Single-particle analysis using inductively coupled plasma atomic emission spectrometry

Abstract

A novel method of single-particle analysis by double viewing position-inductively coupled plasma-atomic emission spectrometry (DVP-SP-ICP-AES) has been developed. Emission intensities at two vertical observation positions of the ICP are measured simultaneously with a single detector. As a single particle travels through the ICP central channel, a pair of transient emission peaks, corresponding to the two observation positions, is produced. Correlation plot of the two intensities is a powerful tool of quality assurance of single-particle analysis. Poor correlation indicates that particles at one or both of the observation positions are poorly vaporized and the measurement is likely inaccurate. Yb2O3 particles of relatively large particle size (200 – 2000 nm) and high boiling point (4343 K) were used as model particles for the investigation of the effects of gas temperature, particle size and boiling point on the analytical performance of SP-ICP measurements. The first observation position was 8.5 mm above the load coil (ALC). The gas-kinetic temperature is comparable to the boiling point of Yb2O3. It is also inhomogeneous across the central channel. The second observation position was 19.5 mm ALC at which the degree of particle vaporization (DOV) of the model particles is substantial. Large scattering in the correlation plot was observed, indicating inconsistent DOV of the model particles at the lower observation position. A sheath gas device was used to confine the model particles either in the center region or the outer region of the ICP central channel. Poor correlation of the intensity distributions of the two regions at low observation position demonstrates that plasma inhomogeneity is a major source of errors of SP-ICP measurements. Computer modelling of particle vaporization in the ICP has been developed to provide the theoretical base of the investigation. The major parameters of the computer model include heat transfer rate from the plasma gas to the sample particles, mass-transfer-limited and heat-transfer-limited particle vaporization rates, analyte diffusion rate, 2-dimensional temperature profile of the ICP, and distribution of particles in the ICP central channel. The model shows that, at low observation position, DOV of the model particles is close to zero and differs significantly, depending on the radial position of the particles and the particle size. The model also shows that, because of high diffusion rate of the analyte ions, a particle of zero DOV at the observation position can give measurable emission intensity after the center of mass of the particle has passed the viewing region. The model provides significant insight into the vaporization and signal production processes of SP-ICP measurements. Guidelines on the selection of optimal observation position for precise and accurate SP-ICP measurement have been established by simulation of particle vaporization of particles of different boiling points and particle sizes. Methods to ensure high degree of particle vaporization at the observation position have also been developed, including the use of mixed He-Ar carrier gas or the reduction of carrier gas flow rate to increase the gas-kinetic temperature and heat transfer rate, and measurement at higher observation position to increase the particle residence time.