ABSTRACT
Various means can be employed to generate aerosols [15]. Primarily due to their simplicity, pneumatic nebulizers are by far the most common means for aerosol generation with analytical instruments employing aerosols. These nebulizers use a high-velocity gas jet to blast a liquid stream into aerosol droplets. With ultrasonic nebulization (USN), the liquid sample flows across the surface of an ultrasonic transducer powered by a radio-frequency (RF) power supply. With thermospray aerosol generation, liquid is pumped through a capillary that is electrothermally heated. If the energy input to the tube is sufficient to cause partial vaporization of the liquid, the fraction of vapor produced from the liquid will also increase as the exit of the tube is approached. Vapor exiting the capillary will be of sufficiently high velocity to convert the remaining liquid to an aerosol. In effect, thermospray acts to preconcentrate the dissolved solids in the liquid phase prior to interaction of the aerosol within the typically turbulent flow of a spray chamber, where the processes favoring the loss of large droplets can occur. Electrospray aerosols are generated by applying a high voltage between the tip of a capillary through which the sample flows and a counterelectrode and have found wide use in mass spectrometry. For the purposes of this discussion, the differences in typical size distributions for these aerosol sources are of interest, as exemplified by the dry particle size number distributions shown for three of these in Fig. 5 [16]. From this figure, electrospray aerosols can be highly monodisperse, whereas those of the thermospray and the pneumatic nebulizer are highly polydisperse. Further, as the thermospray aerosols effectively involve a preconcentration step, thermospray dry aerosols tend to be larger than pneumatic aerosols and, although not indicated, are also generally larger than USN aerosols. Electrospray aerosols can also be made much smaller than the example given, down to low nanometer levels.
