Field Induced Fragmentation (Fif) Spectra of Oxygen Containing Volatile Organic Compounds with Reactive Stage Tandem Ion Mobility Spectrometry and Functional Group Classification by Neural Network Analysis.

Mobility isolated spectra were obtained for protonated monomers of 42 volatile oxygen containing organic compounds at ambient pressure using a tandem ion mobility spectrometer with a reactive stage between drift regions. Fragment ions of protonated monomers of alcohols, acetates, aldehydes, ketones, and ethers were produced in the reactive stage using a 3.3 MHz symmetrical sinusoidal waveform with an amplitude of 1.4 kV and mobility analyzed in a 19 mm long drift region. The resultant field induced fragmentation (FIF) spectra included residual intensities for protonated monomers and fragment ions with characteristic drift times and peak intensities, associated with ion mass and chemical class. High efficiency of fragmentation was observed with single bond cleavage of alcohols and in six-member ring rearrangements of acetates. Fragmentation was not observed, or seen weakly, with aldehydes, ethers, and ketones due to their strained four-member ring transition states. Neural networks were trained to categorize spectra by chemical class and tested with FIF spectra of both familiar and unfamiliar compounds. Rates of categorization were class dependent with best performance for alcohols and acetates, moderate performance for ketones, and worst performance for ethers and aldehydes. Trends in the rates of categorization within a chemical family can be understood as steric influences on the energy of activation for ion fragmentation. Electric fields greater than 129 Td or new designs of reactive stages with improved efficiency of fragmentation will be needed to extend the practice of reactive stage tandem IMS to an expanded selection of volatile organic compounds.

Tandem ion mobility spectrometry at ambient pressure and field decomposition of mobility selected ions of explosives and interferences.

A tandem ion mobility spectrometer at ambient pressure included a thermal desorption inlet, two drift regions, dual ion shutters, and a wire grid assembly in the second drift region. An ion swarm could be mobility isolated in the first drift region using synchronized dual ion shutters and decomposed in a wire grid assembly using electric fields of 1.80 × 104 V cm-1 (118 Td) from a 1.8 MHz sinusoidal waveform. Mobility selected ions that underwent field induced decomposition were NO3-, from PETN·Cl- and NG·Cl-, and NO2- from RDX·Cl-. The extent of decomposition ranged from 60 to 90%, depending on gas temperature, field strength, and ion identity, introducing additional controls to improve selectivity in trace determination of explosives. Ion transmission through the wire grid assembly ranged from 80 to >95% with losses increasing for increased field strength. Studies with pairs of explosives and interfering substances demonstrated decisive detection of explosives and portend reduced rates of false positive using tandem ion mobility spectrometers with a reactive stage.