Aerodynamic Focusing Aerosol Beams

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. 

The variation of S with source pressure can be explained qualitatively as follows, referring to Fig. 4.15. For pa K /Joniin (Fig- 4.15a) the gas density downstream from the 

FifTure 4.14 Dependence of the beam expansion angle S on the reduced source pressure polpomvn for the converging nozzle, pmiiin Mm) - 29 mm punnn (0.365 /xm) 51 mm pomin (1-3 pm) = 330 inmHg. 

This qua) itati ve picture was verified experimentally by blocking part of the nozzle inlet. The pattern of the deposit showed that for po po min the panicle paths crossed between the nozzle inlet and the target (Fig. 4.15a), whereas for po pomin crossing did not occur. 

---

Under carefully engineered condition with low pressure, however, nanoaerosol particles can be separated from the gas phase because of the relatively small amount of molecules surrounding the nanoaerosol. The nanoaerosol can then be focused to a beam after expansion through an orifice, referred to as aerodynamic particle focusing. In this case, as depicted in Fig. 1, the trajectories of the aerosol particles depend on their corresponding Stokes number... 

Figure 4.7 Schematic of the time-of-flight aerosol mass spectrometer (TOF-AMS). Aerosol is introduced into the instrument through an aerodynamic lens focusing the particles through a skimmer and an orifice onto the vaporizer. Particle vapor is ionized and the ions are guided into the TOF-MS, which generates mass spectra at 83.3 kHz repetition rate. For particle size measurement the particle beam is chopped with a mechanical chopper and the detection is synchronized with the chopper opening time [178], Aerosol Science Technology A New Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS) - Instrument Description and First Field Deployment. 39 637-658. Copyright 2005. Reston, VA. Reprinted with permission...

---