Comparison of ESI and APPI for the Detection of Nitro Explosives Using Differential Mobility Mass Spectrometry

Abstract

Rapid and reliable detection of explosive residues is critical for security applications (e.g., counterterrorism) and environmental health contexts (e.g., legacy military site contamination). Key target analytes among nitro-organic explosives encompass 2,4,6-trinitrotoluene (TNT), hexogen (RDX), pentaerythritol tetranitrate (PETN) as well as 2,4-dinitrotoluene (DNT, a common military-grade TNT impurity) and 2-amino-4,6-dinitroluene (2A-DNT, a microbial degradation product of TNT). Ion mobility spectrometry is one of the key technologies for fast and reliable detection of explosives. When coupled with mass spectrometry (MS), it enables high sensitivity and precise analyte identification. In this study, a differential mobility analyzer (DMA) coupled to a triple quadrupole MS was utilized to compare two relatively orthogonal ionization techniques─atmospheric pressure photoionization (APPI) and electrospray ionization (ESI)─for explosive trace detection in calibration standards and soil extracts, applied via liquid injection or via thermal desorption from filters. Both ionization techniques showed good linearity and enabled sensitive detection of the tested explosives. While APPI showed increased sensitivity toward dinitrotoluenes (DNT, 2A-DNT), ESI performed significantly better for TNT, RDX, and PETN but failed to detect DNT via thermal desorption. Both ionization techniques were further applied to the analysis of explosives in soil extracts, providing meaningful insights into their applicability for field sample analysis and revealing distinct effects of matrix components on ion source behavior and detection performance. The study underscores the practical relevance of the approach while offering a balanced evaluation of the methodological strengths and limitations.