Ion steering

It is frequently necessary to alter the path of a beam of ions. This may be required in order to steer the beam through an aperture, perhaps because of the possibility of a slight misalignment of the optical elements, or towards an ion detector. Alternatively, it may be necessary to deflect the ion beam along a completely different path, for example, turning it almost 90° for injection into a flight tube (see Section 3.5.4). 

Whatever the purpose, the most commonly used and simplest form of deflector is a pair of plane-parallel electrodes. With different potentials on the two electrodes, any positive ions that pass between the electrodes will be deflected towards the electrode with the least positive potential. If the velocity of an ion perpendicular to the surface of the plates is initially zero, then as soon as the ion enters between the plates and experiences the electric field it is deflected in a vertical direction, where the vertical direction is perpendicular to the surface of the electrode surface. For an ion with a charge e, the force experienced vertically is given by eE, and thus from Newton s second law the acceleration given to the ions is 

Instead of expressing the deflection in terms of time, we can express it in terms of horizontal distance travelled, jc, where x = vt and v is the horizontal velocity that is unchanged by a force acting in a perpendicular direction. Thus we end up with 

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FIGURE 9.4 A more detailed view of an orthogonal (right-angled) TOF ICP-MS system, showing some of the ion-steering components (courtesy of GBC Scientific Equipment Pty Ltd.). 

Chaimelling phenomena were studied before Rutherford backscattering was developed as a routine analytical tool. Chaimelling phenomena are also important in ion implantation, where the incident ions can be steered along the lattice planes and rows. Channelling leads to a deep penetration of the incident ions to deptlis below that found in the nonnal, near Gaussian, depth distributions characterized by non-chaimelled energetic ions. Even today, implanted chaimelled... 

O- versus C-a kyIation product ratios in the methylation of desoxybenzoin by dimethyl sulphate can be varied between 0.75 and 63 by the choice of catalyst. The reaction can be steered towards enol-ether formation by large, sterically shielded ammonium ions, while C-alkylation is favoured by small ammonium ions (e.g. RMejN" ) and by crown ethers (Dehmlow and Schrader, 1990). 

Lebold TP, YeowEK, Steer RP (2004) Fluorescence quenching of the SI andS2 states of zinc meso-tetrakis(4-sulfonatophenyl)porphyrin by halide ions. Photochem Photobiol Sci 3 ... 

A Ca2+-ion selective rigid -flexible - rigid type bichromophoric sensors based on the conformation liable bis-squaraine dyes 27 works on the principle of Cation-steered folding, which leads to dramatic perturbations in the optical properties as a result of exciton interactions [87],... 

Focusing elements to steer ions into mass analyzer(s)... 

J. Bourson and B. Valeur, Ion-responsive fluorescent compounds. 2. Cation-steered intramolecular charge transfer in a crowned merocyanine,/. Phys. Chem, 93, 3871 (1989). 

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