Int. J. Ion Mobil. Spec. DOI 10.1007/s12127-016-0203-6 ORIGINAL RESEARCH Model calculation for ion mobility in air using the MOBCAL program Kazunari Takaya 1,4 · Takahiro Kaneko 1 · Hajime Tanuma 1 · Tatsuhiro Nishide 2 · Hiroaki Sugiyama 2 · Nobuo Nakano 2 · Hisayuki Nagashima 3 · Yasuo Seto 3 Received: 2 May 2016 / Revised: 8 June 2016 / Accepted: 10 June 2016 © Springer-Verlag Berlin Heidelberg 2016 Abstract We determined new potential parameters for a calculation of ion mobility in air using the MOBCAL pro- gram. In our model, we regard air as a virtual uni-atomic molecular gas. And the Lennard-Jones parameters for vari- ous kinds of atoms in molecular ions were determined with simple and systematic methods. Using these parameters, we have calculated the mobilities of product ions from chem- ical warfare agents, normal saturated hydrocarbons, and illicit drugs. Most of the calculation results showed good agreement with the experimental values. Therefore this cal- culation method will be useful for understanding of the ion mobility spectrometry in air. Keywords Ion mobility in air · MOBCAL · Potential parameters Introduction Ion mobility spectrometry (IMS) has been widely used to analyze and identify chemical species of ionized molecules  Hajime Tanuma tanuma-hajime@tmu.ac.jp 1 Department of Physics, Tokyo Metropolitan Unversity, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan 2 Riken Keiki Co. Ltd., 2-7-6, Azusawa, Itabashi-ku, Tokyo 174-8744, Japan 3 National Research Institute for Police Science, 6-3-1, Kashiwanoha, Kashiwa, Chiba 277-0882, Japan 4 Present address: Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan in a gas based on their mobilities [1, 2]. Recently, IMS is applied for the detection of environmental pollutants, chem- ical warfare agents, and explosives because the IMS devices work under atmospheric pressure, detect agents quickly, and are miniaturized their size easily. We are now working for the development of a new IMS device for the chemical warfare agents in a national collaboration project in Japan. In IMS, a drift velocity of an ion under the influence of an electric field in a drift tube filled with a neutral buffer gas is measured using arrival time spectra from an electric shutter to a ion current detector, which is usually a Faraday plate. Since ions of different geometrical sizes and struc- tures will experience different interactions with the buffer gas molecules, each ion has a different drift velocity and mobility value, even if the these ions have a same mass number. As a tool of the gas analyzer, the IMS device must have an ionizer in it. We employed corona discharge in the atmospheric pressure as an ionization method. In this method, sample molecules are not directly ionized by elec- tron impact in the corona discharge, and they might be ionized mostly by the proton transfer reaction in colli- sions of H 3 O + , which are produced in some sequential ion-molecule reactions in air. But, the mechanism of the ion- ization by discharge in air is extremely complex, and it is hard to predict the product ionic species from some neutral gas samples. In addition, drifting ions under atmospheric pressure can form various cluster ions. For example, in high moisture conditions, some ions easily bond with H 2 O and hetero- dimers might be produced. On the other hand, when the concentration of a sample gas is very high, the protonated dimer ions should be considered. The IMS devices work- ing under atmospheric pressure, which measure only ionic currents as arrival time spectra, have no mass spectrometer.