Combined use of direct analysis in real-time/Orbitrap mass spectrometry and micro-Raman spectroscopy for the comprehensive characterization of real explosive samples

Abstract

Direct Analysis in Real Time (DART™) high-resolution Orbitrap™ mass spectrometry (HRMS) in combination with Raman microscopy was used for the detailed molecular level characterization of explosives including not only the charge but also the complex matrix of binders, plasticizers, polymers, and other possible organic additives. A total of 15 defused military weapons including grenades, mines, rockets, submunitions, and mortars were examined. Swabs and wipes were used to collect trace (residual) amounts of explosives and their organic constituents from the defused military weapons and micrometer-size explosive particles were transferred using a vacuum suction-impact collection device (vacuum impactor) from wipe and swap samples to an impaction plate made of carbon. The particles deposited on the carbon plate were then characterized using micro-Raman spectroscopy followed by DART-HRMS providing fingerprint signatures of orthogonal nature. The optical microscope of the micro-Raman spectrometer was first used to localize and characterize the explosive charge on the impaction plate which was then targeted for identification by DART-HRMS analysis in both the negative and positive modes. Raman spectra of the explosives TNT, RDX and PETN were acquired from micrometer size particles and characterized by the presence of their characteristic Raman bands obtained directly at the surface of the impaction plate nondestructively without further sample preparation. Negative mode DART-HRMS confirmed the types of charges contained in the weapons (mainly TNT, RDX, HMX, and PETN; either as individual components or as mixtures). These energetic compounds were mainly detected as deprotonated species [M–H]−, or as adduct [M + 35Cl]−, [M + 37Cl]−, or [M + NO3]− anions. Chloride adducts were promoted in the heated DART reagent gas by adding chloroform vapors to the helium stream using an “in-house” delivery method. When the polarity was switched to positive mode, DART-HRMS revealed a very complex distribution of polymeric binders (mainly polyethylene glycols and polypropylene glycols), plasticizers (e.g., dioctyl sebacate, tributyl phosphate), as well as wax-like compounds whose structural features could not be precisely assigned. In positive mode, compounds were identified either as protonated molecules or ammonium adduct species. These results clearly demonstrate the complementarity of micro-Raman microscopy combined with DART-MS. The former technique provides structural information on the type of explosives present at the surface of the sample, whereas the latter provides not only a confirmation of the nature of the explosive charge but also useful additional information regarding the nature of the complex organic matrix of binders, plasticizers, polymers, oils, and potentially other organic additives and contaminants present in the sample. Combining these two techniques provides a powerful tool for the screening, comprehensive characterization, and differentiation of particulate explosive samples for forensic sciences and homeland se...

Ambient Pressure Laser Desorption—Chemical Ionization Mass Spectrometry for Fast and Reliable Detection of Explosives, Drugs, and Their Precursors

“…Surface analyzing techniques allow for the observation of different potentially interesting targets in forensic science like luggage, letters, or clothes without time-consuming sample pretreatment. Direct analyses in real time (DART) [9,10], DART-Raman combination [11], desorption electrospray ionization (DESI) [12,13], solvent assisted desorption/ionization mass spectrometry (DI-MS) [14], or a variety of laser desorption/ionization (LDI) [15] systems are common techniques. Two of the most famous setups of these LDI systems are called matrix-assisted laser desorption/ionization (MALDI) [16,17] and surface-assisted laser desorption/ionization (SALDI) [10,18].…”

Easily fabricated ion source for characterizing mixtures of organic compounds by direct analysis in real time mass spectrometry

“…When used with DART, prior studies have utilized gas phase reagents such as chloroform, dichloromethane, ammonia, or triuoroacetic acid to form gas phase adducts. 45,48 Similar chemical modications using DESI have also been reported, using the designation "reactive DESI". That method involves host-guest supramolecular chemistry with either the host or guest in the spray to complex with a target functionality present on the analyte in the sample, or derivatization chemistry involving attachment of a xed charge to the target functionality within the molecule, oen with the reagent molecule being delivered in the spray.…”

“…45 The formation of various positively and negatively charged gas phase adducts from atmospheric molecules such as ammonia are also commonly observed and assist in ionization processes. 45,48 DART analysis has been applied successfully in the majority of applications where other ambient mass spectrometry methods have been utilized. 11,21,47,49 The selection of DESI or DART as the ionization source for a particular analytical application requires consideration of the target analyte's solubility in potential DESI spray solvents, and vapor pressure as well as potential reactivity in known DART ionization mechanisms.…”

Utilizing Surface Acoustic Wave Nebulization (SAWN) for the Rapid and Sensitive Ambient Ionization Mass Spectrometric Analysis of Organic Explosives

“…Of the various ambient ionization techniques DART (direct analysis in real time) has been used for the analysis of both inorganic [9,10] and organic explosives [11][12][13][14] with excellent sensitivity. The design of the DART source also facilitates realtime and high-throughput analysis and seems sufficiently robust and reproducible although ionization can be affected by external factors like humidity [15].…”