Effects in Ion Mobility Spectrometry

The influence of moisture in an IMS measurement begins with the formation and composition of reactant ions available in the reaction region of the drift tube. While the formation of hydrated protons can be assumed when moisture is 1 ppm and above, at levels of moisture of 100 ppb and below, the lifetime of ions such as H2O, N4+, and N2+ may be sufficient for reactions with analyte. At practical extreme moisture levels, 10 ppb and below, metastable ions may exist in helium. Consequently, a continuum of ion populations with moisture can be anticipated in both the reactant ion, and the associated reactions depend on moisture. 

In purified air or nitrogen at ambient pressure and temperature, the principal reactant ions in positive polarity will be hydrated protons, and the product ions will be formed through displacement-type reactions of Equation 11.1  

Generally, only molecules with dipole moments and proton affinities greater than that of water will be able to displace the water molecules and remain intact at moisture levels greater than 10 to 100 ppm and thus provide response in the instrument. This excludes alkanes, alkenes, aromatic hydrocarbons, and perhaps some alcohols. As moisture levels reach 0.1 to 0.5 ppm, charge exchange reactions may be observed as shown in Equation 11.2  

Recently, the mobility spectra for TMA were obtained over a range of moistures, and emphasis was given to quantitative characteristics and how moisture inside the drift tube affected detection. As a molecule with strong proton affinity, TMA was ionized over a range of moistures. When response was based on the monomer ion peak or the sum of peaks generated by the analyte, the sensitivity of detection was not strongly dependent on humidity. The proton-bound dimer was quantitatively 

In given concentrations of sample vapor neutrals, spectra for product ions can be altered by control of temperature, and this was seen at cryogenic tanperature, at which ion clusters not usually observed in mobility spectra were formed and appeared in mobility spectra. For example, proton-bound trimers of alcohols were observed when temperatures were decreased to -20°C and dissociated at temperatures from -20°C to +10°C. Increases in temperatures will lead to dissociation of complex ions, such as proton-bound trimers and proton-bound dimers. As temperature is increased, the intensity of peaks for protonated monomer increase, and the peak abundance of proton-bound dimers decreases. This has been developed and explored for dimethyl methyl phosphonate (DMMP), amines, and ketones. For example, proton-bound dimers of alkyl amines underwent dissociation above -30°C on a 2- to 20-ms time scale, which is within the range of drift times for these ions. Consequently, the dissociation pathway can be observed as a distortion in the peak shape and baseline of a mobility spectrum since an ion entering the drift region as a proton-bound dimer dissociates to a protonated monomer before arriving at the detector. These studies permitted the determination of kinetics of dissociation for thermalized ions and illustrated that the appearance of an ion in a mobility spectrum is governed by ion lifetimes in comparison to ion residence times in drift tubes, and ion lifetimes are controlled by temperature. 

---

Spangler, G.E. Space charge effects in ion mobility spectrometry. Anal. Chem. 1992,64, 1312. 

---