Separation of Ions from Explosives in Differential Mobility Spectrometry by Vapor-Modified Drift Gas

TLDR

Differential mobility spectrometry of nitro‑organic explosives in purified air at 100 °C yields product ions M– or M×NO₂⁻ whose peaks appear within a narrow compensation voltage range of –1 to +3 V, reflecting the low field‑dependence of ion mobility at E/N values up to 120 Td. The enhanced peak separation is explained by a ΔK model in which the low‑field mobility Kl of vapor‑clustered ions differs from the high‑field mobility Kh of declustered core ions. Adding 1000 ppm methylene chloride to the drift gas raises the alpha parameter from –0.005–0.02 to 0.08–0.24 and shifts compensation voltages from +3 to +21 V, demonstrating controllable field dependence.

Abstract

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.