Studies of ion transport properties and ion-molecule interactions in a drift tube mass spectrometer (1984)
AuthorsMcIntosh, Bruce J.show all
Mobilities of ions in helium buffer gas have been determined using a drift tube mass spectrometer. The ions are formed by dissociative charge transfer from electron impact generated He⁺ to traces of compounds (<0.2%) added to helium buffer. The electric field strength to gas number density ratio (E/N) was varied from 10-140X10-²¹Vm². The design, construction and operation of the instrument is described in detail. An important feature of the instrument constructed for this project is the ability to vary drift distance from 1-10 cm by continuous movement of ion source. This feature enables mobilities to be determined for ions formed by one or more reactions along the drift tube provided the reactions are large completed during the early period of ion transit. Mobilities are derived from the slope of a plot of arrival time of maximum signal against drift distance. A mathematical analysis of this procedure is presented for various reaction schemes and used to determine optimum operating conditions and estimate systematic errors. Alternatively, the mobility may be determined iteratively by comparison of the experimental arrival time distributions with those calculated using an appropriate model. The total uncertainty in mobilities obtained by either technique is estimated not to exceed 5% and reproducibility between individual measurements is found to be better than 3%. A series of ions containing -CN group has been studied. Mobilities are presented for CN⁺, HCN⁺, H₂CN⁺, C₂N⁺, HC₂N⁺, C₂H₃N⁺ and C₂N₂⁺ formed from hydrogen cyanide, dicyanogen, methyl cyanide and methyl isocyanide. Momentum transfer collision integrals, Ω (¹,¹), have been calculated as a function of effective temperature in accordance with the two-temperature theory of Viehland and Mason. The mobilities of several selected organic ions formed from different precursor neutrals have been determined. The C₃H₆⁺ ion produced from either cyclopropane or propene was found to exhibit the same mobility regardless of the precursor neutral molecule. This was also found for the C₃H₃⁺ ions and in both cases is believed to signify structural equivalence. Ions of empirical formula C₂H₄O⁺ formed from ethanol and ethylene oxide were found to exhibit different mobilities and C₂H₅O⁺ ions from ethanol and dimethyl ether could be similarly distinguished. The different mobilities reflect structural differences and the use of mobility measurements as a technique for discrimination of isomeric ion structures is discussed.