Faraday plate detector

The lifetimes of gas ions in air at ambient pressure were first measured directly in IMS with a dual-shutter IMS-MS. " A drift region was used to separate ion swarms and a second ion shutter at the end of the drift region was used to isolate an ion (e.g., a proton bound dimer), which then passed into another drift region before reaching the mass spectrometer. There was time enough to observe decomposition and obtain rate constants. Later, purpose-built IMS-IMS was described and included a Faraday plate detector. A schematic of this drift tube is shown in Rgure 6.8b the instrument is now used in determining the lifetime of ions from explosives in air. ... 

AMBIENT DETECTION OF MOBILITY-SEPARATED IONS 7.2.1 Faraday Cup and Faraday Plate Detectors... 

The Faraday plate detectors, the mainstay of detector technology in IMS from its inception as a modern analytical method, are seen as robust and effective for field instruments and relatively small ions. Nonetheless, the poor gain and susceptibility to microphonic noise can also be seen as disadvantages. Probably, these worries and wishes are small compared to the fundamental barrier to improved resolving power, which is established with the ion shutter. The Bradbury-Neilson (BN) or Tyndall-Powell (TP) shutters have and will certainly be the method of choice into the foreseeable future for IMS analyzers. The constraint is ambient pressure based. The limitation induced with fleld-mobility-based injections are large, yet no improved solution has been demonstrated. 

When an ion swarm is injected into the drift region of the drift tube, spatial resolution of ions of differing mobility can be separated as differences in drift velocity as the ions move toward the detector, here at virtual ground. Separate packets or swarms of ions develop with the separation as shown in Fig. 2, where three ion swarms have been resolved in time and space. As ions collide with the detector, commonly a simple metal disc or Faraday plate, neutralization of ions is accompanied by electron flow in the detector plate this is amplified and shown in the inset of Fig. 2. Thisplot of detector response(current or voltage) versus time (in ms) is called a mobility spectrum and is the... 

Fig. 1.23. The electron diffraction apparatus developed by Parks and coworkers includes an rf-ion trap, Faraday cup, and microchaimel plate detector (MCP) and is structured to maintain a cylindrical symmetry around the electron beam axis [147]. The cluster aggregation source emits an ion beam that is injected into the trap through an aperture in the ring electrode. The electron beam passes through a trapped ion cloud producing diffracted electrons indicated by the dashed hues. The primary beam enters the Faraday cup and the diffracted electrons strike the MCP producing a ring pattern on the phosphor screen. This screen is imaged by a CCD camera mounted external to the UHV chamber. The distance from the trapped ion cloud to the MCP is approximately 10.5 cm in this experiment...

By measuring the drift time U needed by ions to overcome the distance between the shutter grid and the detector (a Faraday plate), mobilities K are determined. 

FIGURE 11-3 Faraday cup detector The voltage on Ihe ion suppressor plates is adjusted to minimize differentiat response as a function of mass. 

For drift tube IMS systems, efficient collection of the ions is not the only consideration for ion detection. As a swarm of ions approaches the Faraday plate, it induces a charge on the plate that starts current to flow in the detector. That is, as a swarm of positively charged ions approaches a Faraday plate, electrons in the metal plate... 

Ion detection is provided by Faraday cups, electron multipliers, channel electron multipliers, cryogenic detectors, multichannel plate detectors, and electroop-tical detectors. 

Figure 4.16 Schematic of a Faraday Cup detector along with the incoming ion beam and the secondary electrons (e ) prodnced/trapped by the repellor plate.

Figure 1.2 Typical arrangement for a conventional Ion mobility spectrometer. Ions are produced In the upstream region (left-hand side of the figure), in this case via a radioactive source, and are then drawn from left to right by an electric field applied through a series of electrodes (the guard rings ). Ions are injected in pulses using an electrical shutter (a Bradbury-Nielson (BN) gate) and the time taken to reach the detector is then determined. The ion detector in the figure is a simple Faraday plate (see Section 3.6).

To obtain a mass spectrum, ions need to be converted into a usable signal by a detector. The simplest form of ion detection is a photographic plate or a Faraday cup for the direct measurement of the charge. In a Faraday cup the induced current is generated by an ion which hits the surface of a dynode and emits... 

Figure 10.5 See color plates. Handheld, Portable n -Faraday Differential CHA detector under development at Sandia National Laboratories in April 2006. (Courtesy Philip J. Rodacy, Sandia National Laboratories.)...

Another commonly used detector is the Faraday cup. This detector is an analogue detector and so has poorer sensitivity than a pulse counting electron multiplier. However, it has the advantage of simplicity (it is essentially only a metal plate used to measure ion current), and it does not suffer from burn-out like an electron multiplier (which must be periodically replaced). 

---