| Solids elemental analysis has an important stutus and role in metallurgy,geology, mining,environmental protection,national defense and semiconductor industry. Traditionally,solid samples are dissolved by acids and then analyzed by atomic spectroscopy or mass spectrometry.Recently,direct analysis of solid has attracted great attentions due to it only needs little sample preparation,so it can save a large amount of time,especially when the samples are difficult to be dissolved or hazardous chemicals are commonly involved.What's more,direct analysis of solid can avoid the contamination from the dissolution process and the lost of elements needed to be analyzed,so the results will be more accurate.Mass spectrometry has been favored because of its high accuracy of mass measurement and high sensitivity.Laser ionization mass spectrometry(LIMS) is considered as a sleeping giant.At an irradiance of 109 W/cm2 -1010 W/cm2,the irradiated spot is heated in less than 10-10s,and the temperature reached significantly exceeds the boiling temperature of the samples.The vaporization of the spot is explosive because of the fast increase in temperature over a small sampling area in a short time.Therefore,the selectivity of vaporization is negligible.The plasma reaches a thermal temperature of 20000- 50000 K in the center of the plume and the ionization fraction approaches unity.However,two problems must be solved:(1) the kinetic energy distribution of the ions generated with 109 W/cm2 irradiation can be as high as several hundred electron volts(eV),and conventional TOFMS cannot resolve peaks with such a broad energy distribution;(2) multiply charged ions cause severe interferences in the spectrum.In traditional LIMS instruments,the laser plasma is generated in high vacuum environment such that the ions have great energy distribution.The unsettled ions are introduced to the mass analyzer,as a relsut,the ions with different m/z can not be separated well,which reduce the resolution.Since 1990s,the atomic spectrometry research group of Key MOE lab of Analytical Sciences in Xiamen University has been specializing in the development of TOF-MS.We had successfully made the firstμs-GD ion source TOF-MS in 1994 and it filled the blank of this kind of instruments in China.Doctor He with the cooperation of Professor A.F.Dodonov from Russia made out the second high resolution TOF-MS with electrospray ionization source(ESI-Q-TOF-MS) successfully in 2001; its main performance had reached the national advanced level.Making use of the original time of flight mass analyzer and adding an ion guide cooling cell,this laser ablation time of flight mass spectrometry makes the combination of laser ablation ionization source and time of flight mass analyzer come true.This home-made instrument has a number of features as follows:(1) Using rf-only hexapoles for collisional cooling of energetic ions from the laser plume,which resolves the energetic ions unmatched with mass analyzer.(2) The use of a low vacuum ion source with helium as the buffer gas suppresses interferences from doubly charged matrix ions,which makes the mass spectrums more clearly and easily to be understood.(3) The system has the potential for direct and semi-quantitative elemental analysis of solid samples,which eliminates the need for standard preparation and calibration curve fitting procedures.This paper can be divided into six chapters.In chapter 1,the intruduction about mass spectrometry,the history of laser and the background of this study are introduced.The principle of LA-TOF-MS is discussed in chapter 2.The structure of this instrument and the whole process are described in chapter 3.The optimizations of the parameters for LA-TOF-MS are discussed in chapter 4.The studies which carried out by LA-TOF-MS and some applications are depicted in chapter 5.The last chapter is subjected in the conclusion and problems existed in this LA-TOF-MS,and some improvement methods are advised at the end of this chapter.
|
PDF Full Text Request