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%0 Thesis
%A Schlatter, Nils
%T Quantitative Analyse des anorganischen Lösungsinhalts wässriger Proben mittels portabler laserinduzierter Plasmaspekroskopie (pLIBS) : Entwicklung der Methodik, Anwendung und Evaluation
%I Rheinisch-Westfälische Technische Hochschule Aachen
%V Dissertation
%C Aachen
%M RWTH-2024-07682
%P 1 Online-Ressource : Illustrationen
%D 2024
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2024
%X The inorganic solution content of aqueous samples is currently still analysed almost exclusively in the laboratory using conventional laboratory methods such as ion chromatography (IC) or atomic absorption spectroscopy (AAS). These methods are costly, time-consuming and not always practical. Many of the field methods developed to date lack the ability to quantify a large number of elements simultaneously in real time. The research objective of this thesis is therefore to evaluate whether and how aqueous solutions can be quantitatively analysed on site for inorganic solution content using portable laser-induced plasma spectroscopy (pLIBS). Laser-induced plasma spectroscopy (LIBS) is an atomic spectroscopic method in which a pulsed laser is focused on a small area of the surface of a sample. This creates a plasma, the vaporised sample material is atomised and ionised and the electromagnetic radiation released is then analysed. The first application of LIBS to aqueous solutions took place as early as 1984, albeit with stationary laboratory equipment. Difficulties in directly analysing the liquid surface with LIBS subsequently led to different types of sample preparation. Until now, the analysis of aqueous solutions has been limited to stationary or large, transportable LIBS devices. However, with advancing miniaturisation, analysis is also possible with portable devices. This thesis documents the method development, application and evaluation of a portable method. Using the pLIBS Z-300 (SciAps), liquid-to-solid conversion is used as sample preparation in order to avoid the physical issues associated with analysing liquid surfaces and to reduce the detection limits by concentrating the sample during evaporation. Aluminium foil is used as a substrate because it is inexpensive, readily available and has few spectral interferences. To optimise the distribution of the evaporation residue and prevent the so-called coffee ring effect, a thin pencil layer is applied to the metal surface. The calibrations are created with dilution series from AAS standards. A 3D-printed sample holder guarantees the focusing and analysis of the evaporation residue and makes the method reproducible. Consisting of a base into which the aluminium foil is inserted and a stencil that is placed on top, the device can be mounted during the measurement process on the one hand and automatically focused on the other. For calibration, dilution series with concentrations between 0.1 and 1000 mg/L are prepared from single-element AAS standard solutions. A drop of 0.75 µL is added with a pipette through the stencil onto the surface-enhanced Al-foil and then evaporated on a hot plate. A square grid of 10 * 10 analysis points per vaporised drop with four individual analyses per point guarantees the complete detection of the evaporation residue. In the device-specific software, several lines of the element of interest with as little interference and as high intensity as possible are selected and an intensity ratio is formed with the strongest Al lines for each concentration. These can be used to create calibration lines for the elements of interest in a spreadsheet.When analysing the spectra, which were also used for calibration, it is shown using the calibration curves that the three elements Li, Na and K can be quantified in standard solutions from 0.1 to around 100 mg/L (Li, Na) and 160 mg/L (K). At higher concentrations, the signal is no longer directly proportional to the concentration. In addition, the surface enhancement leads to a significantly improved shape and distribution of the evaporation residue and consequently to more reproducible results. At 0.006 to 0.011 mg/L, the detection limits for the three alkali metals are well below the concentration of 0.1 mg/L of the lowest standard solution used. When applied to mineral waters, with further calibrations for Ca, Mg, Sr, Cl, NO3 and SO4, similar results are obtained. In low mineralised waters up to about 1000 µS/cm, the dissolved ions can be quantified with the exception of NO3. In addition, self-absorption of the emitted light occurs in the plasma, which cancels out the proportionality of concentration and signal intensity. The effect can be investigated in more detail using mixed standard solutions. Divalent ions are more susceptible to self-absorption than monovalent ions.Potentially toxic elements such as Cr, Ni, Cu, Zn, As, Se, Cd and Pb can also be quantified in standard solutions using the method. Although the calculated detection limits for these elements are below 0.03 mg/L, it is not possible to create calibration curves below the concentration of 0.1 mg/L for Zn and As. In addition, when comparing produced and predicted concentrations, only Cr shows plausible results for the concentration range below 0.1 mg/L. Only Cu can currently be reliably quantified in the range of the limit values for drinking water set by the WHO and the German Drinking Water Ordinance. The results show that the method developed cannot compete with laboratory methods such as AAS or ICP-MS in the field of trace analysis. However, it has a major advantage when rapid results or cost-effective preliminary screening are required. The distribution and shape of the evaporation residue can be optimised in the future by further developing the application process or the applied material. Self-absorption prevents the analysis of higher concentrations and must be mathematically minimised, which not only enables the analysis of higher concentrations but also increases reproducibility. The hot plate in combination with the sample holder can also be further developed with a metal version to further facilitate the methodology in the field. The calibration of further elements opens up a broader field of application in different sectors and thus leads to a significant market potential.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2024-07682
%U https://publications.rwth-aachen.de/record/991097