Analysis of Characteristic Parameters of Sub-micron Silicon Dioxide Particles by ICP-MS

Nanoparticles and sub-mciro particles were increasingly adopted in the field of catalyzer, semiconductor, magnetic material, biomedical additives, consumer goods and food. Inductively coupled plasma-mass spectrometer (ICP-MS) is an alternative way to determine characteristic parameters of particle. However, the transient signals duration in a range of 300-800 μs from individual particles is shorter than that offered by the most of ICP-MS instruments. In this work, a method of ICP-MS combined with extra data acquisition and storage devices for measuring the diameter and concentration of sub micrometer particles was established. An Elan 6000 ICP-MS was used with the analog output pin connecting to a current amplifier. The amplified signal was recorded by an oscilloscope. The data acquisition rate of 40 kHz was used to obtain an individual peak for each particle. A data processing algorithm including filtering noise and subtracting the threshold was developed in this work to process the raw data. The silicon dioxide particles in the range of 300-2000 nm were used for the optimization of sample system and instrument conditions, the effect of sample uptake rate and sample gas flow rate on the signal intensity from single particle were discussed as well. The signal intensity increased initially with the gas flow rate which may be due to the increasing of nebulization efficiency and approaching to the optimum ionization position, and then decreased after reaching the maximum because of the ion diffusion. The optimum sample gas flow rate was 0.90-0.95 L/min for sample uptake rate of 100 μL/min. Under the conditions of optimization, the diameter detection limit of SiO₂ particle was 233 nm, which depended on the background interference, instrument sensitivity and electronic noise. The correlation coefficient of linear calibration curves of particle mass for 300-900 nm particles was over 0.99. While, there were significant deviations for 1 500 nm and 2 000 nm particles measurements. The transport efficiency calculated using particle concentration method was in good agreement with the filter trap method. With the increasing of sample uptake rate, the transport efficiency reduced from 33% to 2.2%. The diameter results given by ICP-MS, photon correlation spectroscopy (PCS) and scanning electron microscope (SEM) showed the consistence under 900 nm. In comparison, a slight worse diameter resolution based on ICP-MS was observed. The broadening effects were considered to be caused by variations in droplets size and ionization positions in plasma. The goals will be focused on improving the transport efficiency of sample introduction system in the future.