High Speed Photographic Study of Wet Droplets and Solid Particles in the Inductively Coupled Plasma
Motion pictures of the ICP were taken at 4000 frames s–1 . An intact wet droplet containing yttrium causes the formation of a pale red cloud juxtaposed on the usual ambient emission structure of the plasma. Most of these droplet clouds are shaped like an oval or a comet. A few droplets produce small, bright spheres followed by faint, wispy streaks that point downstream. Such a spherical cloud is caused by some rapid event such as explosion of a droplet in the final stages of solvent evaporation. The faint streaks are some residue, perhaps small solid particles. In a particular frame, a number of these faint streaks protrude from the tip of the initial radiation zone (IRZ) into the normal analytical zone (NAZ). When wet droplets are introduced, that portion of the analyte that travels through the center of the plasma passes through three distinct regions (i.e., the IRZ, the streaks, then the NAZ), rather than directly from the IRZ to the NAZ. Groups of two or three droplets tend to appear together in the same time interval (≈0.12 ms) in the plasma. These droplet clouds are not seen, and the particle streaks are much less evident, when the solvent is removed before the aerosol is injected into the plasma. Aqueous slurries of Y2O3 in various particle sizes (0.1 or 3 µm mean diameter) produce white streaks along the center line of the plasma, which are attributed to individual solid particles. These observations also support the general precept that calibration of the response for slurries using aqueous solution standards is best accomplished by keeping the particle loading such that each wet droplet contains no more than one solid slurry particle.