| Summary: | 碩士 === 國立清華大學 === 原子科學系 === 88 === There are two main topics included in this work. Firstly, the feasibility of using Inductively Coupled Plasma Quadrouple Mass Spectrometry （ICP-Q-MS） as a means for precise and accurate measurement of lead isotope ratio in bullet samples was established. It is well known that mass discrimination and dead time both contribute to the total mass bias and influence the accuracy of isotope ratio measurement. After evaluating the mass bias through measuring SRM (NIST 981) and correcting it by a new calculation method, the accuracy can be effectively improved (error below 0.4%). In addition to statistic error, plasma flicker and variation of sample uptake rate, nebulization, and extraction efficiencies are important sources, which influence the precision of isotope ratio determination. From the experimental results, it indicates that good precision can be achieved by shortening the dwell time and increasing the scanning rate to smooth out signal fluctuations. To a typical sample with low lead content (20ppb), a reasonably good precision (RSD 2.5%) with the use of cross flow nebulizer (CFN) can be obtained under the scanning rate of thousand sweeps per replicate. The precision can be further remarkably improved with the use of USN (ultrasonic nebulizer), under the same instrument settings, to as low as 0.25%, i.e. about ten-fold better than CFN. The developed system is being tested for its applicability in forensic study.
Secondly, applicability of three calibration methods for SA-ICP-OES was evaluated. Firstly, three matrix-matched solid standards were used to quantify the trace elements in low alloy steels. The limits of detection were estimated to be in the range of 10-100 ppm, and the precision of ≦10% was found to all the elements tested. Secondly, calibration study on dry aerosols for SA-ICP-OES was performed. Aqueous standards and a solid standard were used to evaluate the sensitive factors between solid particles from spark ablation and dry aerosols from ultrasonic nebulization. With the dual sample introduction SA-ICP-OES system, the analytical performance was evaluated by NIST SRM 1262b and the results showed that an error of ≦10% was achieved for the determination of Mn, Cu, Ni and Si. And thirdly, calibration of SA-ICP-OES using standard additions with dried solution aerosols was performed. By means of flow injection method hyphenated with USN, this system not only performed on-line standard addition method, but also obtained dry aerosols from calibration standards to match the elemental response of solid particles. Accurate performance of this FIA-USN-SA-ICP-OES was evaluated by NIST SRM 1270 and the results showed that an error of ≦10% could be achieved for the elements investigated.
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