Practical implementations for ion mobility sensor

A system for signalling the presence of selected ion species in a gaseous sample of an ion mobility sensor (10), having a unidirectional field provided between first and second electrodes (16, 18) positioned a selected distance apart. An ionization source (35) generates ions in the sensor and a trigger controller (42) allows ions to enter the region (12) between the first and second electrodes at a time t1. The ions then arrive at the second electrode at a time t2. A transimpedance amplifier (38) converts the ion charges to voltage signals. The presence of selected ion species is determined by circuitry that is activated by the trigger controller at t2. The species are preferably analyzed by directing the amplifier output to an analog to digital converter (56) and then to a digital signal processor (58). The ions may alternatively be analyzed by generating a logic pulse representing a selected ion species when the amplifier output exceeds a preset value. An alarm signal is output when the logic pulse is coincident at an AND gate (46) with a logic pulse representing the species' travel time from the first electrode to the second electrode.