Maximilian Bonta

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.

Application of gold thin-films for internal standardization in LA-ICP-MS imaging experiments

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, highly variable sample conditions and changing instrumental parameters during measurement time aggravate reliable quantification especially in biological tissues. Normally matrix matched standards used for calibration are scarcely available and the manufacturing process thereof is rather complicated. Thus most experiments reported in the literature only delivered qualitative information on the analyte distributions. The use of appropriate internal standards facilitates the preparation of calibrations even without the utilization of matrix-matched standards. In the presented work an approach for providing reliable quantitative bio-images is proposed using gold thin-layers as an internal standard and patterns printed with commercially available inkjet printers as standards. The method development is based on copper from blue ink as the element of interest. It could be shown that gold standardization compensates instrumental drifts, matrix related ablation differences and day-to-day signal changes. Not only was the quality of the obtained images improved by gold standardization; while the relative standard deviation of the measurements was around 15% before standardization it could be decreased to less than 5% by gold standardization. Also quantitative information could be obtained for samples with unknown analyte concentrations. Depending on the used beam diameter limits of detection in the range of some hundreds ng g(-1) were achieved. The presented method is a promising and easy-to-handle alternative to matrix matched standards for signal quantification.

Quantitative LA-ICP-MS imaging of platinum in chemotherapy treated human malignant pleural mesothelioma samples using printed patterns as standard

LA-ICP-MS has often been applied for the analysis of trace elemental distributions in biological tissues. However, the strong matrix dependence of LA-ICP-MS analyses and highly variable matrix conditions aggravate reliable qualitative distribution analyses and thus for obtaining quantitative information elaborate quantification strategies have to be applied. In this work printed patterns on paper with thin gold layers as pseudo-internal standard have been proposed as an alternative approach to the commonly used matrix-matched tissue standards. Besides a major reduction of the workload for standard preparation the presented method allows for compensation of instrumental drifts during measurement as well as a reduction of matrix related effects. The developed method has been verified using matrix-matched tissue standards (deviations from the actual metal content less than 5% with relative standard deviations of less than 7%) and applied to platinum imaging on human malignant pleural mesothelioma samples after administration of individuals with platinum containing cytostatic drugs.

Elemental mapping of biological samples by the combined use of LIBS and LA-ICP-MS

In this study a combination of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Laser Induced Breakdown Spectroscopy (LIBS) was used for laterally resolved elemental analysis of biological samples. In general LA-ICP-MS is an excellent technique for the analysis of many trace elements. However, bulk components such as H or O are not accessible using this technique. In addition to those elements, also some other elements that are difficult or impossible to investigate using LA-ICP-MS (i.e., F, N, Cl, etc.), could be detected by LIBS. In this work, the simultaneous use of LIBS and LA-ICP-MS (tandem LA/LIBS) for the analysis of biological samples is presented, opening the door for the possibility of complete analysis of the elemental composition of a human tumor sample. Results show good correlation with the histological stainings. The obtained distribution images provide a valuable basis for further medical interpretation.

Application of dried-droplets deposited on pre-cut filter paper disks for quantitative LA-ICP-MS imaging of biologically relevant minor and trace elements in tissue samples

In this work, a novel calibration approach for minor and trace element quantification in LA-ICP-MS imaging of biological tissues is presented. Droplets of aqueous standard solutions are deposited onto pre-cut pieces of filter paper, allowed to dry, and sputtered with a thin gold layer for use as pseudo-internal standard. Analysis of the standards using LA-ICP-MS is performed using radial line-scans across the filters. In contrast to conventionally used preparation of matrix-matched tissue standards, the dried-droplet approach offers a variety of advantages: The standards are easy to prepare, no characterization of the standards using acid digestion is required, no handling of biological materials is necessary, and the concentration range, as well the number of investigated analytes is almost unlimited. The proposed quantification method has been verified using homogenized tissue standards with known analyte concentrations before being applied to a human malignant mesothelioma biopsy from a patient who had not received any chemotherapeutic treatment. Elemental distribution images were acquired at a lateral resolution of 40 μm per pixel, limits of detection ranging from 0.1 μg g(-1) (Mn, Ni, Cu, Zn) to 13.2 μg g(-1) (K) were reached.

Improvements in the direct analysis of advanced materials using ICP-based measurement techniques

Advanced materials are nowadays used in various industrial and scientific contexts, due to their particular and sometimes unique properties. In many cases, those properties are closely linked to the composition of the materials. An integral part in the characterization of advanced materials is therefore to determine their precise elemental composition, as well as to detect possible contaminants. With this information, the production as well as the properties of the final products can be optimized. To obtain such information in a routinely way, ICP-OES or ICP-MS are versatile tools, since those techniques allow sensitive multi-element analysis in a variety of matrices (e.g., high-purity materials, semiconductors, electronic components, metals, alloys, ceramics, and polymers). However, if using ICP-based techniques in their regular configuration, conversion of the solid material into a liquid solution is necessary. For this purpose, procedures such as acid digestion, fusion or dry ashing have been reported. However, although being well established, the application of these approaches is related with some drawbacks. Besides the problem of jeopardizing information on spatial distribution of analytes, some further shortcomings are risk of sample contamination and/or analyte losses, as well as increased time demand for sample preparation (especially in case of materials with high chemical resistance). Analyzing the solid sample directly is therefore an attractive alternative to conventional liquid analysis. Solid-sampling techniques which are frequently applied in combination with ICP-OES or ICP-MS detection are electro-thermal-vaporization (ETV) and laser-ablation (LA). Besides offering the mentioned advantages in sample preparation, solid-sampling techniques often allow for improvements in sensitivity, since unnecessary sample dilution could be avoided. Furthermore, LA (with restrictions also ETV) offers the possibility of spatially resolved analysis and depth profile analysis, providing information about the distribution of major, minor and trace constituents within the sample. The aim of this review is to discuss new analytical developments in ETV and LA in combination with ICP techniques for the quantitative determination of bulk, trace and ultra-trace elements in the routine analysis of advanced materials.

Characterization of recombinant human diamine oxidase (rhDAO) produced in Chinese Hamster Ovary (CHO) cells

Human diamine oxidase (hDAO) efficiently degrades polyamines and histamine. Reduced enzyme activities might cause complications during pregnancy and be involved in histamine intolerance. So far hDAO has been characterized after isolation from either native sources or the heterologous production in insect cells. Accessibility to human enzyme is limited and insect cells produce non-human glycosylation patterns that may alter its biochemical properties. We present the heterologous expression of hDAO in Chinese Hamster Ovary (CHO) cells and a three step purification protocol. Analysis of metal content using ICP-MS revealed that 93% of the active sites were occupied by copper. Topaquinone (TPQ) cofactor content was determined using phenylhydrazine titration. Ninety-four percent of DAO molecules contained TPQ and therefore the copper content at the active site was indirectly confirmed. Mass spectrometric analysis was conducted to verify sequence integrity of the protein and to assess the glycosylation profile. Electronic circular dichroism and UV-vis spectra data were used to characterize structural properties. The substrate preference and kinetic parameters were in accordance with previous publications. The establishment of a recombinant production system for hDAO enables us to generate decent amounts of protein with negligible impurities to address new scientific questions.

Direct imaging of elemental distributions in tissue sections by laser ablation mass spectrometry

We present a strategy for imaging of elements in biological tissues using laser ablation (LA) mass spectrometry (MS), which was compared to laser ablation inductively coupled plasma (LA-ICP) MS. Both methods were adopted for quantitative imaging of elements in mouse kidney, as well as traumatic brain injury model tissue sections. MS imaging (MSI) employing LA provides quantitative data by comparing signal abundances of sodium from tissues to those obtained by imaging quantitation calibration standards of the target element applied to adjacent control tissue sections. LA-ICP MSI provided quantitative data for several essential elements in both brain and kidney tissue sections using a dried-droplet approach. Both methods were used to image a rat model of traumatic brain injury, revealing accumulations of sodium and calcium in the impact area and its peripheral regions. LA MSI is shown to be a viable option for quantitative imaging of specific elements in biological tissue sections.

A metric for evaluation of the image quality of chemical maps derived from LA-ICP-MS experiments

For laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) imaging experiments – as well as other techniques used for elemental or molecular mapping – the accordance of the measured distribution with the actual distribution is of utmost importance to guarantee reliability of the obtained images. In most experiments reported in the past, the experimental conditions have been chosen so that washout effects and signal carry-over are minimized by scanning the sample surface very slowly. Therefore, measurement times become very long and decently resolved images will require acquisition times of several hours up to more than one day. To increase the application range of LA-ICP-MS for imaging it is important to decrease the measurement times, which is best accomplished by increasing the scanning rates. However, depending on the instrumentation, this can lead to blurring and compromised image quality. In this work, we present a metric to compare the measured elemental distribution with their actual distribution based on a sample with visually distinguishable features. This approach allows quantitative determination of the image quality and enables comparison of multiple measurement conditions. This information can be used for method optimization, to get a reasonable tradeoff between image quality and measurement time.

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

AbstractLaser 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. Graphical abstractSample preparation strategies for biological tissues are compared with regard to the elemental distributions and average trace element concentrations.