Studies of single-particle inductively coupled plasma mass spectrometry

Abstract

The transient ICP-MS intensity of a particle depends on the rates of particle vaporization and ionization and diffusion of the analyte atoms in the ICP. In this study, a mathematical model has been developed to investigate the relative contribution of these three processes on the relative ICP-MS intensity along the ICP central channel. Gold nanoparticles were used as the model particles. The input parameters of the computer model are optimized by comparing the calculated depth profiles of ICP-MS intensity of the model particles to the experimental profiles.

The computer model previously developed in this research group has been updated in this study. The rates of vaporization of the model particles in the ICP by both heat-transfer-limited and mass-transfer-limited vaporization mechanisms are compared for each time step. The slower process is used to calculate the number of gold atoms vaporized in that time step. The degree of ionization of the vaporized gold atoms is then calculated using Saha equation. The ionization temperature was determined experimentally by comparing the intensity ratio of Cd and I. The profile of ionization temperature is sigmoidal and gives more accurate estimation of the ICP-MS intensity than the hypothetical linear ionization temperature profile used in the previous study. The vaporized atoms diffused rapidly to form a plume. The number of ions in the ion plume that passes through the sampler cone is calculated as the ICP-MS intensity. The effect of gas dynamics is taken into consideration by introducing a new parameter of effective aperture of the sampler cone. The previous model suffers from underestimation of the number of ions sampled by using the physical diameter of the sampler cone in ion sampling calculation. With the improvements on the ionization and diffusion calculation, the shape and relative sensitivity of the simulated depth profiles of ICP-MS intensity of the model gold nanoparticles agree with the experimental results.

Vaporization and ionization are the main processes in the computer model that determine the ion production rate. The limiting process for the smaller particles is ionization while that for the larger particles is vaporization. The ionization-limiting process gives the linear calibration curves. The depth profiles of ICP-MS intensity for the particles and standard solutions are also identical and the sensitivity ratio of solution to particles is constant along the sampling depth for the smaller particles. The particle diameter at which the limiting process changes from ionization-limiting to vaporization-limiting is the transition diameter. The calibration curves roll off for particle diameter larger than the transition diameter. The linear dynamic range of the calibration curves, or the transition diameter, is dependent on molecular weight, density, ionization potential, and boiling point of the particles. The transition diameter also depends on the ICP forward power and carrier gas flow rate. High gas flow rate and low power are preferred for larger linear dynamic range. Particles within the linear dynamic range can be calibrated using standard solution with the sensitivity ratio between solution and particle corrected.