No description available.

published_or_final_version / abstract / Chemistry / Master / Master of Philosophy

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. / published_or_final_version / Chemistry / Master / Master of Philosophy

Transient emission of a particle in inductively coupled plasma-atomic emission spectrometry (ICP-AES) depends on the fundamental processes of aerosol desolvation, particle vaporization and atomization, ionization, excitation and diffusion of the analyte. Ideally, the rate of the above processes can be determined from the evolution of the transient emission as the ion plume moves along the central channel of the ICP. However, the dimension of the ion plume is significantly smaller than the central channel. The signal-to-background and signal-to-noise ratios suffer when the entire channel is imaged. Deconvolution of the temporal profile is required to determine the emission intensity of the ion plume versus observation height. Small aperture can be used to locate the vertical emission position accurately, but the evolution of the plume emission is lost. In this study, a double-slit method has been developed to pin-point two vertical positions of an ion plume. An ion plume travelling along the ICP central channel produces two peaks in the temporal emission profile. The temporal evolution of emission intensity can be correlated to delineate the degree of particle vaporization at the two positions. The relative widths and separation of the two peaks in a double-peak can be used to determine the analyte diffusion rate and particle velocity in the ICP, respectively. An unicellular green algae, chlorella vulgaris, was used as the test particles. The average Mg content of the algae is equivalent to MgO particles of diameter of 265nm. The strong ionic emission at wavelength of 279.55nm was monitored using a ¼ -m monochromator equipped with a PMT detector. Method of curve fitting was used to filter out the noise with minimum distortion of the peak shape for accurate determination of peak height and peak width. The merits of curve fitting versus methods of smoothing such as moving average and Savitzky-Golay filtering will be discussed. All transient emissions from the algal cells were detected with sufficient signal-to-noise ratio using a single-slit setup with slit height of 1mm at observation height of 18 mm above the load coil and ICP forward power of 1400 W. However, using the double-slit setup, less than half of the expected double-peaks were observed. One of the peaks in the double-peak can be below the detection limit and the double-peak is lost. An innovative development of this study is that the relative sensitivity corresponding to the 2 slits can be varied to enhance the intensity of the weaker emission peak. The peak with insufficient signal-to-noise ratio for detection can be enhanced to a level above the limit of detection. The number of observed double-peaks in increased and the observed particles are more representative of the population. Two types of double-peaks are categorized according to the relative intensity of the first peak to the second peak. A computer model was used to estimate the intensity ratio of the two emission peaks at different observation position of the ICP. The experimental and theoretical ratios agree generally. The theoretical ratio also shows the bias in the population sampled by the double-slit setup. / published_or_final_version / Chemistry / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2008. / Also available in print.

published_or_final_version / Chemistry / Master / Master of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Master / Master of Philosophy

published_or_final_version / Chemistry / Master / Master of Philosophy