Faraday plate

Figure 6. Diagram of our 1-atm ion mobility spectrometer (IMS) apparatus (a) stainless steel source gas dilution volume, (b) septum inlet, (c) needle valve, (d) Nj source gas supply, (e) source and drift gas exhaust, (f) flow meter, (g) pressure transducer, (h) insulated box, (i) drift tube, (j) ion source, (k) Bradbury-Nielson gate, (I) Faraday plate/MS aperture, (m) drift gas inlet, (n) universal joint, (o) electrostatic lens element, (p) quadrupole mass filter, (q) 6"-diffusion pump, (r) first vacuum envelope, (s) channeltron electron multiplier, (t) second vacuum envelope, (u) 3"-dif-fusion pump, (v) Nj drift gas, (w) leak valve, (x) on/off valves, (y) fused silica capillary, (z) 4-liter stainless steel dilution volume, (aa) Nj gas supply.

IM reaction. Instead, the waveform simultaneously obtained by the Faraday plate shown in Figure 7A is used for this purpose, because the sensitivity of the Faraday plate to these two ions is identical. 

The device resembles a cylindrical differential mobility analyzer (DMA) in that a sample flow is introduced around the periphery of the annulus between two concentric cylinders, and charged particles migrate inward towards the inner cylinder in the presence of a radial electric field. Instead of being transmitted to an outlet flow, the sample is collected onto a Nichrome filament located on the inner cylinder. The primary benefit of this mode of size-resolved sampling, as opposed to aerodynamic separation into a vacuum, is that chemical ionization of the vapor molecules is feasible. Because there is no outlet aerosol flow, the collection efficiency is determined by desorption of the particles from the filament, chemical ionization of the vapor, separation in a mobility drift cell, and continuous measurement of the current produced when the ions impinge on a Faraday plate. 

Finally, the ions (lowing down ihc quadrupole strike the Faraday plate defector. In some cases. Ihc signal is amplified further by an electron multiplier. Thus, there is obtained a spectrum of signal intensity versus m/e value. Each molecule has a unique fragmentation pattern so that a spectrum can he used as a fingerprint lor compound idenlilication. In addition, it is possible lo quantitate the amount of a particular compound by comparing sample signal intensity with the intensity produced hy a known amount of ihe compound. 

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... 

The combination of a chromatographic column with the IMS enables multidimensional data analysis, and allows peak identification by the use of chromatographic data (retention times) and also by use of the specific ion mobility data (arrival time at the Faraday plate) of the ions formed... 

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. 

Nickel is a transition element and has a variable valence. Using a nickel salt, 2.00 faradays plate out 39.2 g of nickel. What form of nickel ion is in the solution of this salt ... 

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... 

Ion collectors at ambient pressure, however, do not require a cup design because the impinging ion on the collector does not have sufficient energy to produce secondary ions thus, the potential loss of secondary ions is not a problem for quantification. Under ambient pressure conditions, ions can be collected with a simple flat-plate design called a Faraday plate. Faraday plates can be easily interfaced to an IMS as a flat, terminal plate after the ion separator. 

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... 

Teflon Block Niiich I Holds the Faraday Plate While Isolating from Rest of the Electrode... 

FIG U RE 7.2 Faraday plate assembly with aperture grid. (Drawn by ManujaLamabadusuriya.)... 

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... 

FIGURE 7.3 Differential ion mobility spectrometer (DIMS) showing both positive and negative Faraday plates used for detecting the ions that pass through the tunable ion filter. Because ions are detected continuously, no aperture is required. (From Sionex Corporation.)... 

Figure 8.2 shows a mobility spectrum in which the ion current is plotted as a function of arrival time obtained from an IMS DTIMS used for explosive detec-tion.2 In this negative-polarity spectrum, the intensity of the current at the Faraday plate is plotted as a function of the time after the ion shutter is opened to introduce an ion swarm into the drift region of the IMS. When an ion swarm arrives at the electrode, an increase in current produces a peak representing the arrival time of the swarm. The arrival time axis is normally recorded in milliseconds, and the measured mobility is determined by the relation... 

On the other hand, the DMA instrument, which operates similarly to the alMS, contains only one collector electrode. Ions are focused onto the Faraday plate as a function of scanning the orthogonal voltage through which the ions traverse. Spectra are plotted as the ion current on the Faraday plate as a function of UK, where K is the mobility of the ion. In the DMA literature, Z is used to denote ion mobility instead of K. As with the alMS, resolution power is low however, monomers can be separated from dimers. For example. Figure 8.4 shows a typical ion mobility spectrum of a 9.2-kDa polystyrene resin obtained from a DMA." ... 

FIGURE 8.3 Ion mobility spectrum from an aspiration-type ion mobility spectrometer. Spectrum is an array of Faraday plates producing a histogram-type display. ... 

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. 

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