Evolution of drop size distributions for pneumatically generated aerosols in inductively coupled plasma-atomic emission spectrometry

Abstract

Drop size distributions of pneumatically generated aerosols used for inductively coupled plasma-atomic emission spectrometry have been measured using a laser Fraunhofer diffraction system at several locations along their trajectories in a spray chamber. The influence of: (1) nebulizing gas and liquid flows; (2) design and dimensions of nebulizers; and (3) physical properties of solvents on drop size distributions are reported. Droplet coalescence and evaporation appear to be the main factors influencing the aerosol size distributions in the stages immediately following primary aerosol formation. Differences in particle and gas velocities can also influence evaporation rates and coagulation processes. In the middle of the spray chamber, impaction loss processes predominate. The magnitude of such losses increases as the droplet mean size increases. At the bottom wall of the spray chamber, the aerosol distribution is modified mainly through inertial losses. For a given chamber, the coarser the aerosol distribution just before this point is, the greater the efficiency of the inertial loss process will be. For a fixed geometry spray chamber any factors which tend to increase the mean drop size will also tend to increase the efficiency of impaction, gravitational and inertial loss processes. Factors which tend to increase the mean drop size are low nebulizing gas flow and high liquid flow, high surface tension, low volatility, large cross-sectional area for the gas outlet, and the use of a cross-flow design nebulizer. Volatilization and, to a lesser extent, coalescence processes are increased by the factors that tend to decrease mean drop size. The higher ratio of surface area to mass enhances evaporation and the greater particle density increases the likelihood of particle collisions.