Aspirator Drift Tubes

While time-dispersive ion mobility devices of the type used for drift tube IMS require aperture grids prior to the Faraday plate to preserve the resolving power of the instrument, ion filters and scanning mobility spectrometry such as differential mobility spectrometry (DMS), field asymmetric IMS (FAIMS), differential mobility analysis (DMA), and aspiration IMS (alMS) do not require an aperture grid and can efficiently detect ions with a simple Faraday plate. In these devices, ions do not travel as a discrete swarm, and the exact arrival time of the ions is not critical. Figure 7.3 shows a schematic of a typical differential ion mobility spectrometer (DIMS) in... 

As with chromatography, the position of the peak in IMS provides qualitative information. The location of the ion swarm as it exits a drift region is dependent on the type of instrumentation used For drift tube instruments, it is the arrival time of the ion swarm at the Faraday plate or mass spectrometer orifice for DMS, it is the compensation voltage required to create a stable path through the instrument, and for aspiration-type instruments, it is the location of the Faraday plates as a function of the strength of the electric field. All of these qualitative measurements can be related to the mobility of the ion swarm, although in some cases this relation is complex and not well understood. Nevertheless, the relationships of K, K, and fl to ion mobility spectra have been described elsewhere in this book and serve as the qualitative basis of IMS. Until the fundamental relation of ion-molecule interactions can be understood sufficiently to model ion behavior in IMS instruments, IMS standards will serve to calibrate the various IMS platforms. 

In summary, ion mobility separations occur by a variety of methods. In all cases, there is an instrumental scan parameter that controls the separation of the ion. For example, in the drift tube spectrometers, it is the arrival time of the ion for the aspiration spectrometers, it is the position of the faraday plates for the mobility analyzers, it is the strength of the orthogonal voltage and for the DMSs, it is the compensation voltage. The relative value of these scan parameters for two ions is called the separation factor a, and the resolving power of a spectrometer is determined by the ratio of the scan parameter to the width of the scan parameters for a packet of ions. 

As described in previous chapters, there are many different types of ion mobility methods. These include drift tube ion mobility spectrometry (DTIMS), traveling wave ion mobility spectrometry (TW-IMS), differential mobility spectrometry (DMS), differential mobility analysis (DMA), and aspiration ion mobility spectrometry (alMS). All of these IMS methods have been interfaced to MSs. 

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