Aerosol Chambers

McMurry, P. H., and D. J. Rader, Aerosol Wall Losses in Electrically Charged Chambers, Aerosol Sci. Technol., 4, 249-268 (1985). 

Three main processes appear to control the modification and loss (or transport) of analyte aerosol in the spray chamber droplet-droplet collisions resulting in coagulation, evaporation, and impact of larger droplets into the walls of the spray chamber. Aerosol droplets can be lost (impact the walls and flow down the drain) as a result of several processes in the spray chamber [11,20]. Because turbulent gas flows are key to generating aerosols with pneumatic nebulizers, the gas in the spray chamber is also turbulent. Droplets with a variety of diameters... 

Aerosol beams are directed streams of small particles in a low pressure gas. They are formed when an aerosol expand.s from a high-pressure reservoir through a no/xle into a low-pressure chamber. Aerosol beams were discussed early in this chapter in connection with studies of particle rebound from surfaces. Such beams are also used to introduce particles into mass. spectrometers for single-particle chemical analysis (Chapter 6). The characteristics of the aerosol beam depend on particle size, nozzle configuration (converging or capillary), and skimmer arrangements. 

As a first stage, the stream of liquid from an HPLC eluant is passed through a narrow tube toward the LINC interface. Near the end of the tube, the liquid stream is injected with helium gas so that it leaves the end of the tube as a high-velocity spray of small drops of liquid mixed with helium. From there, the mixture enters an evacuation chamber (Figure 12.1). The formation of spray (nebulizing) is very similar to that occurring in the action of aerosol spray cans (see Chapter 19). 

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

The first form of aerosol modifier is a spray chamber. It is designed to produce turbulent flow in the argon carrier gas and to give time for the larger droplets to coalesce by collision. The result of coalescence, gravity, and turbulence is to deposit the larger droplets onto the walls of the spray chamber, from where the deposited liquid drains away. Since this liquid is all analyte solution, clearly some sample is wasted. Thus when sensitivity of analysis is an issue, it may be necessary to recycle this drained-off liquid back through the nebulizer. 

To assist evaporation of solvent, the argon stream carrying the aerosol can be passed through a heated tube called a desolvation chamber, operated at temperatures up to about 150°C. 

These generators vaporize a liquid (oil/mineral oil or glycol and water), which then condenses into a fine aerosol on contact with cooler air. The amount of smoke produced should be controllable by the liquid feed rate and the temperature of the heating chamber, but in practice the output is not ea.sy to control. They will, however, produce a large amount of smoke over a long periled, dhe generators are relatively expensive (several hundred ECUs), are bulky, are not generally portable, and require an electrical connection. 

In brief, the experimental technique is to create abacterial population in a close chamber, obtain a quantitative assessment of the viable airborne bacterial population by means of a suitable sampling device, submit the population to the disinfeetant action, whether ultraviolet tight, chemical vapour or aerosol, and then determine the airborne population at suitable intervals. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

Two-Filter Method (TF). The Two Filter Method was first introduced by Thomas and LeClare (Thomas and LeClare, 1970). A modified 66.3 liter decay chamber was used with a filter on each end. Air was sampled at 40 /min. The inlet filter removed all of the radon daughters and aerosol particles but allowed radon gas to pass. 

Porstendorfer, 1984). Knutson et al. (1983) measured similar results in their chamber investigation. The results show that the values of the deposition velocity of the free radon daughters are about 100 times those of the aerosol radon progeny. But there are no information about the effective deposition surface S of a furnished room for the calculation of the plateout rates qf and qa by means of Vg and Vg. For this reason the direct measurements of the plateout rates in rooms are necessary. Only Israeli (1983) determined the plateout rates in houses with values between qf = 3-12 h"1 and qa = 0.4-2.0 h"1, which give only a low value of the... 

It has been found that the "unattached" fraction is an ultrafine particle aerosol with a size range of 0.5 to 3 nm. In order to initiate studies on the formation mechanism for these ultrafine particles, a series of experiments were made in the U.S. Bureau of Mines radon chamber. By introducing SO into the chamber, particles were produced with an ultrafine size distribution. It has been found that the particle formation mechanism is supressed by the presence of radical scavengers. These experiments suggest that radiolysis following the decay of Rn-222 gives rise to the observed aerosol and the properties of the resulting aerosol are dependent on the nature and the amount of reactive gas present. 

A TSI Condensation Nuclei Counter model 3020 is used to continuously monitor the aerosol concentration in the chamber atmosphere. The chamber air is drawn through a port in the chamber wall into the counter and returned to the chamber through another port. The chamber interior temperature and humidity are monitored with a commercial hygrometer system. 

---