Over the past few years, laser-induced breakdown spectroscopy (LIBS) has earned a lot of attention in the field of online polymer identification. Unlike the well-established near-infrared ...spectroscopy (NIR), LIBS analysis is not limited by the sample thickness or color and therefore seems to be a promising candidate for this task. Nevertheless, the similar elemental composition of most polymers results in high similarity of their LIBS spectra, which makes their discrimination challenging. To address this problem, we developed a novel chemometric strategy based on a systematic optimization of two factors influencing the discrimination ability: the set of experimental conditions (laser energy, gate delay, and atmosphere) employed for the LIBS analysis and the set of spectral variables used as a basis for the polymer discrimination. In the process, a novel concept of spectral descriptors was used to extract chemically relevant information from the polymer spectra, cluster purity based on the k-nearest neighbors (k-NN) was established as a suitable tool for monitoring the extent of cluster overlaps and an in-house designed random forest (RDF) experiment combined with a cluster purity–governed forward selection algorithm was employed to identify spectral variables with the greatest relevance for polymer identification. Using this approach, it was possible to discriminate among 20 virgin polymer types, which is the highest number reported in the literature so far. Additionally, using the optimized experimental conditions and data evaluation, robust discrimination performance could be achieved even with polymer samples containing carbon black or other common additives, which hints at an applicability of the developed approach to real-life samples.
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Abstract
Polymers are used in a variety of different areas, including applications in food packaging, automotive and the semiconductor industry. Information about degradation of these materials ...during application, but also uptake of pollutants from the surrounding environment is therefore of great interest. Conventional techniques used for polymer characterization such as FT-IR or Raman spectroscopy, but also thermo-analytical techniques offer insights into degradation processes but lack the possibility to detect uptake of inorganic species. Moreover, these techniques do not allow the measurement of depth profiles, thus information about degradation or pollutant uptake with sample depth is not accessible. In this work, we propose LA-ICP-MS and LIBS as powerful analytical tools for polymer characterization, overcoming the limitations of conventional analytical techniques used for polymer analysis. Applicability of the developed procedures is demonstrated by the analysis of artificially weathered polyimides and modern art materials, indicating that the degradation of the polymer but also the uptake of corrosive gases is not limited to the sample surface. Finally, a tandem LA-ICP-MS/LIBS approach is employed, which combines the advantages of both laser-based procedures, enabling the simultaneous analysis of polymer degradation and cadmium uptake of polystyrene after exposure to UV radiation and treatment with artificial sea water.
The ever-increasing speed of exchange of ideas, information, and culture allows contemporary art to be in constant growth, especially concerning the choice of artistic materials. Their ...characterization is not only crucial for the study of artistic techniques but also for research into the stability of the material and, consequently, the best preservation practices. For this aim, an analytical method should have the advantages of not requiring sample preparation, performing superficial micro-analysis, and obtaining detailed spectral information. For this study, laser-induced breakdown spectroscopy (LIBS) was employed. It was used for the identification of modern paints composed of inorganic pigments and organic binders, such as acrylics, alkyds, and styrene-acrylics. Principal component analysis (PCA) was used to classify the different pure materials, above all, the polymeric binders. To distinguish the paint mixtures, whose LIBS spectral results were more complex due to the pigment/binder interaction, a statistical method recently employed in the cultural heritage field was chosen, namely, random decision forest (RDF). This methodology allows a reduction of the variance of the data, testing of different training data sets by cross-validation, an increase of the predictive power. Furthermore, for the first time, the distribution of different inorganic pigments and organic binder materials in an unknown sample was mapped and correctly classified using the developed RDF. This study represents the first approach for the classification of modern and contemporary materials using LIBS combined with two different multivariate analyses. Subsequent optimization of measurement parameters and data processing will be considered in order to extend its employment to other artistic materials and conservation treatments.
The present study demonstrates the importance of converting signal intensity maps of organic tissues collected by laser-induced breakdown spectroscopy (LIBS) to elemental concentration maps and also ...proposes a methodology based on machine learning for its execution. The proposed methodology employs matrix-matched external calibration supported by a pixel-by-pixel automatic matrix (tissue type) recognition performed by linear discriminant analysis of the spatially resolved LIBS hyperspectral data set. On a swine (porcine) brain sample, we successfully performed this matrix recognition with an accuracy of 98% for the grey and white matter and we converted a LIBS intensity map of a tissue sample to a correct concentration map for the elements Na, K and Mg. Found concentrations in the grey and white matter agreed the element concentrations published in the literature and our reference measurements. Our results revealed that the actual concentration distribution in tissues can be quite different from what is suggested by the LIBS signal intensity map, therefore this conversion is always suggested to be performed if an accurate concentration distribution is to be assessed.
Synthetic polymers and plastics have become one of the most important materials in our modern world and everyday life with all kinds of applications mainly due to their wide range of excellent and ...tuneable properties. Several novel materials consisting of multiple different synthetic polymers or composite materials like natural-fiber-reinforced polymer composites have already been reported in literature. Additionally, materials consisting of multiple synthetic polymers already found their way in our daily lives (e.g. double-sided adhesive tape). With emerging materials consisting of different structured synthetic polymers, the need for analytical methods characterizing these kinds of sample also arises. Conventionally, analytical techniques such as FT-IR or Raman spectroscopy are used for polymer classification. Although, these techniques offer laterally resolved investigations they lack the possibility of analyzing depth profiles. In this work, we present laser induced breakdown spectroscopy (LIBS) as a novel and powerful analytical method for spatially resolved polymer classification. As a feasibility study, two exemplary structured synthetic polymer samples (2D structured and multilayer system) have been analyzed using LIBS and the spatial distribution of 5 different synthetic polymers, namely acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyethylene (PE), polyacrylate (PAK) and polyvinylchloride (PVC) have been successfully classified using multivariate statistical approaches (principal component analysis (PCA) and k-means clustering). Spatially resolved classification results were validated by comparing the obtained distribution of the 2D structured sample to the elemental distribution of a contamination present in one of the synthetic polymers. Classification of the polymeric multilayer system was validated by comparing the obtained results to a microscopic cross-section. It was shown that LIBS cannot only be used to investigate 2D structured polymer samples but also for direct analysis of depth profiles. Besides synthetic polymer classification, LIBS provides simultaneous analysis of the elemental composition of the sample, which increases the total amount of information accessible with only one measurement.
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•LIBS combined with multivariate statistics can be used for polymer classification.•Imaging of the distribution of a sample consisting of 2 polymers using LIBS.•3D depth profiling of a sample consisting of multiple polymer layers.
In the last years, LA-ICP-MS has become an attractive technique for analyzing solid samples from various research fields. However, application in material science is often hampered by the limited ...availability of appropriate certified reference materials, which are a precondition for accurate quantification. Thus, frequently in-house prepared standards are used, which match the sample's composition and include all elements of interest at the required concentration levels. However, preparing and characterizing such standards is often labor-intensive and time-consuming.
This work proposes a new approach for the fabrication of matrix-matched standards based on the concept of standard addition. In the first step, the analytes of interest are homogeneously deposited onto the sample surface using liquid standards and a spraying device. For analysis, the generated thin layer is ablated simultaneously with the underlying sample. Thereby deviations in the ablation process and particle transport can be avoided. It could be shown that the developed method is highly versatile and could be easily adapted to the actual needs. Using silicon, silicon carbide, copper, aluminum, and glass as a matrix, excellent linear correlations between observed signal intensities and deposited amounts were found for the elements Zn, Ag, In, and Pb (R2 - values greater than 0.99).
The method was applied to determine the content of sulfur, zinc, silver, indium, and lead in a commercial Kapton® polyimide film. The obtained results could be verified based on the homogeneously distributed sulfur by conventional liquid ICP-MS analysis after sample digestion, showing similar precision and accuracy. Lead was found to show a very inhomogeneous distribution in the Kapton® film, with concentration below the LOD at most measured locations and irregularly occurring spots with significantly higher concentrations. Finally, a quantitative depth profile of sulfur in a Kapton® film has been measured to assess the uptake of SO2 after a weathering experiment.
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•A new method for matrix-matched standard preparation for quantitative LA-ICP-MS analysis was developed.•Spray deposition of liquid standards result in evenly distributed residues on the sample surface.•The method is applied for quantitative determination of sulfur in a polymer, results are verified with a reference method.•The method is applied for quantitative elemental mapping of Pb in a polymer, showing an inhomogeneous distribution.•Versatile applicability of the approach is demonstrated with multielement standards on various sample materials.
Epoxy molding compounds are the most commonly used composite material for encapsulation in the semiconductor industry. They consist of a polymer-based matrix, inorganic filler particles, and a wide ...range of (in)organic additives to fine-tune the properties. As the encapsulation material is in direct contact with the delicate semiconductor components, the reliability and lifetime of electronic devices heavily depend on applying an appropriate type of encapsulation material. Semiconductor manufacturers typically obtain epoxy molding compounds from specialized external suppliers. Therefore, quality control checking if the correct molding compound was obtained and whether its properties deviate from previous batches is of great interest to the semiconductor industry. In this work, we investigate the capabilities of a Tandem LA-ICP-MS/LIBS approach to comprehensively characterize epoxy molding compounds to construct a classification model capable of distinguishing between different molding compound types. Tandem LA-ICP-MS/LIBS enables detection of the elemental composition of all relevant components within epoxy molding compounds: LIBS can detect polymer-specific signals and the major and minor elements present in the matrix (inorganic filler particles and additives). LA-ICP-MS provides information about elements present at trace levels (e.g., contaminations), which can provide information about batch-to-batch variations. We analyzed 29 samples of 20 molding compound types from 4 different suppliers. Using exploratory data analysis (PCA and HCA), we investigated the spectral fingerprint of the different molding compound types. Finally, a Random Decision Forest-based classifier is optimized and characterized, and a model is constructed. The final classifier is tested with independent samples that were not part of the training set, revealing a satisfying performance and highlighting some molding compound types that are difficult to distinguish.
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•Tandem LA-ICP-MS/LIBS method for quality control in the semiconductor industry.•Exploratory data analysis and development of a classification model.•Machine learning based elemental fingerprinting.
Microplastics (MPs) are currently one of the major environmental challenges within our society. With the awareness of the impact of MPs on the environment increasing over the last years, the need for ...increased monitoring as well as comprehensive analysis to better understand the fate and impact of MPs has become more and more important. A major aspect of MP characterization is the assignment of the polymer type of individual particles. Here, per- and poly-fluoroalkyl substances (PFAS), originating from fluor-containing polymers, have gained a lot of attention due to the severe environmental impact. Additionally, quantitative analysis of the metal content is of great interest in the field, since MPs are prone to either leaching (in)organic additives into the environment or taking up and accumulating hazardous components (e.g., heavy metals). In this work we demonstrate the capabilities of a simultaneous LIBS/LA-ICP-MS setup for the analysis of MPs. In the first part, we demonstrate the potential of targeted LIBS analysis for the imaging of fluor-containing polymers. Using a laser spot size of 5 μm combined with highly sensitive ICCD detection enables analysis of particles in the low μm range. In the second part we combine the polymer-identification capabilities of LIBS with the high sensitivity of ICP-MS to perform matrix-matched quantification of the metal content of individual MPs. In this case we use a spot size of 50 μm facilitating polymer classification with a broadband spectrometer, resulting in detection limits of 0.72 μg/g for Pb and 9.5 μg/g for Sn simultaneously measured using ICP-MS.
•A versatile procedure for polymer testing was developed using a weathering setup in combination with quantitative LA-ICP-MS analysis.•Differences in the uptake of H2S and SO2 in polymer films were ...investigated.•The influence of temperature and relative humidity on SO2 uptake characteristics in polymer films was investigated.•Quantitative LA-ICP-MS depth profiles of sulfur give unprecedented insight into SO2 diffusion profiles into polymer films.
In polymer coating applications for technological products exposed to harsh environmental operating conditions, it is essential to provide and extend the protection capability of the coating material against corrosive gases, such as hydrogen sulfide or sulfur dioxide. Therefore, applied materials are subject to ongoing developments to improve their properties. Such developments aiming to establish improved polymer materials can only be performed accompanied by suitable testing procedures and analysis methods, for which a versatile approach is proposed in the presented work.
We developed a testing approach, combining a mixed flowing gas setup (MFG) that allows flexibly adjustable exposure conditions for weathering experiments, and laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) for subsequent sample analysis, allowing quantitative and spatially resolved determination of the sulfur uptake derived from H2S or SO2 weathering with high sensitivity. The testing procedure is demonstrated on four different polymer types – polyimide (PI), polyvinylpyrrolidone (PVP), polystyrene (PS), and polyvinylacetate (PVA). Results from bulk measurements showed a significantly higher uptake of SO2 compared to H2S, and that the uptake increases with higher exposure temperatures and relative humidity levels. Further, quantitative sulfur depth profile measurements reveal diffusion profiles into the polymer films. The gas uptake was observed mostly withing the uppermost 1–2 µm of the surface, with quickly decreasing concentrations into the bulk of the materials. However, strong variations of the spatial distribution of sulfur between the different polymer types and between different exposure conditions were determined. The results provided give a good insight into what possibilities the proposed investigation procedure opens up for specifically dedicated research compared to standard methods that are used to determine gas permeation.
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