An apparatus and method for measuring low or trace concentrations of compounds and mixture of gases. A method and apparatus of the invention permits separating ions of different mobilities by passing them through an abrupt change or step in electric field magnitude. By using the separation method, the compounds of interest may be measured with less interference from other compounds of the gas mixture, which reduces or eliminates the need for prior separation of the components of the gas mixture. Several embodiments of the invention are described including the use of current amplifiers on one, or more, parts of the apparatus. While a single screen can provide a chamber which is divided into two regions of different electric fields, it is within the scope of the invention to include multiple screens to provide several steps in the electric field permitting it to be possible to trap and measure ions with successively higher mobilities. The gases used include halogenated compounds, including fluorocarbons, and most preferable SF6.