Mobility drift

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. 

With normal metals it is just the reverse (for example, copper). Foreign atoms in the lattice disturb the regularity of the periodic potential in the lattice, like the thermal vibrations, and decrease the mobility (drift velocity) of the electron (waves) through the lattice because these are scattered. Substances with a low conductivity which, however, decreases with incr. temperature, such as some alloys, carbides and nitrides of titanium, vanadium etc., may be called semi-metals. 

Increasing crystallinity causes a fall in the conductivity of a polymer because the mobility (drift velocity per unit field) of the ions in the crystalline regions is lower than that of the ions in the non-crystalline regions. When designing materials for high conductivity it is therefore desirable to... 

FIGURE 3.7 Graphical representation of CGC retention time (in minutes), mobility drift time (in milliseconds), and total ion intensity (in arbitrary nnits) of lavender oil. The highlighted peaks (in white boxes) show separation of analyte peaks by GC only, IMS only, and both CGC and IMS this dnal CGC and IMS techniqne enables a greater degree of separation for a complex mixture than with either technique alone. (From Crawford et al.. The novel use of gas chromatography-ion mobility-time of flight mass spectrometry with secondary electrospray ionization for complex mixtnre analysis, Int. J. Ion Mobil. Spectrom. 2010, 14, 23-30. With permission.)... 

After mobility separation, the ions exit the ion mobility drift tube and are shaped into a ribbon trajectory with a combination of Einzel and DC-quadrupole lenses so that the ion beam traverses the 25-cm long focusing region and passes through a 1.6 X 12.7 mm slit into the source region of a TOF-MS. 

When possible, the first method is preferable because there is always the danger of mass discrimination, ion/neutral association, or collisional dissociation when ions pass from the high pressure of a mobility spectrometer to the vacuum of a mass spectrometer. Despite these complications, mass spectrometry has value for identifying ions because otherwise ion identity must be deduced intuitively by reference to known or anticipated reactions and ion behavior in a mobility drift tube. 

Schematic of a typical ion mobility spectrometer is shown in Fig. 1. An ion mobility spectrometer consists of an ionization source, an ion mobility drift tube, a detector, and supporting electronics. The samples are usually ionized by radioactive Nickel-63, electrospray ionization source, corona discharge, or photoionization source. The ions travel through the drift tube while colliding with the medium molecules, usually air or nitrogen, at atmospheric pressure. The resulting ion velocity is proportional to the applied electric field and mobility of the ion.

The initial setup was developed into a hybrid quadrapole-ion-mobility-TOF instrument [148], The collision cell region of this instrument features three traveling-wave stacked ring ion guides, of which the middle one is used as ion-mobility drift tube and the other two may be used as collision cell, when applicable. The 185 imn long IMS part is operated at pressures up to 1 mbar with up to 200 ml/min Ar whereas the collision cells are 100 mm long and operated at 10 mbar with up to 10 ml/min gas [148]. The system can be used for a wide variety of applications. 

Ion-mobility mass spectrometry (IM-MS) has emerged as an important analytical method in the last decade [74]. In IM-MS, ions are generated by pyrolysis, electrospray, laser desorption, or other ionization techniques prior to their entry into a gas-filled mobility drift cell. In this cell, ions drift at a velocity obtained from an electric field based on their shapes or dipoles in the case of differential mobility spectrometry (DMS). The greater the cross section of an analyte is (i.e., the larger the ion... 

Mobility Drift Time ( isec) Mobility Drift Time ( isec)... 

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