Recent advances in quantitative LA-ICP-MS analysis : challenges and solutions in the life sciences and environmental chemistry

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely accepted method for direct sampling of solid materials for trace elemental analysis. The number of reported applications is high and the application range is broad; besides geochemistry, LA-ICP-MS is mostly used in environmental chemistry and the life sciences. This review focuses on the application of LA-ICP-MS for quantification of trace elements in environmental, biological, and medical samples. The fundamental problems of LA-ICP-MS, such as sample-dependent ablation behavior and elemental fractionation, can be even more pronounced in environmental and life science applications as a result of the large variety of sample types and conditions. Besides variations in composition, the range of available sample states is highly diverse, including powders (e.g., soil samples, fly ash), hard tissues (e.g., bones, teeth), soft tissues (e.g., plants, tissue thin-cuts), or liquid samples (e.g., whole blood). Within this article, quantification approaches that have been proposed in the past are critically discussed and compared regarding the results obtained in the applications described. Although a large variety of sample types is discussed within this article, the quantification approaches used are similar for many analytical questions and have only been adapted to the specific questions. Nevertheless, none of them has proven to be a universally applicable method

The properties and the formation mechanism of the stellar counter-rotating components in NGC 4191

We disentangle two counter-rotating stellar components in NGC 4191 and characterize their physical properties (kinematics, morphology, age, metallicity, and abundance ratio). We performed a spectroscopic decomposition on integral field data to separate the contribution of two stellar components to the observed galaxy spectrum across the field of view. We also performed a photometric decomposition, modelling the galaxy with a S\'ersic bulge and two exponential disks of different scale length, with the aim of associating these structural components with the kinematic components. We measured the equivalent width of the absorption line indices on the best fit that represent the kinematic components and compared our measurements to the predictions of stellar population models. We have evidence that the line-of-sight velocity distributions (LOSVDs) are consistent with the presence of two distinct kinematic components. The combined information of the intensity of the LOSVDs and photometry allows us to associate the S\'ersic bulge and the outer disk with the main kinematic component, and the inner disk with the secondary kinematic component. The two kinematic stellar components counter-rotate with respect to each other. The main component is the most luminous and massive, and it rotates slower than the secondary component, which rotates along the same direction as the ionized gas. We also found that the two kinematic components have the same solar metallicity and sub-solar abundance ratio, without the presence of significant radial gradients. On the other hand, their ages show strong negative gradients and the possible indication that the secondary component is the youngest. We interpret our results in light of recent cosmological simulations and suggest gas accretion along two filaments as the formation mechanism of the stellar counter-rotating components in NGC 4191 (Abridged).Comment: 10 pages, 10 figure. Accepted for publication in Astronomy and Astrophysic

Elemental imaging using LA-ICP-MS on biological samples

Zsfassung in dt. SpracheDie Darstellung der Verteilung (Imaging) von Heteroelementen auf biologischen Proben (meist Gewebeschnitte) mittels laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) stellt ein immer wichtiger werdendes Gebiet im Bereich der Biowissenschaften dar.Die Kenntnis der Verteilung von Hauptkomponenten wie auch von Spurenelementen kann dazu beitragen, fundamentale biologische Prozesse näher zu verstehen. Selbst bei Untersuchungen rein qualitativer Natur sind zahlreiche Limitationen zu berücksichtigen. Obwohl ICP-MS eine quantitative massenspektrometrische Methode ist, werden in der Literatur hauptsächlich qualitative Verteilungsbilder präsentiert.Besonders für Untersuchungen der lateralen Verteilung von Elementen auf biologischen Proben wird ein interner Standard benötigt, um Artefakte in den erstellten Verteilungsbildern zu vermeiden. Sowohl Änderungen im Materialabtrag durch den Laser als auch sich verändernde Plasmabedingungen und Sensitivität des Detektors während der Messung sollten durch diesen internen Standard eliminiert werden. Dabei ist die gleichmäßige Verteilung des internen Standards über die Probenoberfläche ein wichtiger Faktor. Während der Messung auftretende instrumentelle Drifts und durch Änderungen der Matrix bedingte Unterschiede im Materialabtrag können durch die Verwendung geeigneter Konzepte zur Signalnormalisierung kompensiert werden. Ein weiterer Punkt, der sich durch die Verwendung von internen Standards ergibt ist eine Vergleichbarkeit von Proben, die an unterschiedlichen Tagen gemessen wurden.Zur Quantifizierung in LA-ICP-MS Imaging-Experimenten wird in der Literatur am häufigsten die Verwendung von "matrix-angepassten" Standards (matrix-matched) beschrieben. Hierbei wird ein Gewebehomogenat mit einer bekannten Menge an Analyt versetzt, wieder in feste Form gebracht und danach werden - wie von der Probe auch - Dünnschnitte angefertigt. Jedoch ergeben sich aus der Verwendung eben solcher Standards zahlreiche Einschränkungen. Durch die Entwicklung neuer Quantifizierungsstrategien sollen die vorhandenen Probleme eliminiert und gleichzeitig die Vertrauenswürdigkeit der erhaltenen Ergebnisse erhöht werden. Die Verwendung von geeigneten internen Standards ermöglicht die Erstellung von Kalibrationen ohne matrix-matched Standards.In der vorliegenden Arbeit wird eine Methode zur internen Standardisierung in LA-ICP-MS Imaging-Experimenten vorgestellt. Muster, die mit konventionellen Tintenstrahldruckern auf organische Matrices (z.B. normales Papier) geduckt werden, werden als Ersatz zu biologischen Proben zur Entwicklung und zum Testen von Standardisierungsmethoden verwendet. Das untersuchte Element ist Kupfer (in blauer Druckerfarbe enthalten), dessen Verteilung durch den kontrollierten Druckprozess bekannt ist. Alle anderen Elemente zeigen konstante Verteilung über die gesamte Probe. Durch die Reduktion der betrachteten Analyten und die Minimierung der Probenkomplexität kann die gesammelte Datenmenge auf ein übersichtliches Maß reduziert werden. Diese Bedingungen erleichtern die Methodenentwicklung und evaluierung vor der Anwendung auf reale biologische Proben mit unbekannten Analytgehalten.Die Verwendung von dünnen Goldschichten als internen Standard wurde evaluiert und mit Messungen ohne internen Standard und mit Kohlenstoff (in der Literatur häufig verwendet) als Standard verglichen. Weiters wurde die Homogenität der aufgebrachten Schichten untersucht, um das Einbringen möglicher Artefakte durch inhomogene Goldschichten zu vermeiden.Die tatsächlichen Kupferkonzentrationen auf den Proben konnten leicht durch einen Säureaufschluss der geduckten Muster und anschließende ICP-MS Messung der Proben bestimmt werden. Die hohe Reproduzierbarkeit des Druckvorgangs stellt sicher, dass die Mengen an aufgebrachtem Kupfer innerhalb einer vernachlässigbaren Varianz liegen.Die Ergebnisse der präsentierten Arbeit unterstreichen die Tatsache, dass Goldschichten mit Dicken im Nanometerbereich ein geeignetes Hilfsmittel zur Signalnormalisierung in LA-ICP-MS Imaging-Experimenten sind. Es konnte gezeigt werden, dass die Abscheidung der Goldschichten homogen ist und, dass die Normalisierung auf das Goldsignal die eine Eliminierung von instrumentellen Drifts und matrixabhängigen Ablationsunterschieden ermöglicht. Durch eine Reduktion der Standardabweichung der Messungen nach der Gold-Normalisierung konnte die Verlässlichkeit der erhaltenen Images gesteigert werden. Ohne Standardisierung ergaben sich auf unterschiedlichen Matrices veränderte absolute Analytsignale. Durch Standardisierung auf das Goldsignal konnten diese Unterschiede eliminiert werden. Des Weiteren konnte eine verlässliche Quantifizierungsmethode für LA-ICP-MS Imaging-Experimente entwickelt werden. Es war möglich, quantitative Informationen über Analytverteilungen auf unbekannten Proben zu erhalten. Dabei können die Druckmuster als Kalibration verwendet werden.LA-ICP-MS imaging experiments are of growing interest within the field of biosciences. Revealing the distributions of major components as well as trace elements in biological samples can help to understand fundamental biological processes. However, obtaining accurate images and reliable quantitative information is in most cases a sophisticated task and requires properly developed methods. Measurement methods employed in the past only offer partly satisfactory results with several points of criticism. Highly variable sample conditions and changing instrumental parameters during measurement time aggravate even obtaining reliable qualitative information when measuring biological tissues. Despite ICP-MS is a quantitative mass spectrometric method most of the reported experiments only delivered qualitative information about the analyte distributions. Especially investigations on the spatial distribution of trace elements in biosamples require an internal standard to compensate measurement artifacts. Both variations in the material desorption by the laser and changes of plasma conditions and detection sensitivity during measurement have to be monitored and eliminated by this internal standard having a constant distribution across the sample surface.Appearing instrumental drifts during measurement time and matrix related ablation differences can be compensated employing elaborate concepts for internal standardization. Furthermore the factor of day-to-day comparability of the performed experiments can be earned being an important point when measurements have to be performed in several parts or different samples should be compared.When quantification is reported in literature matrix-matched standards have been utilized for quantification. For this method a tissue homogenate is spiked with a known amount of analyte, the tissue is brought to a solid form again and thin-slices are cut from the standard in the same way as from the sample material. So, this artificially created standard can be used for calibration. However, the use of matrix-matched standards has several limitations. By the development of novel standardization strategies these limitation should be overcome and the reliability of the obtained quantitative information will be increased. The use of appropriate internal standards facilitates the preparation of calibrations even without the utilization of matrix-matched standards.In this work the development of an accurate internal standardization method using an inkjet pattern on different organic matrices is described, being a suitable alternative for the analysis of biological samples for testing of the internal standardization method. The spatial distribution of copper (from blue printing ink) is known due to the controlled printing process, other elements show homogenous distribution over the measured area. By the reduction of the monitored analytes and diminishing the complexity of the sample the amount of acquired data is reduced. These conditions ease the method development and evaluation before application to biological samples with unknown trace element concentrations. The use of deposited gold thin-layers as an internal standard is evaluated and compared to measurements without an internal standard and with carbon as internal standard being often described and used in literature. Furthermore the homogeneity of the gold application is monitored to emphasize the usability of gold layers for signal normalization and quantification.The actual amounts of copper could be easily determined by mineralization of the printed patterns and liquid ICP-MS measurement.High reproducibility of the printing process ensures that the copper amounts on all samples are only within a minor variance.The results of this work underline the fact that gold layers with thicknesses in the nanometer range are a powerful tool for signal normalization in imaging LA-ICP-MS experiments. It could be shown that the gold deposition is homogenous and that normalization to the gold signal can compensate for variances in the experimental conditions such as instrumental drifts or matrix dependent changes in material ablation.The image quality could be drastically increased by reducing the standard deviation of the measurements and by compensating for matrix dependent ablation differences. Different sample matrices showed changed absolute analyte signals. This effect could be eliminated by the gold standardization. Furthermore a reliable method for signal quantification in LA-ICP-MS imaging experiments was developed. Using gold normalization it was possible to quantitatively determine elemental concentrations on unknown samples; the printed patterns were used as calibrations.6

Quantification strategies for elemental imaging of biological samples using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)

Zusammenfassung in deutscher SpracheLaser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is a method excellently suited for the laterally resolved analysis of trace elements in tissue samples. Information about the distribution of metals in biological samples - either endogenous or exogenous to an organism - can reveal important information about biological processes. Within this PhD thesis, simple and accurate quantification strategies of trace elements in tissue samples could be successfully developed. Single- as well as multi-element quantification of trace metals could be achieved. In further experiments, LA-ICP-MS was used in parallel with Laser Induced Breakdown Spectroscopy (LIBS); such Tandem LA/LIBS setup showed to also allow access to biological bulk elements such as carbon, oxygen, or hydrogen in the laterally resolved analysis of tissues. Besides developing and optimizing approaches for quantification and extending the multi-element capabilities, also the topic of image quality was addressed. It was possible to define a metric which is capable of quantitatively defining image quality of elemental distribution images - a useful feature for optimization of the instrumental parameters for a measurement. Developed analytical procedures could be successfully applied to questions of medical relevance. Especially investigations on platinum distributions in tumor tissues delivered results which excellently complement traditional approaches for diagnosing drug uptake and efficacy.10

A comparison of sample preparation strategies for biological tissues and subsequent trace element analysis using LA-ICP-MS

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is one of the most commonly applied methods for lateral trace element distribution analysis in medical studies. Many improvements of the technique regarding quantification and achievable lateral resolution have been achieved in the last years. Nevertheless, sample preparation is also of major importance and the optimal sample preparation strategy still has not been defined. While conventional histology knows a number of sample pre-treatment strategies, little is known about the effect of these approaches on the lateral distributions of elements and/or their quantities in tissues. The technique of formalin fixation and paraffin embedding (FFPE) has emerged as the gold standard in tissue preparation. However, the potential use for elemental distribution studies is questionable due to a large number of sample preparation steps. In this work, LA-ICP-MS was used to examine the applicability of the FFPE sample preparation approach for elemental distribution studies. Qualitative elemental distributions as well as quantitative concentrations in cryo-cut tissues as well as FFPE samples were compared. Results showed that some metals (especially Na and K) are severely affected by the FFPE process, whereas others (e.g., Mn, Ni) are less influenced. Based on these results, a general recommendation can be given: FFPE samples are completely unsuitable for the analysis of alkaline metals. When analyzing transition metals, FFPE samples can give comparable results to snap-frozen tissues.180518141