Method Development for Selective and Nontargeted Identification of Nitro Compounds in Diesel Particulate Matter

Nitro-aromatic compounds are associated with a host of adverse human health and ecological outcomes; however, current methods of detection are limited by the lack of accuracy for the nontargeted identification of nitro compounds. This paper describes the development of a novel, accurate, and selective method of identifying nitro compounds, especially nitro polycyclic aromatic hydrocarbons (PAHs), in complex soot mixtures. For the first time, high-performance liquid chromatography was used in combination with Orbitrap mass spectrometry for the nontargeted identification of nitro compounds. This method was validated on a mixture of 84 standard molecules containing 23 nitro compounds and then applied to a complex soot sample, the National Institute of Standards and Technology standard reference material (SRM) 1650a, to successfully identify 105 masses, 62% of which were monosubstituted nitro compounds. All nitro compounds reported in the latest certificate of analysis (COA) for SRM 1650b were successfully identified except for 1,3-dinitropyrene and 1,6-dinitropyrene. Compounds not reported in the COA of the SRM 1650b, including but not limited to 1,8-dinitropyrene, alkylated nitro-molecules of all masses in the COA of SRM 1650b, nitro-phenylnaphthalene isomers, dinitronaphthalene, nitro-phenols, nitro-keto-PAHs, nitro-carboxylic-PAHs, and other nitro partial polar compounds, were also tentatively identified. Future work will consider a larger set of classes, including isomers. This will help identify the chemical composition of mixtures in order to take proactive approaches to prevent health and environmental hazards.