Improving the Sensitivity and Resolution of Miniature Ion Mobility Spectrometers with a Capacitive Trans Impedance Amplifier

Abstract

The selectivity and sensitivity of ion mobility spectrometry (IMS) to explosives was first demonstrated by Karasek in 1974.1 Airport security has always been a concern in the United States, especially since September 11th, 2001, and as a result IMS is commonly used to screen airline passengers and their luggage at all major airports. Portable IMS systems are now widely available for a variety of applications, but as the overall size of the IMS instrumentation decreases, the sensitivity typically decreases as well. A new ion detector read out technology, a capacitive trans-impedance amplifier (CTIA), coupled to a traditional Faraday plate has shown increased sensitivity over a Faraday plate read by a conventional current to voltage converter when used in mass spectrometry. Sandia National Laboratories sponsored a project to determine whether the CTIA technology could be coupled to an IMS, and to determine the potential increase in sensitivity that could be provided to a miniature IMS equipped with the new read out technology.Sandia first provided a full size IMS, a Phemto-Chem PCP-110, which was modified to support the first generation of CTIA (CTIA1). The CTIA1 was coupled to the IMS and was successfully used to detect explosives. Next, Sandia provided miniature IMS drift tubes, but incompatibilities necessitated the design of new miniature systems. At first, only the drift tube itself was redesigned, but eventually a complete miniature IMS, including the ionizer, circuitry, and read out, was designed and built. During the design phase a new ion-beam shutter capable of increased resolution was also implemented. The second generation of CTIA was coupled to a custom drift tube and the system demonstrated increased resolution and drastically increased sensitivity to the common explosives TNT and RDX when compared to the sensitivity of the system provided by Sandia. A custom miniature drift tube coupled to a CTIA will be placed into the peripheral equipment for Sandia's MicroHound II instrumentation to provide a portable IMS with sensitivity equal to or better than bench top IMS systems.