Tandem differential mobility spectrometry

Ion mobility spectrometry (IMS) provides knowledge of the ions shape and size through the measurement of the speed of ion swarms derived from a substance in electric fields. In one embodiment of IMS, termed differential mobility spectrometry (DMS), ions are characterized and separated through differences in mobilities at high and low electric fields. DMS analyzers exhibit poor resolution between peaks for ions masses from 150 to 300 Da. An advantage of small, planar DMS drift tubes operating in air at ambient pressure, however, is a continuous duty cycle for ion transmission, making DMS an ion filter. This feature favors the development of DMS analyzers in tandem configurations to improve peak capacity or selectivity of response in the current practice of DMS. Mobility measurements with independent control of ion motion in two sequential stages of DMS drift tube were achieved in a first demonstration of tandem DMS. Combinations of separation and compensation fields at characteristic or specific pairs for two DMS stages were used to isolate individual substances from others in the mixture of volatile organic compounds using based field dependent mobilities of ions in DMS. The selectivity of the measurement expressed as the ratio of analyte signal over the sum of signals of twenty two interfering compounds was improved 31X for dimethyl methylphosphonate and 106X for 1-hexanol in a tandem DMS measurement. Chemical modification of proton-bound dimers of dimethyl methylphosphonate and tributyl phosphate in tandem DMS was achieved using vapors of isopropanol introduced to the supporting atmosphere after ion selection in a first stage. Ions isolated near 0 Townsend in a compensation field in a first DMS stage underwent displacement reactions under the influence of vapor modifier which resulted in increased peak capacity. Finally, proton-bound dimers were dissociated through electric field heating of ions into protonated monomer ions in air at ambient pressure at 25°C using a tandem DMS instrument where the first stage was an ultrahigh field asymmetric waveform ion mobility spectrometer (ultraFAIMS). Protonated monomers were measured in a second stage from dissociation of proton-bound dimer ions of acetone, ethyl acetate, and dimethyl methylphosphonate. Fragment ions were formed from propyl acetate by heating proton-bound dimer ions at 90 Td. This provides a route to improve peak capacity in DMS without gas modifiers. An additional advantage of tandem DMS with fixed pairs of compensation and separation fields should be higher (∼10 ms) speed of response. These achievements establish a foundation for triple stage DMS with ion pre-selection, fragmentation, and characterization as an ambient pressure analog to tandem mass spectrometry.