Static targets

The kinetic energy for internal energy transfers is governed by the laws concerning collisions of a mobile species (the ion) and a static target (the collision gas). 

Although the term mechanism of action itself implies a classification according to the dynamics of drug substance effects at the molecular level, the dynamics of these interactions are only speculative models at present, and so mechanism of action can currently only be used to describe static targets, as discussed above. 

When the liquid target is not a static pool but, rather, a continuous stream of liquid, the added description of dynamic is used. Thus, dynamic FAB and LSIMS refer to bombardment of a continuously renewed (flowing) liquid target. 

In dynamic FAB, this solution is the eluant flowing from an LC column i.e., the target area is covered by a flowing liquid (dynamic) rather than a static one, as is usually the case where FAB is used to examine single substances. The fast atoms or ions from the gun carry considerable momentum, and when they crash into the surface of the liquid some of this momentum is transferred to molecules in the liquid, which splash back out, rather like the result of throwing a stone into a pond (Figure 13.2). This is a very simplistic view of a complex process that also turns the ejected particles into ions (see Chapter 4 for more information on FAB/LSIMS ionization). 

In fast-atom bombardment (FAB), an atom gun is used to fire heavy fast atoms at the static surface of a target solution (also called a matrix). 

An even more ambitious goal is to characterize an unsupported catalyst, because the surface is extremely rough and the target rapidly deteriorates under bombardment. Energy deposition leads to enormous erosion, because the substrate cannot get rid of the energy deposited, owing to the low heat conductivity. As a consequence static LEIS conditions have to be used to obtain information on the surface alone. In Fig. 3.60a we show a series of LEIS spectra obtained with 5 keV Ne" ions on a... 

This means that if the intensity of one of the forms is zero (static quenching), such anisotropy sensor is useless since it will show anisotropy of only one of the forms. The account of fractional intensity factor R Fb // , (the ratio of intensities of bound and free forms) leads to a more complicated function for the fraction of bound target,/ ... 

The nonelectric firing system is simple and requires a minimum of equipment. Once initiated, however, it passes from the control of the operator. The electric system can be retained in the control of the operator up to the actual firing. Electric caps also are more waterproof. On the other hand,-the electric system is more complicated, employs more equipment, and may require the operator to remain near the scene at the time of firing. In addition, electric systems can be accidentally activated by static electricity and are hazardous to use in some target situations. 

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