Mechanical Ion Injection

A remarkable adaptation of technology borrowed from another analytical method was demonstrated for an electrospray ionization (ESI) IMS instrument ion injection to a drift tube was achieved through modulation with a mechanical chopper as found in atomic absorption spectrometry. The chopper was a disk with a small hole that would align with the source and drift tube and would operate as an ion injector. The disk had a second window that was used with optical sensors to synchronize ion injection and drift time, and ion injections were made at pulse rates of 5 to 200 Hz with pulse widths of 200 to 500 ps. 

The drift tube dimensions or characteristics were as follows length of 45.0 cm, a voltage divider with 3.34-Mft resistors, electric field of about 400 V/cm, and 78 drift rings (0.12 cm thick, 4.90 cm outside diameter, 2.55 cm inside diameter). The front flange of the drift tube was placed at ground potential, and the detector, and housing of the drift tube, was floated to -20.0 kV. Thus, the capillary of the ESI source was operated only at +5.0 kV with +500 V applied to the chopper wheel. The distance between the ESI source and inlet window of the chopper was 2 mm, and that between the inlet window and inlet flange of the drift tube was 5 mm. 

An attractive feature of this chopper-based method was the exclusion of ESI spray from the drift tube except for the brief period of ion injection. Aerosols were blocked from entering the drift tube most of the time, protecting the drift tube from the large burden of mass flux this made this design inherently clean, unlike other ESI IMS designs, for which a large demand exists for a drift tube to accept an aerosol-rich flow, which must be desolvated and swept from the drift tube. Injection pulse widths 

4 ION INJECTION TO DRIFT REGIONS WITHOUT WIRE-BASED ION SHUTTERS 

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