Nitrate ester derivatization for the generation of structurally informative ions via gas chromatography-mass spectrometry

Abstract

Forensic investigations of explosives in post-blast and trace scenarios often utilize gas chromatography-mass spectrometry (GC–MS). Many high explosives (e.g., trinitrotoluene) provide structurally significant ions so that a compound can be confidently identified by GC–MS; however, GC–MS only provides functional group identification for the presence of nitrate esters (e.g., nitroglycerin (NG)), a class of high explosives that is frequently encountered in forensic casework. Nitrate esters vary drastically and are much more accurately identified by their component alcohol structure rather than merely the detection of the nitrate and nitrite ions. Herein, we demonstrate a two-step derivatization reaction requiring no purification steps or advanced sample preparation and characterize several of the reaction byproducts created along with the desired reaction product. In this process, nitrate esters are initially reduced to their component alcohol, and then subsequently silylated. The resulting product, when submitted to GC–MS with positive ion mode chemical ionization, provides mass spectra that characterizes the component alcohol by the pseudo-molecular ion of the reaction product. GC–MS with negative ion mode chemical ionization confirms the presence of a nitrate functional group by the characterization of the unreacted starting material. These results combine to identify nitrate esters in forensic settings, and as a proof of concept, we demonstrate the applicability of the derivatization reaction to pentaerythritol tetranitrate (PETN), NG, and erythritol tetranitrate (ETN) standards, as well as a formulated composition containing PETN-SEMTEX 1A, homemade ETN, and post-burn samples.