Field induced displacement reactions with proton bound dimers of organophosphorus compounds in a tandem differential mobility spectrometer.

Endothermic displacement reactions between proton bound dimers of organophosphorus compounds (OPCs) and isopropanol (IPA) were enabled in air at ambient pressure with tandem differential mobility spectrometry (DMS). Proton bound dimers (M2H+) were mobility isolated in purified air with a first DMS stage, mixed with IPA at ≥100 ppm in a middle reactive stage at 106 to 160 Td from a symmetrical 4 MHz waveform, and mobility analyzed in a second DMS stage. Although the enthalpy for displacement of M by IPA in M2H+ is unfavorable by +44 to 50 kJ mol-1, formation of the heterogenous proton bound dimer, MH+(IPA) arises from field induced dissociation of M2H+ to MH+ and addition of IPA. While peak dispersion for M2H+ of OPCs is limited to -2.25 to -0.5 V compensation voltage, peaks for MH+(IPA) were located at -10.5 to -8.25 V through a combination of ion transformation and mobility-based vapor modification. This inaugural use of ion reactions in air at ambient pressure demonstrates that multi-stage sequential processing of ions can improve significantly the analytical performance in a mobility spectrometer.

Dissociation Enthalpies of Chloride Adducts of Nitrate and Nitrite Explosives Determined by Ion Mobility Spectrometry.

The kinetics for thermal dissociations of the chloride adducts of the nitrate explosives 1,3-dinitroglycerin (1,3-NG), 1,2-dinitroglycerin (1,2-NG), the nitrite explosive 3,4-dinitrotoluene (3,4-DNT), and the explosive taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB) have been studied by atmospheric pressure ion mobility spectrometry. Both 1,3-NG·Cl(-) and1,2-NG·Cl(-) decompose in a gas-phase SN2 reaction in which Cl(-) displaces NO3(-) while 3,4-DNT·Cl(-) and DMNB·Cl(-) decompose by loss of Cl(-). The determined activation energy (kJ mol(-1)) and pre-exponential factor (s(-1)) values for the dissociations respectively are 1,3-NG·Cl(-), 86 ± 2 and 2.2 × 10(12); 1,2-NG·Cl(-), 97 ± 2 and 3.5 × 10(12); 3,4-DNT·Cl(-), 81 ± 2 and 4.8 × 10(13); and DMNB·Cl(-), 68 ± 2 and 9.7 × 10(11). Calculations by density functional theory show the structures of the nitrate ester adducts involve three hydrogen bonds: one from the hydroxyl group and the other two from the two nitrated carbons. The relative Cl(-) dissociation energies of the nitrates together with the previously reported smaller value for glycerol trinitrate and the calculated highest value for glycerol 1-mononitrate are explicable in terms of the number of hydroxyl hydrogen bond participants. The theoretical enthalpy changes for the nitrate ester displacement reactions are in agreement with those derived from the experimental activation energies but considerably higher for the nitro compounds.