Ion velocity

Magnetic sector instruments typically operate with ion sources held at a potential of between 6 and 10 kV. This results in ions with keV translational kinetic energies. The ion kinetic energy can be written as zt V = Ifur and thus the ion velocity is given by the relationship... 

After the skimmer, the ions must be prepared for mass analysis, and electronic lenses in front of the analyzer are used to adjust ion velocities and flight paths. The skimmer can be considered to be the end of the interface region stretching from the end of the plasma flame. Some sort of light stop must be used to prevent emitted light from the plasma reaching the ion collector in the mass analyzer (Figure 14.2). 

For stand-alone or hybrid TOF mass spectrometry, the ions examined must all start from some point at the same instant. From this zero time, the ions are accelerated through a short region by applying a short pulse of electric potential of several kilovolts. The acceleration gives the ions velocities that vary in proportion to the square root of their m/z values. 

Taking into account that neutralization means tunneling of a target conduction-band electron to the ion, the time integral can easily be replaced by integration over the distance from the surface, s, by use of the identity dt = ds/v , where Vj is the component of the ion velocity perpendicular to the surface. Prom this, the velocity-dependence of the survival probability, P , is obtained ... 

The experimental data of Friedman show a strong energy dependence of [Pg.120]

Reaction 27 is exothermic by 38 kcal./mole, Reaction 28 by 60 kcal./ mole, and Reaction 29 by 16 kcal./mole while C2H3+ appears to be somewhat endothermic (8). Table II shows the spectrum of these ions at two different kinetic energies. The more extensive decomposition at higher impacting ion velocity is clear evidence for the conversion of kinetic energy to internal energy. 

Elastic collisions and chemical reactions in the sheath lead to a broad angular distribution of the ion velocity at the electrode. Examples for SiH and SiH are the abstraction reactions SiH -I- SiH4 SiH4 -f- SiH [330] and SiH - -SiH4 —> SiH + SiH3 [331], Due to the small acceptance angle of the EQP, these processes are only in part reflected in the lED [163, 332]. 

To sum up, the basic idea of the Doppler-selected TOF technique is to cast the differential cross-section S ajdv3 in a Cartesian coordinate, and to combine three dispersion techniques with each independently applied along one of the three Cartesian axes. As both the Doppler-shift (vz) and ion velocity (vy) measurements are essentially in the center-of-mass frame, and the (i j-componcnl, associated with the center-of-mass velocity vector can be made small and be largely compensated for by a slight shift in the location of the slit, the measured quantity in the Doppler-selected TOF approach represents directly the center-of-mass differential cross-section in terms of per velocity volume element in a Cartesian coordinate, d3a/dvxdvydvz. As such, the transformation of the raw data to the desired doubly differential cross-section becomes exceedingly simple and direct, Eq. (11). 

Fig. 4.49. Motion of positive ions in a uniform magnetic field B. (a) The radius is a function of ion velocity, but the frequencyof circulation is not. (b) Excitation of the ions by an RF electric field oscillating at their cyclotron resonance frequency. Adapted from Ref. [196] by permission. John Wiley Sons, 1986.

Note The need for almost perfect vacuum, i.e., extremely long mean free paths, in FT-ICR mass spectrometers arises from the combination of high ion velocities of several 10 m s observation intervals in the order of seconds, and the effect of collisions on peak shape. 

Although the initial velocity of the desorbed ions is difficult to measure, reported values generally are in the range of 400-1200 m s" The initial velocity is almost independent of the ionic mass but dependent on the matrix. [33,36-38,46,50,51] On the other hand, the initial ion velocity is not independent of the compound class, i.e., peptides show a behavior different from oligosaccharides. [51]... 

The essential independence of mean ion velocities on the molecular weight of the analyte leads to an approximate linear increase of the mean initial kinetic energies of the analyte ions with mass. High-mass ions therefore carry tens of elec-tronvolts of translational energy before ion acceleration. [33,41,50] The initial velocity of the ions is superimposed onto that obtained from ion acceleration, thereby causing considerable losses in resolution with continuous extraction TOP analyzers, in particular when operated in the linear mode. 

Karas, M. Bahr, U. Fournier, L Gltick-mann, M. Pfenninger, A. The Initial Ion Velocity As a Marker for Different Desorption-Ionization Mechanisms in MALDI. Int. J. Mass Spectrom. 2003, 226, 239-248. 

Gluckmann, M. Karas, M. The Initial Ion Velocity and Its Dependence on Matrix, Analyte and Preparation Method in Ultraviolet MALDI. J. Mass Spectrom. 1999, 34,461 All. 

Figure 2. Schematic diagram of a solid surface exposed to a plasma. Typical values for important parameters are indicated. The symbols AK, and vi represent the sheath thickness, sheath potential, ion concentration, and ion velocity, respectively.

This force is contrasted by an opposite force (F ), due to the viscosity resistance of the solution, which increases as the ion velocity (v) increases ... 

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