Simultaneous detection and quantification of explosives by a modified hollow cathode discharge ion source

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Chemical Sensors

Background:

• Trace-level detection of explosives is critical due to rising global security threats.
• Previous hollow cathode discharge (HCD) ion sources have been used for explosive detection.
• Existing methods require modification for simultaneous analysis of multiple explosive compounds.

Purpose of the Study:

• To modify an HCD ion source for dual pressure operation and simultaneous analysis of explosives.
• To evaluate the sensitivity and performance of the modified HCD ion source for detecting trinitrotoluene (TNT), nitroglycerin (NG), pentaerythritol tetranitrate (PETN), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX).
• To investigate the ionization mechanisms and adduct formation under different operating pressures and temperatures.

Main Methods:

• Modification of a hollow cathode discharge (HCD) ion source for dual pressure operation.
• Coupling the modified HCD ion source to a linear ion trap mass spectrometer.
• Analysis of model explosive compounds (TNT, NG, PETN, RDX) in negative mode ionization at high (~28.0-30.0 Torr) and low (~0.8-1.0 Torr) ion source pressures.
• Utilizing air as a carrier gas and investigating reactions with reagent ions like NO3- and NO2-.

Main Results:

• The modified HCD ion source achieved simultaneous detection and quantification of explosives with improved sensitivity.
• At higher pressure, NG, PETN, and RDX formed adduct ions with NO3-, while TNT showed a [TNT + NO3-HNO2]- ion.
• At lower pressure, TNT exhibited its molecular ion [TNT]-•, and RDX showed fragment ions, while NG and PETN were not detected.
• Stable adduct ions were formed with NG, PETN, and RDX, and TNT at higher temperatures (140-200 °C) due to reagent ions in the HCD plasma.
• Formation of [TNT-H]- and [TNT+H]- ions was observed at higher pressures, with the latter attributed to Birch reduction-type reactions on the inner metallic surface.

Conclusions:

• The modified HCD ion source is effective for simultaneous detection and quantification of explosive compounds at trace and ultra-trace levels.
• The dual pressure system allows for selective detection of different explosive types based on their ionization behavior.
• The use of air as a carrier gas and the HCD plasma chemistry contribute to the efficient formation of characteristic ions for explosive identification.