Aerosols condensation method

Figure 1 Schematic of evaporation-condensation method of aerosol generation.

In the condensation method, a sample of vapour-saturated gas is subjected to a rapid volume expansion. This lowers the temperature and causes a state of supersaturation. Condensation of the vapour will then take place on any particles or ions present in the sample. If the sample is free of particles and ions, there will still be collisions of vapour molecules that create clusters, called embryos, which can serve as nuclei for condensation. Although the seeding nuclei maybe of different sizes, they grow by diffusion of the condensable vapour to ultimate diameters that are almost independent of the original nuclei sizes. The condensation method can be used to make aerosols having diameters from about 36 nm to just over 1 pm and concentrations from about 10 to 10 particles/cm [64]. 

Evaporation/condensation reactors involve evaporating a solid, such as a metal, and then mixing it with a cool gas to induce condensation as an aerosol. This method has been used to produce nanometre-sized metal particles. 

Another conditioning method, adaptable to scrubber systems, consists of inducing condensation of water vapor on the aerosol particles as nuclei, increasing the size of the particles and making them more susceptible to collec tion by inertial deposition. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

There are a number of techniques for generating aerosols, and these are discussed in detail in the LBL report (1979) and in volumes edited by Willeke (1980) and Liu et al. (1984). We briefly review here the major methods currently in use these include atomizers and nebulizers, vibrating orifices, spinning disks, the electrical mobility analyzer discussed earlier, dry powder dispersion, tube furnaces, and condensation of vapors from the gas phase. 

Gas-phase reactions can also be used to produce products of low volatility that condense to give an aerosol. The reaction of gaseous NH3 with HC1 to form particles of solid ammonium chloride and the reaction of gaseous S03 with water vapor to form H2S04 are typical examples. Such methods tend to give submicron particles. 

Another electrical method involves an analysis of the current voltage characteristics of a parallel plate condenser through which the aerosol is passed continuously (2N). 

Gas-phase and aerosol methods, such as by spray pyrolysis, laser pyrolysis, plasma, or vapor phase evaporation followed by condensation. 

Rather than produce an atomic vapor by evaporation from a solid surface, an aerosol may be generated from an aqueous salt solution by an atomization procedure. The aerosol can then be evaporated so that the salt condenses into a particle. This is known as the spray-pyrolysis technique. The flame decomposition method is a modification of this technique, in which the aerosol is introduced into a high-temperature flame (1200-3000 K). The precursor is vaporized and oxidized to form metal-oxide particles. 

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