Drift tubes for characterizing atmospheric ion mobility spectra using ac, ac-pulse, and pulse time-of-flight measurement techniques

Abstract

Two drift tubes constructed of insulating cylinders with conductive guard rings on the inside walls are examined to determine their suitability for measuring ion mobility spectra at atmospheric pressure. One drift tube is of the pulse time-of-flight (TOF) type with adjustable drift distance, and the other is an ac-TOF drift tube similar in principle to devices reported by Tyndall and Powell [Proc. R. Soc. A 129, 162 (1930)] and Van de Graaff [Philos. Mag. 6, 210 (1928)]. The latter drift tube is evaluated using sinusoidal and alternating-polarity pulse-voltage waveforms for gating the shutters. Methods for determining the drift velocity of an ion from theoretical fits of the TOF spectrum are described for drift tubes of fixed length exhibiting ‘‘end effects.’’ Mobility values with uncertainties less than ±1% can be obtained with the pulse-TOF drift tube. Comparable results are obtained with the ac drift tube if an alternating-polarity pulse-voltage waveform is used for gating the shutter.