Sample molecules, collision between

Further explanation of this Cl process can be found in Chapter 1. Briefly, Cl results from collision between sample molecules and specially produced reagent gas ions such that ions are formed from sample molecules by various processes, one of the most important of which is the transfer of a proton (H+, Figure 9.2). 

Chemical ionization (Cl) is a softer technique than El, ions being produced by collisions between sample molecules and ions generated by a reagent gas such as methane or ammonia. Three stages are involved. For methane, for... 

After being formed as a spray, many of the droplets contain some excess positive (or negative) electric charge. Solvent (S) evaporates from the droplets to form smaller ones until, eventually, ions (MH+, SH+) from the sample M and solvent begin to evaporate to leave even smaller drops and clusters (S H n = 1, 2, 3, etc.). Later, collisions between ions and molecules (Cl) leave MH+ ions that proceed into the mass analyzer. Negative ions are formed similarly. 

Figure 18 illustrates the difference between normal hydrodynamic flow and slip flow when a gas sample is confined between two surfaces in motion relative to each other. In each case, the top surface moves with speed ua relative to the bottom surface. The circles represent gas molecules, and the length of an arrow is proportional to the drift velocity for that molecule. The drift velocity variation with distance is illustrated by the plots on the right. When the ratio of the mean free path to the separation distance between surfaces is much less than unity (Fig. 18a), collisions between gas molecules are much more frequent than collisions of the gas molecules with the surfaces. Here, we have classical fluid flow or viscous flow. If the flow were flow in tubes, Poiseuille s law would be obeyed. The velocity of gas molecules at the surface is the same as the velocity of the surface, and in the case of the stationary surface the mean tangential velocity of the gas at the surface is zero. 

The primary ion impact is believed to start a cascade of collisions between the impacting particle and the atomic nucleii in the sample, resulting in ejection of neutral molecules and ions through so-called sputtering (Fig. 2.8) [119]. The SIMS... 

Simple collision theory recognizes that a collision between reactants is necessary for a reaction to proceed. Does every collision result in a reaction Consider a 1 mL sample of gas at room temperature and atmospheric pressure. In the sample, about 10 collisions per second take place between gas molecules. If each collision resulted in a reaction, all gas phase reactions would be complete in about a nanosecond (10 s)—a truly explosive rate As you know from section 6.2, however, gas phase reactions can occur quite slowly. This suggests that not every collision between reactants results in a reaction. 

The Cl source operates at low pressure. Ion-molecule reactions occur and are needed for sample ionization. The MALDI source is under vacuum, but during the ionization process the pressure increases in the plume close to the target and ion-molecule reactions occur. The various sources operating at atmospheric pressure include ESI, APCI, APPI and AP-MALDI. All these sources operate at sufficient pressure to have numerous collisions between ions and molecules, and reactions between these species are observed. 

So far we have said nothing about the range of velocities actually found in a gas sample. In a real gas there are large numbers of collisions between particles. For example, when an odorous gas such as ammonia is released in a room, it takes some time for the odor to permeate the air, as we will see in Section 5.7. This delay results from collisions between the NH3 molecules and 02 and N2 molecules in the air, which greatly slow the mixing process. 

Immediately after their production, the ions in the humid atmospheric-pressure ion source will attract solvent molecules by ion-dipole interactions. These solvated ions must be desolvated prior to entering the mass analyser. This is achieved by ion-molecule collisions in the transition region, especially between the ion-sampling aperture and the skimmer. A small potential difference between the nozzle and the skimmer is applied to enhance declustering. 

In this discharge, electrons and positive ions are formed, and they interact with neutral sample molecules flowing through the discharge. Collisions between electrons and neutral sample molecules produce more sample ions. The newly produced ions collide frequently with other neutral... 

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