Characterization of signal-production processes of single particles inICP by time-resolved ICP-AES

Abstract

The research in this thesis aims to characterize the signal-production processes of single particles in the ICP using time-resolved ICP-AES. Signal-production processes, including desolvation, vaporization, atomization, ionization, and diffusion, determine the temporal emission intensity of a single particle. Bimetallic nanoparticles of BaTiO3 (average diameter = 115 nm) were used as test particles. The particles were introduced into the ICP by nebulization of the suspension of the particles in water. As the ion plume of a particle moves up in central channel of the ICP, a temporal emission peak of the analyte atoms in the plume is produced. The emission intensity at any point of time in the temporal profile is related to the degree of vaporization and excitation of the particle at the corresponding vertical position of the ICP. The signal-production processes can, in principle, be studied by measuring the temporal emission profiles. However, the emission intensity of single particles is typically low. Continuous integration of the entire ICP central channel further reduces the signal-to-background ratio (SBR).

A novel double-slit method has been developed to measure the temporal emission intensity of a single particle at two pre-defined ICP vertical positions. Two horizontal slits of slit height of 1 mm were placed in front of the monochromator. As the ion plume passes through the double-slit, two peaks in the temporal emission profile are produced. The configuration of the double-slit (slit height and distance between the two slits) was optimized for maximum signal-to-noise ratio (SNR) and temporal resolution of the double-peaks.

Fast data sampling rate (50,000 Hz) was used in proper sampling of the temporal emission peaks. Large data sets were obtained. Custom programs were developed to extract the relatively weak double-peaks from the temporal emission profiles. The data treatment strategy includes smoothing of the temporal profile to increase SNR and automated peak extraction based on the characteristics of the double-peaks (peak height, peak width, time-difference of the peak pair, and SNR). Four smoothing methods, including Moving Average Filtering, Savitzky-Golay Filtering, Fast Fourier Transform (FFT) and Wavelet Transform, were tested. FFT was adopted because the method requires only one parameter (the cutoff frequency) and is relatively easy to optimize.

Hundreds of double-peaks were obtained in a typical temporal profile of time duration of approximately 120 s. The emission intensity and peak width of the peak pair are correlated to determine the degree of vaporization of the analyte atoms, the extent of diffusion of the analyte atoms and the plume size, and the velocity of the plume in the ICP. Two types of double-peaks are identified. The relative peak height and peak width of the double-peaks in each type are related to the degree of vaporization of the single particles. Simulation of the evaporation rate of water droplets that enclose the single BaTiO3 particles shows that the time required for complete evaporation of water is a major factor that determines the degree of vaporization of BaTiO3 particles at the double-slit. Aggregation of BaTiO3 particles in the suspension was also investigated.