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Identifier 000434031
Title Βελτιστοποίηση πειραματικών παραμέτρων και μεθόδου επεξεργασίας του φασματικού σήματος στη Μικροσκοπία Πλάσματος Επαγόμενου από Λέιζερ (micro-LIBS). Εφαρμογή στη στοιχειακή χαρτογράφηση βιο-ορυκτών και την ανάλυση μεταλλικών κραμάτων
Alternative Title Experimental conditions and signal extraction optimization in Laser Induced Breakdown Spectroscopy microscopy (micro-LIBS) for the elemental imaging of biogeochemical archives and the elemental analysis of metal alloys
Author Μαλεγιαννάκη, Ειρήνη Ν.
Thesis advisor Άγγλος, Δημήτριος
Reviewer Δημάδης, Κωνσταντίνος
Περγαντής, Σπυρίδων
Abstract Elemental composition fluctuations of geological materials and bio-minerals reflect changes in their environment. The application of Laser Induced Breakdown Spectroscopy (LIBS) in characteristic cross-sections of these bio-minerals serves as a valuable tool for mapping the variations of chemical composistion and examine their relation with climate proxies which are time and space specific. The matrix of the bulk material consists of calcium carbonate (CaCO3), while elements such as magnesium (Mg) or strontium (Sr) incorporate in the matrix as impurities. The analytical method is based in the recording of elemental ratio Mg/Ca, which is related with the environmental temperature variation of the analyzed sample, which might be speleothems or minerals composed by biological organisms, such as stalagmites or sea shells. In the present work, a customade LIBS microscopy system was used (micro-LIBS), which enables the sea shell cross-section analysis (some cm2) with high spatial resolution (50 μm/ spot area) and fast acquisition time (2000 pixels / hr). These characteristics of micro-LIBS mapping facilitates surface scanning of a large number of samples that contributes to the exploration of the correlation between elemental ratio fluctuations and environmental temperature variations. The study of Mg/Ca fluctuation across the growth lines of a sea shell is carried out by monitoring the LIBS intensity ratio fluctuation of calcium (Ca) and magnesium (Mg) characteristic atomic lines. It has been proven that intensity ratio provides reliable imformation that reflect compositional variations of the sample. Given the fact that atomic emission signal in LIBS depends on plasma characteristic properties, we examine how experimental parameters affects accuracy and precision of the measurement in each plasma event (measurement), and so as the accuracy of quantitative elemental analysis. For that reason, the magnitude and the origin of signal fluctuation of different atomic lines were investigated. There are intrinsic factors of a plasma event that lead to fluctuation of the atomic emission signal, such as plasma temperature and the amount of ablated mass, or experimental factors, such as signal collection and recording and signal extraction of experimental data. The main purpose of the research is understanding the effect of the above parameters in the recorded LIBS signal, their subsequent elimination and finally their correction. Monitoring the LIBS signal assigned to spectral lines that depend, in a similar way, on plasma physical parameters, for example lines of the same species that come from the same multiplet, is the procedure proposed in this research in order to investigate the origin and the magnitude of measurement uncertainty and their dependence of experimental conditions. Particularly, calcium ion (Ca II) emission lines at 370.603 nm and 373.690 nm that come from the same multiplet, so as lines at 315 nm and 317 nm, were chosen. The intensity ratio of these lines, I370/I373 and I315/I317, does not depend on the physical plasma parameters, because emission starts from the same energy level. Although, the effect of experimental conditions (Laser Pulse Energy, Delay time, Gate time, ICCD-MCPGain, Spectrometer Entrance Slit) on the intensity ratio fluctuation was investigated and for the accuracy of the results two signal collection-recording systems were used for simultaneous recording of the same plasma event. When all experimental parameters investigated were set in the appropriate value that leads to high measurement signal and the integrated intensity-signal extraction method was applied, the measurement uncertainy was reduced up to 2%. Elemental ratio of two different elements present in a biomineral sample it is not easily obtained with the application of the calibration curve method, because the preparation of matrix-matched standard samples. The proposed method which is commonly referred in the bibliography as Calibration-Free LIBS method needs no standard sample, but has as a prerequisite the accurate determination of plasma temperature. Plasma temperature measurement was based on Boltzmann plot method, by the use of atomic emission lines that come from singly ionized calcium atoms (Ca II) with characteristic emission wavelength at 370.603nm, 373.690 nm, 315.887 nm και 317.933 nm. Measurements on calcite (CaCO3) and dolomite (CaMg(CO3)2) solid samples were carried out and Boltzmann plot method was applied on LIBS data for temperature extraction in order to investigate the accuracy of the proposed method. The Saha-Boltzmann equation was used for the quantitative analysis of Mg/Ca elemental ratio in these samples. Number density of a specific species can be obtained from the above equation, when characteristic spectral line intensity, assigned to the elements, and plasma temperature value is known. The quantitative results were correlated with the results from the system uncertainty investigation, in order to determine the accuracy limit of the proposed methodology for both one spot measurement and elemental mapping procedure applied to samples of some cm2 surface size. The proposed methodoly was applied to both standard bronze samples (Cu-Sn) of low Sn content ([Sn] < 10%wt) and archaeological findings of similar composition, that were found in Phaistos archaeological region. The micro-LIBS system, that has been already referred, and a mobile LIBS instrument was used for the measurements in standard samples, while only the latter instrument was used for the measurements of the findings in the archaeological site of Phaistos. The aim of this research was elemental composition determination of tin (Sn) in the samples and the subsequent testing of the accuracy of the quantitative results.
Language Greek
Subject Elemental analysis
LIBS
Spectroscopy
Στοιχειακή ανάλυση
Φασματοσκοπία
Issue date 2020-11-27
Collection   School/Department--School of Sciences and Engineering--Department of Chemistry--Post-graduate theses
  Type of Work--Post-graduate theses
Permanent Link https://elocus.lib.uoc.gr//dlib/7/f/7/metadata-dlib-1605698871-135429-1462.tkl Bookmark and Share
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