1.1 A brief introduction of LA-ICP-MS
Laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) is a powerful micro-analytical technology by combining a laser ablation system (in situ solid sampling) to the inductively coupled plasma-mass spectrometer (ICP-MS) (signal detection). LA-ICP-MS begins with a laser beam focused on the sample surface to generate fine particles - a process known as “Laser Ablation”. The ablated particles are then transported to the secondary excitation source (ICP) for atomization and ionization. The charged ions in the plasma torch are subsequently introduced to a mass spectrometer detector for both elemental and isotopic analysis.
Gray (1985) firstly demonstrated that a laser ablation system could be coupled to the inductively coupled plasma-mass spectrometer (ICP-MS) for in situ elemental analysis of solid samples. After 30 years of development, LA-ICP-MS has proven to be an extremely valuable analytical tool in Earth science. Its capability of multi-element analysis, ultra-high sensitivity, speed, and modern cost have led it to widespread applications including glass, mineral, inclusion micro-analysis, geochronology (e.g. U-Pb system), isotopic compositions (e.g. Li, Sr, Nd, Hf, Pb) and 2D/3D elemental mapping. Nowadays LA-ICP-MS is considered as an essential microanalysis technique in the geoanalytical laboratory.
1.2 Scope and structure of the thesis
Even though the great success has been achieved in diverse fields; there are still some aspects that need further investigations for the improvement of LA-ICP-MS capability. Among them, most critical are: Laser ablation process, Elemental fractionation, Sample preparation technique (for bulk analysis), Data reduction protocol, Strengthening plasma, Development of reference material, etc.
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There are some of the above-listed topics involved in this thesis corresponding to three individual chapters. These include:
(1) Data reduction protocol based on bulk normalization as 100% (wt), which is implemented within Iolite software package;
(2) Further improvement of LA-ICP-MS capability from sample preparation technique to strengthening plasma;
(3) Pushing the LA-ICP-MS analytical capability to sub-micron spatial resolution.
The thesis starts with an introductory chapter for a briefly description of the slope and structure of the thesis, then comes three scientific chapters that covering the Ph.D. projects.
Each scientific chapter comprises of two publications or manuscripts. Thus a total of six publications or manuscripts are included in this thesis. Conclusion remarks and Outlook as an additional chapter (at the end) are to summary the major contributions of this thesis as well as outlook the nearby future of LA-IC-MS.
1.3 Individual manuscript and Author contributions
Chapter 2
regards to the development of data reduction protocols for elemental analysis, which is based on bulk normalization as 100% (wt) and implemented within Iolite software package. The quality of LA-ICP-MS data depends not only on the instrumentation but also the protocols used for data processing. This chapter includes two manuscripts as follows,Chapter 2.1
Shitou Wu and Klaus Simon. (2017)
LA-ICP-MS transient signal quantification of NIST, MPI-DING, USGS, and CGSG glass reference materials by Ratioing, Standardization, and Normalization (RSN) (submitted to “Geostandards and Geoanalytical Research”).
A quantitative reduction strategy consisting of Ratioing, Standardization, and Normalization (RSN) was proposed to process the LA-ICP-MS transient signal of NIST, MPI-DING, USGS
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and CGSG glass reference materials. The RSN strategy allows the quantitative reduction without knowing the concentration of internal standard prior to LA-ICP-MS analysis.
In this manuscript, I carried out the experiments and wrote the manuscript. Klaus Simon made the algorithms and compiled the code script for accomplishing the proposed protocol.
He also improved the manuscript.
Chapter 2.2
Shitou Wu, Yaping Wang, and Chunxue Xu. (2017)
Laser ablation-inductively coupled plasma-mass spectrometer: A quantification strategy based on two reference materials and bulk normalization as 100% (wt). Chinese Journal of Analytical Chemistry.
45(7), 965–972.
A quantification strategy based on two reference materials (NIST610 and StHs6/80-G) and bulk normalization as 100 %(wt) were proposed to reduce LA-ICP-MS transient signals, which eliminates the deficiencies encountered with the quantification strategy using single reference material, such as the extremely low content or large uncertainty of some elements.
In this manuscript, I proposed the quantification strategy and conducted the experiment and wrote the manuscript. The other two authors edited and improved the manuscript.
Chapter 3
refers to the bulk analysis of granite by LA-ICP-MS and strengthening plasma with the addition of nitrogen and hydrogen into plasma in combination with guard electrode.These two projects lead to the further improvement of LA-ICP-MS capability.
Chapter 3.1
Shitou Wu, Volker Karius, Burkhard C Schmidt, Klaus Simon and Gerhard Wörner. (2017)
Comparison of ultrafine powder pellet and flux-free fusion glass for bulk analysis of granitic rock samples by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry. (submitted to
“Geostandards and Geoanalytical Research”).
Two approaches are developed that include an optimized wet-milling protocol for granites
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and a flux-free fusion protocol for producing homogeneous granitic glasses. Comparison of ultrafine powder pellet and flux-free fusion glass for LA-ICP-MS bulk analysis of granitic rock samples was carried out.
In this manuscript, I was involved in developing this idea and wrote the manuscript. Volker Karius supervised the wet-milling experiment and edited the manuscript. Burkhard C.
Schmidt managed the flux-free fusion experiment and modified the manuscript. Klaus Simon helped with the interpretation of the data. Gerhard Wörner and Volker Karius initially suggested the idea and improved the manuscript.
Chapter 3.2
Shitou Wu and Klaus Simon. (2017)
Signal enhancement in LA-ICP-MS analysis by guard electrode and the addition of nitrogen and hydrogen into carrier gas: A perspective from experiment (in preparation for “Journal of Analytical Atomic Spectrometry”).
The signal enhancement in LA-ICP-MS by using guard electrode and the addition of small amount nitrogen/hydrogen into carrier gas was investigated. The plasma strength was evaluated under different experimental conditions.
In this manuscript, I designed and carried out the experiments and wrote the manuscript.
Klaus Simon helped with the interpretation of the data and edited the manuscript.
Chapter 4
involves the pushing the LA-ICP-MS analytical capability to sub-micro spatial resolution. Elemental fractionation and laser ablation rate are the two bottlenecks that hamper this application. This chapter covers two manuscripts, which are focusing on elemental fractionation at high spatial resolution and laser ablation rate, respectively.Chapter 4.1
Shitou Wu, Yaping Wang, Chunxue Xu and Jihai Yuan. (2016)
Elemental fractionation studies of 193 nm ArF excimer laser ablation system at high spatial resolution. Chinese Journal of Analytical Chemistry. 44(7), 1035–1041.
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Limits of detection (LODs), mass load effect, downhole fractionation and matrix effect of 193 nm ArF excimer laser ablation system at high spatial resolution were systematically investigated. Trace elements in GSD-1G, StHs6/80-G, and NIST612 were analyzed at 10 m spot size.
For this study, I proposed the idea, conducted the experiments, and wrote the manuscript.
Other co-authors helped with the interpretation of the data and improved the manuscript.
Chapter 4.2
Shitou Wu, Chunxue Xu, Klaus Simon, Yilin Xiao and Yaping Wang. (2017)
Ablation behaviors of 193nm ArF excimer laser system for the selected substrates. Rock and Mineral Analysis. Accepted. (in Chinese with English abstract).
Ablation behaviors of 193 ArF excimer laser for silicate glasses, common minerals, and powder pellets were investigated. Ablation rates influenced by laser parameters (including spot size, energy density, and laser frequency) were evaluated. Data for ablation rates of 43 different substrates are presented.
In this manuscript, I designed the experiment and wrote the manuscript. Klaus Simon helped to collect the topographic images of laser craters. Yilin Xiao provided samples. Other co-authors helped with the interpretation of the data and improved the manuscript.
“Nothing can be obtained in geochemistry without careful analytical work.”
by C.J.Allegre
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