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.

Reactive Tandem Ion Mobility Spectrometry with Electric Field Fragmentation of Alcohols at Ambient Pressure.

A tandem ion mobility spectrometer at ambient pressure with a reactive stage produced fragment ions by water elimination from protonated monomers of alcohols with carbon numbers three to nine. Protonated monomers of individual alcohols were mobility isolated in a first drift region and were fragmented to carbocations at 64 to 128 Td and 45 to 89°C. Precursor and fragment ions were mobility characterized in a second drift region. Enthalpies for fragmentation of ROH2+ to primary carbocations were calculated as 76 to 97 kJ/mol and enthalpies for subsequent charge migration to 2° carbocations were -49 to -58 kJ/mol. Plots of drift times for pairs of protonated monomer and fragment ions from alcohols, esters, alkanes, and aldehydes produced distinctive trend lines attributed to fragmentation paths characteristic of chemical class. Specific combinations of drift times for fragments and precursor ions provide additional chemical information for spectral interpretation in ion mobility spectrometry.