Particle beam, focusing

Fig. I. Experimental setup the clusters are emitted from the cluster condensation cell, passing as a particle beam through a differential pumping stage into the focus of a time-of-flight mass spectrometer, where they are ionized by a laser pulse.

Electrospray has been successful for numerous azo dyes that are not ionic salts. Several anthraquinone dyes have been analysed by LC-ESI-MS [552]. Electrospray achieves the best sensitivity for compounds that are precharged in solution (e.g. ionic species or compounds that can be (de)protonated by pH adjustment). Consequently, LC-ESI-MS has focused on ionic dyes such as sulfonated azo dyes which have eluded analysis by particle-beam or thermospray LC-MS [594,617,618]. Techniques like LC-PB-MS and GC-MS, based on gas-phase ionisation, are not suitable for nonvolatile components such as sulfonated azo dyes. LC-TSP-MS on... 

Elemental mass distribution - The aerosol sampled by the LPI for elemental analysis was impacted on coated mylar films affixed to 25 mm glass discs. The mylar had been coated with Apiezon L vacuum grease to prevent particle bound. The LPI samples were sent to Crocker Nuclear Laboratory for elemental analysis by PIXE using a focused alpha particle beam of 3 to 4 mm diameter. Nanogram sensitivities for most elements were achieved with the focused beam. A detailed description of the PIXE focused beam technique applied to LPI samples can be found in Ouimette (13). Based upon repeated measurements of field samples, the estimated measurement error was about 15-20% or twice the minimum detection limit, whichever was larger. 

LSIMS is nearly identical to FAB except tliat the primary particle beam is an ion beam, usually cesium ions, rather than a neutral beam. Tlie ions can be focused and accelerated to higher kinetic energies than are possible for neutral beams, and sensitivity is improved for higher masses. 

Figure 14.12 Net focusing effect on a diverging charged particle beam from the combination of a converging and diverging pair of magnetic quadrupole lenses. The key feature of the doublet is that the particles move along paths that are closer to the optical axis in the defocusing element of the pair. (From Harvey, 1964.)...

The basic requirement is a diffusion-pumped chamber attached to an ion gun. The ion gun is filled with high-purity inert gas such as Ar. This gas is accelerated between two electrodes with a high potential difference. This ionizes the gas and a beam of focused and collimated gas particles is aimed at the specimen surface. Modern electronics allows very precise manipulation of the ion beam in several ways. Most ion milling machines are singlebeam systems where one surface of the specimen is thinned. Alternatively, double-sided machines are also available where there are two ion beams focused on either side of the same specimen that is milled from both the top and bottom surfaces. 

In the SSC and in other accelerators, the electromagnets are used to bend the streams of particles and keep them on the rails, so to speak, until they near the speed of light. The magnetic fields are also used to store particle beams and to detect and identify elementary particles. Two kinds of magnets are generally fitted into the system dipoles to contain and bend the beam, and quadrupoles to focus it to a narrow diameter. 

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