equipped with a HePI source . Mass calibration ( m/z 10‐1500) was performed using a solution of sodium formate (100 ppm).…”

“…For HePI, a stream of high‐purity helium at a flow rate of about 30 mL/min was passed through a stainless‐steel capillary needle (100 μm i.d.) held at an electrical potential of +3.50 kV.…”

“…Spectra were acquired on a Synapt G2 HDMS instrument (Waters, Milford, MA, USA) equipped with a HePI source. 14…”

Competitive homolytic and heterolytic decomposition pathways of gas‐phase negative ions generated from aminobenzoate esters

“…The source temperature was maintained at 100–150 °C, and the cone voltage was set to 5–10 V and hexapole I at 10 V. For HePI, sample deposits on glass slides obtained by evaporating sample solutions in dichloromethane were placed individually about 1.0 cm away from the helium plasma region in an enclosed ion source. To facilitate negative ion formation, a cotton wad soaked in aqueous ammonia (28% NH 3 in H 2 O) was placed in the ion source …”

“…To facilitate negative ion formation, a cotton wad soaked in aqueous ammonia (28% NH 3 in H 2 O) was placed in the ion source. [17,18] CID mass spectra were recorded on a triple-quadrupole tandem mass spectrometer. For CID experiments, the pressure of the argon in the collision cell was held at 116 Pa, and laboratory-frame collision energy was kept at about 20 eV (unless otherwise stated).…”