General Considerations Concerning Apparent Mobilities in Mixed Ion Populations: Drift Velocities of Mass-IdentifiedN+,N2+,N3+, andN4+Ions in Nitrogen

Abstract

This paper summarizes the difficulties inherent in the measurement of ionic drift velocities in gases, emphasizing the need for careful control of the ionization conditions in the ion source and the necessity for mass analysis of the ions at the end of their flight path. Mass analysis provides a positive determination of the molecular composition of the ions reaching the detector, detects the possible presence of a mixture of charge carriers of various molecular compositions, and is essential to help determine whether or not the charge carriers which have a given molecular composition when they arrive at the detector have had that same composition throughout their transit through the drift region. If examination of the arrival-time spectrum shows that a detected species is involved in an ion-molecule reaction scheme such that the charge carrier has spent an appreciable portion of its total drift time in a different ionic form, then the "apparent drift velocity" derived from the measurement of the transit time and drift distance cannot be meaningfully compared with the existing theory. A drift-tube mass spectrometer used to study the drift of mass-analyzed ions is described, and experimental results are presented for nitrogen ions in the parent gas. The drift-tube pressures used here ranged from 0.05 to 0.25 Torr. The $\frac{E}{{p}_{0}}$ range covered was 7 to 50 V/cm Torr for ${\mathrm{N}}^{+}$ and ${\mathrm{N}}_{3}^{+}$ ions and was 7 to 70 V/cm Torr for ${\mathrm{N}}_{2}^{+}$ and ${\mathrm{N}}_{4}^{+}$ ions. The drift distance was varied from 9 to 34 cm. The transit-time spectra for ${\mathrm{N}}^{+}$ indicate that its drift velocities measured here are the "true" values characteristic of the ${\mathrm{N}}^{+}$ ion undergoing no reactive scattering collisions. The zero-field, reduced mobility of ${\mathrm{N}}^{+}$ in ${\mathrm{N}}_{2}$ is 2.47 ${\mathrm{cm}}^{2}$/V sec. Ion-molecule reactions involving the other ions prevented the determination of their true drift velocities, but the measured apparent drift velocities yield useful information about their drift behavior and the reactions linking the various ions.