A method of selecting ions includes generating a group of ions, accelerating the group of ions through a flight region towards an electronic mass selector grid, and selectively varying a voltage applied to the electronic mass selector grid, such that only a selected subset of the group of ions passes through the grid. An apparatus for selecting ions includes an ion generator, an ion accelerator for accelerating ions into a flight region, and an electronic mass selector grid responsive to an applied voltage to pass a subset of the ions from the flight region. An apparatus for detecting a threat molecule includes an ion generator for generating ions from a mixed gas stream, an ion accelerator for accelerating the ions into a flight region, and an electronic mass selector grid. The grid passes only a subset of the ions, such as ions and/or ionized fragments of the threat molecule.