Ion drift

Upon acceleration through an electric potential of V volts, ions of unknown m/z value reach a velocity v = f2zeV/m]" ). The ions continue at this velocity (drift) until they reach the detector. Since the start (to) and end (r) times are known, as is the length d of the drift region, the velocity can be calculated, and hence the m/z value can be calculated. In practice, an accurate measure of the distance d is not needed because it can be found by using ions of known m/z value to calibrate the system. Accurate measurement of the ion drift time is crucial. 

In time-of-flight (TOF) mass spectrometers, a pulse of ions is accelerated electrically at zero time. Having attained a maximum velocity, the ions drift along the flight tube of the analyzer. The times of arrival of ions at a detector are noted. 

For example, if each bin represents 0.3 nsec and bin number 200 has been affected by an ion arrival, then the flight time must have been 200 x 0.3 = 60 nsec. Knowing the length of the drift tube, the ion drift velocity can be calculated, and from that calculation its m/z value can be deduced. 

Fast concentration and sample injection are considered with the use of a theory of vibrational relaxation. A possibility to reduce a detection limit for trinitrotoluene to 10 g/cnf in less than 1 min is shown. Such a detection limit can by obtained using selective ionization combined with ion drift spectrometry. The time of detection in this case is 1- 3 s. A detection technique based on fluorescent reinforcing polymers, when the target molecules strongly quench fluorescence, holds much promise for developing fast detectors. 

There are, however, also similarities between the two discharge types. Positive ions drifting toward the opposite electrode form a space charge that affects the electric field and the corona current principally in the same way as in a negative corona discharge. Also, the charging of particles takes place almost in the same way for both types of corona discharge. 

Electron tunnelling through the stable oxide film to the adsorbed oxygen which sets up a potential and causes ion drift, thus resulting in logarithmic oxide growth. 

The Cu+2 ion drifts away into the solution but the electrons remain in the copper rod. They move up through the copper anode, through the wire, and enter the silver cathode. At the surface of this rod, the electrons encounter Ag ions in the solution. The electrons react with Ag+ ions to give neutral silver atoms which remain on the rod as silver metal ... 

Gieniec, J. Mack, L.L. Nakamae, K. Gupta, C. Kumar, V. Dole, M. ESI Mass Spectroscopy of Macromolecules Application of an Ion-Drift Spectrometer. Bio-med. Mass Spectrom. 1984,11, 259-268. 

There are several difficulties in the application of this technique to the analysis of sodium barrier properties of these polyimide films. First, as we have seen above, large shifts in the surface potential characteristics of MPOS structures can be associated with electronic conduction in the polyimide and charging of the polyimide-oxide interface. These shifts are not readily separable from any that might be caused by the inward drift of sodium ions. Second, the effect of the electronic charging process is to buck out the electric field in the polyimide which is needed to drive the ion drift mechanism. As seen in Figure 6, the electric field is reduced to very small values in a matter of minutes or less, particularly at the higher temperatures where ion drift would normally be measured. 

A new development, the selected-ion-drift tube or SIFT technique (Smith and Adams, 1979), enables the isolation of reactions due to a given set of ions, of a selected mfe value, with a neutral. Ions are produced outside the flow cell under high vacuum, and injected into the flow tube through a mass filter. This filter eliminates all ions except the ones selected (according to a specific m/e value), and removes the neutral precursor of the ion in question by differential pumping. 

All of these ion sources emit beams of positive ions at relatively low velocities, the ions drift or are pulled out from the ionization region with relatively small electrostatic potentials (U 20 kV). These beams of charged particles can be focused and transported in vacuum to the main accelerating machines. 

A modification of the conventional flowing afterglow apparatus, in which a drift section is incorporated, is shown schematically in Fig. 6.46i-141 In the so-called flow-drift apparatus reactant ions are produced in the upstream section just as in the conventional afterglow system, but the downstream section, where reactions with neutrals occur, is a drift tube, in which a uniform electric drift field is applied. In the latter section ions can be accelerated from thermal kinetic energies to several electron volts. The two sections of the apparatus are separated by an electronic ion shutter, which makes it possible to admit narrow pulses of ions into the drift region at specified times. This permits measurements of ion-drift velocity and, in... 

Positive ions drift to the sheath edge where they encounter the strong field. The ions are then accelerated across the potential drop and strike the electrode or substrate surface. Because of the series capacitor or the dielectric coating of the electrodes, the negative potentials established on the two electrodes in a plasma system may not be the same. For instance, the ratio of the voltages on the electrodes depends upon the relative electrode areas... 

A triple-quadrupole MS consists of three sets of quadrupole analyzers. As ions drift through the space between quadmpoles, pairs of rods of opposite polarity, the voltage can be varied so that only certain ions of specific m/z-ratios pass through the filter, whereas others are diverted. The applied voltage can be varied over time (scanned) and the number of ions exiting the filter can be analyzed as a function of a selected m/z value. In a triple-quadrupole MS, the second quadrupole is run in RF (transmitting all the ions present in the mixture) mode only, without preselection of m/z ratios, to induce fragmentation of ions by collisions with inert gas (CID, collision-induced dissociation). 

In a simple linear TOF instrument (Fig. 20), ions are formed within the source region(s) in the presence of an electrical field (V). The field accelerates the ions into a field-free region (d), which the ions all ideally enter with the same kinetic energy (KE) defined by the accelerating potential. Subsequently as the ions drift... 

Recently, the ion drift tube mobility studies from Bowers group (von Helden et al. 1991) have provided a means for separating carbon cluster ions with different structures because the reciprocal of the ion mobility is proportional to the collision cross-section. Thus a single cluster mass often consists of clusters with several different chemical structures and thus different cross-sections. 

Snowden-Ifft, D. R, Martoff, C. J., Burwell, J. M. 2000. Low pressure negative ion drift chamber for dark matter search, Phys. Rev. D61, 101301 Spergel, D. N. Steinhardt, P J. 2000. Observational Evidence for Self-Interacting Cold Dark Matter, Phys. Rev. Lett. 84, 3760 Spergel, D. N., et al. 2003. First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations Determination of Cosmological Parameters, ApJS 148,175... 

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