Effects of gas temperature gradient on the accuracy of single-particle inductively coupled plasma (SP-ICP) measurements

Abstract

Double-viewing-position single-particle inductively coupled plasma-optical emission spectrometry (DVP-SP-ICP-OES) has been used to investigate the effects of temperature gradient in the ICP central channel on SP-ICP intensity. The correlation plot of the intensities of individual Yb₂O₃ particles at observation positions of 8.5 and 19.5 mm above the load coil shows extensive scattering. The lack of correlation is probably due to the relatively large gradient of ICP gas temperature in the radial direction at the lower observation position. The hypothesis is supported by the results of SP-ICP-OES measurements using a sheath gas device to confine the particles in either the center or the outer region of the ICP central channel. Particles in the two regions show distinct patterns in the correlation plot. Computer simulation shows that the amount of heat required to bring the Yb₂O₃ particles to boiling is substantial and the duration of the heat transfer process is not negligible but increases with particle mass. As a result, large particles have a relatively small degree of vaporization (DOV) at low observation positions where SP-ICP-OES intensity is not proportional to particle mass. The calibration curve is concave. Particle size determination by SP-ICP measurement at low observation positions, therefore, can be erroneous. The article concludes with a simulation of particle vaporization with a wide range of particle sizes and boiling points. A quick lookup graph of the observation position for DOV = 70% versus particle diameter is provided for the selection of the initial observation position for SP-ICP measurement.