In an IMS, ions formed from sample vapors are pulsed into an electric field gradient where they are accelerated against a counter-flow of neutral drift gas molecules. The ions are then separated according to their relative mobilities and identified by plotting the current produced as ions strike a collecting electrode versus their characteristic ion arrival times. The result is a spectrum of ion mobilities.
FT-IMS improves upon existing IMS technology, resulting in greater sensitivity, better resolution, and faster processing time. IMS is used by militaries worldwide to detect chemical warfare agents; by security agencies to detect explosives and narcotics; and by industry to detect toxic industrial chemicals. With FT-IMS, the detection limits show up to a 100-fold improvement over conventional IMS technology.
In current military/commercial IMS systems, the ability to distinguish unique ion mobilities depends on the width of the entrance gate opening pulse that admits ions into the spectrometer as well as the resolving power of the drift region, where differences in ion velocity are established. A wider gate pulse would admit more ions and increase the signal, but the limited resolving power of the drift region would be overwhelmed. This trade-off between sensitivity and resolution is fundamental to current IMS instruments, which require a very narrow gate pulse followed by signal-averaging of multiple scans to achieve adequate signal-to-noise. As a result, current IMS systems suffer from relatively poor resolution.
With FT-IMS the first gate admits ions into the drift region, while the second “external exit” gate pulses against the signal produced by the streaming ions. An interferogram is created as the ions move into and out of phase with the external exit gate pulse. This interferogram is Fourier transformed to generate a more accurate, precise, high intensity ion mobility spectrum. With FT-IMS the entrance gate operates at a 50% duty cycle (compared to 1% with IMS) and so affords a 7-10 fold increase in sensitivity. Second, the entrance gate frequency is variable and can be altered to increase the spectral resolution, whereas conventional IMS operates at a fixed frequency limiting resolution. Finally, fractional second processing time allows higher sampling throughput than IMS.
