Rapid Analysis of Geological Drill-Cores with LIBS

Transcription

1 Rapid Analysis of Geological Drill-Cores with LIBS On the use of laser-induced breakdown spectroscopy Lukas Streubel, Lars Jacobsen, Sven Merk, Michael Thees, Dieter Rammlmair, Jeannet Meima and David Mory Spectroscopy the study of interaction between electromagnetic radiation and matter is a continually developing branch of science that facilitates a number of research and industrial applications. As a state-ofthe-art technique, spectroscopy offers many experimental possibilities to acquire compositional information of inorganic materials, including core samples, minerals and geological specimens. Laser-induced breakdown spectroscopy (LIBS), which provides fast, accurate and high-resolution measurements, is one such spectroscopic technique. LIBS is primarily suited for phase-independent, simultaneous, qualitative and quantitative analysis of samples, dovetailing the technique perfectly with geological standards. Geology fundamentals, motivation and state of the science Humanity s need for prospecting the search for previously unknown deposits of minerals and valuable geological Fig. 1 Front view of LIBS-based core-sample-scanner, with a x- and y-axis movable laserhead behind a window for laser safety material and therefore the entire field of geology grows commensurately with our need for raw materials. Current standard practice for estimating the amount and variety of metals or other resources in ore deposits or mine residues relies on the scanning of drill core samples. With those data, it is possible to calculate complete models for mineral deposit content. Therefore, determination of element concentrations should be as precise as possible. Presently, there are few methods available which can provide (provi- Company / Institution LTB Lasertechnik Berlin Berlin, Germany LTB Lasertechnik Berlin GmbH is an innovative developer and manufacturer of short-pulse lasers in the whole optical spectral range, and of different spectrometers and laser-based measuring technique, marketing its products worldwide. Our mission is to anticipate needs within the widest array of applications and incorporate them into our product development. Our products are characterised by a high innovation level and influence the global device standard. Apart from the high quality and know-how that our customers have come to expect and rely on, our products are economical, practical, mobile, and efficient. LTB is certified according to DIN EN ISO 9001:2008 (TGA-ZM ). Bundesanstalt für Geowissenschaften und Rohstoffe Hannover, Germany Geosciences are an integral part of daily life much more than we are probably aware of. Clean drinking water, sand and clay for building houses, energy and heat are just as much a part of geoscientific research as protecting human lives from geo-risks. The Federal Institute for Geosciences and Natural Resources is committed to sustainable use of natural resources and protection of the human habitat. As a neutral institution feeling responsible for the future, BGR advises ministries and the European Community and acts as partners in industry and science. The leading motive of BGR s daily work is Improvement of Living Conditions by Sustainable Use of the Geo-Potentials. 23

2 the ability to classify minerals, and quantify chemical elements to the extent possible. Remarkably, all of these requirements are feasible through the implementation of LIBS in a single measurement system. Fig. 2 Illustration of the fundamental functionality of LIBS using an echelle-grating. Sample-surface with laser-induced plasma (1); pulsed laser beam (red), detected plasma light (green) (2); all chemical elements emit light after recombining from ionization, wavelengths are characteristic for each element (atomic emission lines, 3); laser-head (Nd:YAG, 1064 nm, short pulsed and focused, 4); path of detected light (5); mirror with focus on measuring site (6); optical fiber (7); entrance slit of the spectrometer (8); Spectrometer (9), collimator mirrors (9.2) focus is corrected by folding mirror (9.1), parallel beam (red), horizontal dispersion by prism (9.3) and vertical dispersion by echelle-grating (9.4), detector-mirror (9.5) focuses diffracted and refracted signal on chip of CCD (10), computer calculates signal to spectrum (11) sional) element quantification and accurate two dimensional mapping of core sample surfaces. One example is energy dispersive X-ray fluorescence (EDXRF). This technology is based on the stimulation of atoms in (core-) samples by X-radiation and the subsequent detection and energetic evaluation of emitted fluorescence quanta. Aside from the accuracy advantages offered by EDXRF, there are some considerable disadvantages: the relatively low measurement speed for systems large enough to accommodate cases of core samples, prerequisite sample preparation, strong focal dependence, the limited variety of measurable chemical elements (from atomic number Z = 5 or Z = 13 without shielding gas) and the limitations commensurate with regulations and safety requirements for X-ray use. The impetus behind the development of a LIBS-based geological sample scanner is therefore largely driven by its potential to better meet the needs of the industry. The aim was to construct a system to provide not only high spatial resolution no matter the sample surface structure, but also a system that is both mobile and fast to take measurements. In addition, the apparatus should be able to handle a high throughput, have LIBS fundamentals and echelle spectrometers LIBS is a type of atomic emission spectroscopy. The technology is based on the understanding of the behaviour of chemical elements in particular, the specific wavelengths of light that elements emit after recombining from ionization. In effect, the idea is to detect and measure this plasma emission and to analyse the captured photons while noting their respective intensities as a function of exact wavelength. The technology works as it follows: a high-energy, short-pulsed and strongly focused laser (typically a Nd:YAG solid-state laser that emits at 1064 nm) ablates and vaporizes a small area of an object of any phase. The emerging gas ionizes and becomes a plasma. Particles recombine after some time (~ 10 µs), and then emit elementally characteristic light. During the ionization of the stimulated material by the laser pulse, one can even hear the loud crack of the laser-induced shockwave when conducting experiments in air. This fact that every chemical element emits clearly self-identifying wavelengths is one of the most important things to know about atomic emission spectroscopy. Fig. 3 Spectrum of steel enhanced from 259 nm to 277 nm. Red and black sections mark orders of the grating this software suggests a chemical element to every detected peak in the spectrum 24 Optik&Photonik 5/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 The collection optics acquire the plasma-emitted light and transfer it to a spectrometer s entrance slit. As already mentioned, a spectrum is the graphical representation of intensity (counts of photons) as a function of wavelength. Inside the spectrometer, dispersive elements like prisms or gratings cause deflection of the collected light by refracting or diffracting light. In this case, an echelle spectrometer (Fig. 2). The echelle spectrometer gets its name from the French word for stairs 200 mm 20 mm y Fig. 4 Principle sketch of the core sample scanner: Movable (x- and y-dimension) laser head with included exhaust system (Q-switched Nd:YAG 1064 nm, energy: mj, pulse-duration 5 6 ns (1); echelle spectrometer (Aryelle150 from LTB Lasertechnik Berlin with resolution , measuring 25 full spectra per second with wavelength bandwidth nm (2); core sample with visible laser ablation spots (3); laser-beam (red) and emitted plasma light (green; 4), laser-induced-plasma (5) Photo Ag Al Cr Fe Mn Na Ni Pb Si Fig. 5 2D mapping profile of a 200 mm 20 mm sector of a core sample s surface. On the left a photograph of the sample followed by location-dependent intensity integrals for selected emission wavelengths for silver, aluminium, chromium, iron, manganese, sodium, nickel, lead and silicon. x 3 2 or ladder because of the nature of the grating it employs to divide light into its orders. These gratings provide high resolution output with a relatively low intensity loss. The incident light on the echelle grating is reflected by integrated steps (with widths similar to the wavelength of the incident light) and cause diffraction. Every step works as a single slit whose diffraction pattern is reflected away according to geometric optics. In addition to the grating, a prism is used which causes a two dimensional (vertical and horizontal diffraction and refraction) interference pattern. This helps to avoid overlapping diffraction orders. A CCD camera using the pixel determined position of incident intensity peaks detects, digitizes, calculates and evaluates the spectral signal. In short: with LIBS it is possible to measure any material and/or surface; to provide fast measurements in comparison to pre-existing core-sample-scanning-methods; and even to detect chemical elements with low mass numbers. LIBS is also quasi non-destructive and there is no need for sample preparation. In spite of these advantages, there remain some disadvantages. Namely, LIBS provides only surface analysis and effectively incapable of measuring exact quantities of chemical elements without calibration. This point is especially relevant while analysing minerals for the purposes of prospecting and geologic sample testing. LIBS and Calibration Because the digitized spectra from the CCD represent only relative intensity values for each wavelength, it is necessary to do a calibration in order to actually quantify the chemical elements in a sample. Because every material has a different temperature of laser-induced plasma and different electron density, quantitative analysis is material specific. These criteria also have an influence on the shape and profile of the detected spectral lines. Therefore, intensity is not merely defined as the maximum value of a peak, but as the integral of the spectral lines. Alternating laser energy due to instabilities in the power source or lasing system itself also influences detected emissions. In order to minimize calculation errors caused by these effects, the integral of the spectral line to be analysed is divided (i.e. normalised) by a spectral line integral of the base element of the sample [1]. In order to get specific and quantified statements about a particular sample LIBS needs reference data. Most often, this comes in the form of a variety of already quantified samples of the same material containing similar (but necessarily different) quantities of chemical elements. Without this, LIBS is only a qualitative measuring technique. 25

4 Mechanics Fig. 4 shows a schematic sketch of the constructed prototype. The laser head includes an exhaust system to avoid ionizing small particles between laser head and focus point on the core-sample to be measured. This assembly is installed on an x-y axis (ca. 2 m 1 m) and can be moved above the sample with position accurate to 100 µm. Ablation size for each spot is approximately 200 µm in diameter. Depending on the number of accumulated shots on a single spot, this crater-size can get even bigger. Generally, core samples of differing sizes are transported in cases ( cm, 5 10 cores per meter). The prototype scanner is capable of accepting an entire sample case directly into the laser-safe measurement chamber. The prototype has been designed to be highly transportable and all of the scanner components can be stored in the chamber itself. Its overall size simplifies transport, as it is relatively lightweight and small enough to fit through standard doorways or into airplane undercarriage. It can be easily carried by four people, set up in a very short time and can be deployed and take measurements directly on the work sites of mines or potential mineral deposits. Measurements can either be manually observed via the side panel through laser safety glass or via a camera installed directly on the laser head itself. Prototype: core sample scanner To evaluate a core-sample, many spectra need to be measured, recorded and evaluated. Fig. 5 shows a 2D mapping of a mm sample with a spatial resolution of 500 µm and 30 laser shots per spectrum (100 Hz shot-frequency). The figure displays intensity mapping after the scanner analysed 16,441 measuring sites on the sample surface. Although the measurement time at this high resolution was eight to nine hours, a sample can also be measured at 2 mm resolution requiring only 1,111 spectra and 30 minutes time. It is important to consider again that without calibration, the intensities of spectral lines carry no quantitative information about absolute content of a chemical element in the sample. There remain several challenges for the final implementation of this prototype. Heterogeneity is the main challenge in analysing core samples. Individual minerals are intimately intertwined and a quantification of each mineral requires its own calibration. High resolution scans require an immense amount of processing capability due to the size of the corresponding data-filled matrices. Furthermore, the intensity for a specific element increases or decreases with changes of the host material, even if the bulk chemical content is similar. Therefore, it is not possible to compare the content of one lithol- Authors Lukas Streubel studied medical physics at Beuth Hochschule für Technik Berlin (BEng) where he completed his Bachelor thesis: Setup, Optimization and Implementation of a Raman Measuring System and Combining Raman and Laser-Induced Breakdown Spectroscopy (translated from German). Streubel is now studying photonics (MEng) at Technische Hochschule Wildau. He joined LTB Lasertechnik Berlin GmbH in 2015 as a student assistant and is currently heading the development of both the aforementioned Raman measuring system and the prototype geological drill-core scanner described in this article. Lars Jacobsen completed a B.A. Language Studies (Mandarin) at the University of California at Santa Cruz in After having some success as co-founder and lead fabricator of Stalk Bicycles a custom bamboo bicycle company in Oakland, California Further authors Michael Thees, Dieter Rammlmair, Jeannet Meima he determined to develop his understanding of engineering. In 2013, he moved to Germany and began a BEng in engineering physics at the Carl von Ossietzky Universität Oldenburg. He joined LTB Lasertechnik Berlin as an intern in 2016 and is currently completing his bachelor thesis, Construction, implementation, and optimization of a LIBSbased control system for GTA-welding. Sven Merk studied chemistry and completed his doctorate at Humboldt Universität zu Berlin. His thesis on Plasmadiagnostics and Quantification for the Laser- Induced Breakdown Spectroscopy (translated from German) was written at the Federal Institute for Materials Research and Testing (BAM) in Berlin. He joined LTB Lasertechnik Berlin GmbH in 2012, and is now working on the development of automated data analysis techniques for LIBS systems for industrial and laboratory applications. David Mory After successful completing a Dipl. Eng. in electrical engineering from the Hochschule für Technik und Wirtschaft Berlin, he began as a Development Engineer at LTB Lasertechnik Berlin GmbH where he worked to develop echelle spectrometers. In 2004, his focus shifted to advancing LIBS technology and became the Manager of Research & Development for LTB in Mory has since been responsible for the development of state-of-the-art measuring systems and techniques for both industrial and research applications. Lukas Streubel, LTB Lasertechnik Berlin GmbH, Am Studio 2c, Berlin Adlershof, Germany, lukas.streubel@ltb-berlin.de, phone: +49 (0) or +49 (0) (David Mory) Bundesanstalt für Geowissenschaften und Rohstoffe, Geozentrum Hannover, Stilleweg 2, Hannover, Germany, phone: Dr. Rammlmair: +49 (0) , Dr. Meima: +49(0) Optik&Photonik 5/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 ogy or mineral to another in Fig. 5, even in the same core sample. The mapping profiles show only where those elements occur. Therefore, LTB Lasertechnik Berlin started a cooperation with the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) to provide LTB s device related know-how with geological fundamentals. This cooperation has already proved fruitful as BGR [2] has managed to identify minerals from their LIBS spectra. In the future, these findings can serve as the foundation for generating calibration matrices for the varying types of core sample. Future Research The prototype and the measurements performed by it will continue to be utilised and analysed in order to provide both qualitative and quantitative results of both point and multi-dimensional LIBS scans of core samples. Further development will also allow the observation camera and correspondingly programmed software to assist in region-of-interest analysis. For example operators will be able to select a sector in a displayed photograph of the sample with a simple mouse-controlled interface, and be able to initiate an automated measurement of the selected region. Further work will also allow laser and scanning parameters to be pre-defined in order for different minerals in the same sample to have comparable results. With some additional development, this prototype scanner has the potential to be a fast and comprehensive measurement system for analysing geological core-samples. DOI: /opph [1] M. Müller: Neue Wege zur Quantifizierung mit der laserinduzierten Plasmaspektroskopie (LIBS), Dissertation Humboldt-Universität zu Berlin, BAM Bundesanstalt für Materialforschung und -prüfung (2010) [2] J. A. Meima et al.: Generation of element and mineral distribution maps by scanning Laser Induced Breakdown Spectroscopy (Lecture on EMC Frankfurt 2012, p. 9), BGR Bundesanstalt für Geowissenschaften und Rohstoffe 27