Capillary Aerosol Generator

In the capillary aerosol generator (CAG) system, the aerosol is formed by pumping the drug formulation through a small, electrically heated capillary. Upon exiting the... 

Shen, X., Hindle, M., and Byron P. R. (2004), Effect of energy on propylene glycol aerosols using the capillary aerosol generator, Int. J. Pharm., 215,249-258. 

The commonest form of sample introduction is by means of an aerosol generated using a pneumatic nebulizer. Several types of nebulizer can be used. All-glass concentric nebulizers (Fig. 4.7a) operate in a similar manner to those used for FAAS. Cross-flow nebulizers (Fig. 4.7b) operate by directing a high-velocity stream of gas across the mouth of a capillary... 

Experimental methods presented in the literature may prove of value in combustion studies of both solid and liquid suspensions. Such suspensions include the common liquid spray. Uniform droplets can be produced by aerosol generators, spinning disks, vibrating capillary tubes, and other techniques. Mechanical, physicochemical, optical, and electrical means are available for determination of droplet size and distribution. The size distribution, aggregation, and electrical properties of suspended particles are discussed as well as their flow and metering characteristics. The study of continuous fuel sprays includes both analytical and experimental procedures. Rayleigh s work on liquid jet breakup is reviewed and its subsequent verification and limitations are shown. 

Fig. 1 illustrates the two mechanisms proposed for the processes of liquid disintegration and aerosol generation within ultrasonic nebulizers. The capillary-wave theory relates to the production of capillary waves in the bulk liquid. These waves constructively interfere to form peaks and a central geyser. When the amplitude of the applied energy is sufficiently high, the crests of the capillary waves break off, and droplets are formed. The rate of generation of capillary waves is dependent on both the physicochemical properties of the nebulized fluid and the intensity of the ultrasonic vibration. Mercer used Eq. (1) to calculate the threshold amplitude for the generation of capillary waves ... 

The second mechanism proposed for aerosol generation is based on the piezoelectric crystal operating at low frequency and imparting vibrations to the bulk liquid. This results in the formation of cavitation bubbles, which move to the air-liquid interface.The internal pressure within the bubbles equilibrates with that of the atmosphere, causing their implosion. When this occurs at the liquid surface, portions of the liquid break free from the turbulent bulk liquid, resulting in droplet formation. The dependence of atomization on cavitation phenomena has been demonstrated for frequencies between 0.5 and 2.0 MHz.Boguslavskii and Eknadiosyants combined these theories with-their proposal that droplet formation resulted from capillary waves initiated and driven by cavitation bubbles. 

The concentric pneumatic nebulizer, used for solvent nebulization, consists of a 100-pm-ID fused-silica capillary for liquid introduction at flow-rates in the range of 0.1-0.5 ml/min, and a circumventing high helium flow (1-3 Fmin). The relative positions of the nebulizer jacket and the liquid capillary outlet can be adjusted to optimize the spray performance. Micro-flow aerosol generators for introduction of 1-5 pFminof hquid were described by Cappiello and Bruner [83-84]. 

A pulsed sample introduction (PSI) interface and laser-induced multiphoton ionization have been also used to couple LC with TOF instruments [56]. The interface consists of a heated capillary for aerosol generation and a high-temperature pulsed nozzle for sample vaporization. The LC effluent enters the heated capillary at a flow rate between 0.5 and 1.6 mLmin A solenoid allows the sample vapor to enter the mass spectrometer in a pulse form. 

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