Particle Beam

The original particle beam interface was introduced by Willoughby and Browner in the mid-1980s using the acronym MAGIC. It relies on the nebulization of the chromatographic eluent followed by desolvation and then ionization of the resultant microparticles. A schematic of a typical particle beam interface is shown in Figure 1.

The initial nebulization of the eluent is accomplished with the aid of a dispersion gas (usually helium); thus a fine and homogeneous aerosol can be generated from mobile-phase flow rates ranging from 0.1 to 2.0 mL min"1. Several designs of nebulizer are available, some utilizing heat or ultrasound in addition to a gas, to create the aerosol. The resultant mixture of gas and solvent droplets passes directly into a desolvation chamber where the droplets are converted into solvent-free particles before reaching the exit nozzle. To aid faster evaporation of solvent

Figure 1 Simplified schematic of a particle beam interface.

molecules, the temperature of the chamber is maintained slightly above ambient. Momentum separation of the resultant stream of gas, solvent vapour and solute microparticles occurs between the desolvation chamber and the ion source. This is achieved by a series of skimmers placed in line with the nebulizer jet and exit nozzle. Differential pumping is effected in the regions between the skimmers. Expansion into the lower pressure regions leads to the formation of a high velocity jet of solute microparticles. Most of the helium and solvent vapours are removed in these lower pressure regions, leading to solute enrichment. The solid solute microparticles enter a conventional EI/CI ion source and are rapidly converted to the gas phase by flash vaporization upon contact with the heated walls of the source. Subsequent ionization by electron impact or chemical ionization follows.

The particle beam interface offers the advantage of producing library-searchable mass spectra but there are limitations of volatility and thermal stability for the analytes. In common with most LC interfaces for MS, the use of involatile buffers is best avoided, as is the use of mobile phases with a high water content. Disadvantages of the particle beam interface lie in the lack of sensitivity compared to other techniques which rely on 'soft ionization' methods, but careful optimization can lead to detection limits in the nano-gram range for full scan acquisitions and use of selected ion monitoring can improve this to picograms. The development of particle beam interfaces capable of operating at lower flow rates would enable an increase in sensitivity to be achieved.

Solar Panel Basics

Solar Panel Basics

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