Optical Depth of Field Effects in Particle Size Analysis by Laser Diffractometry

Abstract

A nonlinear overestimate of the median droplet diameter measured by laser diffractometry in aerosols of interest in atomic spectrochemical analysis is found to result if the aerosol path traversed by the laser exceeds the optical depth of field characteristic of the diffractometer. The depth of field of a Leeds and Northrup diffractometer is found to be 4.4 cm. The magnitude of the error in apparent median droplet diameter is +21% for aerosols emergent from 10 cm slot burners used in flame spectrochemical analysis. Additional data are included for a variety of aerosol path lengths over the range 0.5 to 20 cm. Although a nonlinear overestimate in median diameter results from use of aerosol path lengths exceeding the optical depth of field (4.4 cm), no appreciable change in the width of the apparent droplet size distribution was found to occur for path lengths up to 18 cm.