Low-Pressure Detection of Mobility-Separated Ions

Low-pressure ion detectors for IMS are typical of those used in mass spectrometry. In fact, whenever low-pressure ion detection methods are used with IMS, the IMS is interfaced to a mass spectrometer. Mass spectrometers after IMSs serve as mass filters or mass dispersive devices prior to low-pressure ion detection. Mass discrimination is not necessary, however, as in the case of a quadrupole mass spectrometer operating in the radio frequency (RF)-only regime, where complete ion mobility spectra can be obtained. A more detailed discussion of the advantages 

Secondary electron multiplier (SEM) detectors replaced Faraday cup detectors for scanning mass spectrometers. The electron multiplier is based on the concept of a photomultiplier except that there is no glass membrane, so ions (electrons) can enter the amplification region of the detector. Because electron multipliers are not sealed and are open to the atmosphere, they must be operated under vacuum conditions and therefore cannot be used directly in atmospheric pressure IMS. 

A multichannel plate (MCP) is a type of CDEM in which a series of microchannels on a disk-shaped device are coated with an electron-emissive material to generate 10 to 10 amplification as the electrons cascade through the microchannels. MCPs can be stacked to increase amplification or focused onto a fluorescent surface for ion-beam imaging. Because of the short electron pulse widths ( 1 ns) obtained with MCPs, they are the ion detector of choice for time-of-flight mass spectrometers. 

Whether an electron multiplier has a discrete-dynode or continuous-dynode design, it can be operated in both analog and pulse-counting modes. For analog detectors. 

FIGURE 7.7 Discrete-dynode electron multiplier. (From Babis et al.. Performance evaluation of a miniature ion mobility spectrometer drift cell for application in handheld explosives detection ion mobility spectrometers, Anal. Bioanal. Chem. 2009, 395, 411-419. With permission.) 

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