Separation of ions from explosives in differential mobility spectrometry by vapor-modified drift gas.

Differential mobility spectrometry (DMS) of nitro-organic explosives and related compounds exhibited the expected product ions of M- or M x NO2- from atmospheric pressure chemical ionization reactions in purified air at 100 degrees C. Peaks in the differential mobility spectra for these ions were confined to a narrow range of compensation voltages between -1 to +3 V which arose through a low dependence of mobility for the ions in electric fields at E/N values between 0 and 120 Td (1 Td = 10(-17) V cm2). The field dependence of ions, described as an alpha parameter, ranged from -0.005 to 0.02 at a separation field of 100 Td. The alpha parameter could be controlled through the addition of organic vapors into the drift gas and was increased to 0.08-0.24 with 1000 ppm of methylene chloride in the drift gas. This modification of the drift gas resulted in compensation voltages of +3 to +21 V for peaks. The improved separation of peaks was consistent with a model of ion characterization by DeltaK or Kl - Kh, where Kl is the mobility coefficient of ions clustered with vapor neutrals during the low-field portion of the separation field waveform and Kh is for the same core ion when heated and declustered during the high-field portion of waveform.