An electrospray aerosol generator with X-ray photoionizer for particle charge reduction

Abstract

A prototype single-capillary electrospray (ES) aerosol generator having a soft X-ray photoionizer for charge reduction has been constructed and its performance under various configurations has been investigated in this study. The prototype essentially consists of spray and charge reduction chambers having an orifice disk plate as the partition. Two soft X-ray irradiation directions (i.e., 90° and 180° relative to the spray direction) and two disk plates (with the orifice diameters of 0.25in. and 1.25in.) were tested for the prototype configuration. In the investigation, the spray current as a function of ES voltage was recorded. It is shown that the presence of soft X-ray irradiation in the close proximity of the electrospray process resulted in the increase of applied voltage needed to operate the spray while the spray current at the cone-jet mode remained unchanged. The fluorescence analysis was applied in this study to quantify the transmission efficiency and the charge fractions of particles exiting from the prototype generator. A maximal 47% transmission efficiency could be achieved by the prototype having the setup of 180° irradiation and use of 0.25in. D-orifice plate, and operated at carrier gas flow rate of 8.0lpm. Under the above setup and operation the charge fraction of particles with the sizes ranging from 30 to 300nm was further characterized. It is found that the measured charge fractions of particles exiting from the generator is in good agreement with the calculated Fuchs bipolar charge factions for the cases having the particle sizes less than 110nm. The deviation from the Fuchs charge fraction increases as the particle size increases. It may be because (1) more ions were needed to reduce the charge level on particles of large sizes; (2) spatial non-uniformity of aerosol concentration and ions in charge reduction chamber; and (3) because of the direct photoionization of particles under the X-ray irradiation. At last, the size measurement of ES particles via a scanning mobility particle sizer (SMPS) shows the good quality of monodispersity for particles generated by the prototype when operated at the cone-jet mode.