Analytische Chemie
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Analytical Sciences has developed from Ostwald’s “unentbehrlichen Dienstmagd” to a chemical discipline at the core of many of today’s fundamental and applied scientific problems and innovations. An atomic or molecular understanding of basic processes in chemistry, soft matter physics, materials and life science is enabled only through new analytical methods and instrumentation. Similar observations can be found for pressing sociopolitical conflicts of the future: A rational discussion of global climate change or new energy sources is only possible with reliable analytical results. Progress in Analytical Sciences is only possible if the underlying interdisciplinary character is acknowledged and valued. The talk will illustrate the scope of modern Analytical Science through examples from process analysis relevant to modern process intensification and industry 4.0 to bioanalysis and the use of synchrotron radiation to elucidate fundamental reactions materials.
Modeling is an important tool for understanding a physical phenomenon. It helps to interpret results of experiments and optimize experimental parameters for obtaining a desirable result. Modeling laser induced plasma is beneficial for many scientific and industrial fields, e.g., analytical chemistry, pulsed laser deposition, plasma enhanced chemical vapor deposition, laser welding, additive manufacturing etc. In this presentation, a personal experience in development of a physical model of laser induced plasma will be given in a chronological sequence starting from early 2000th and until now.
Over the time, the model evolved from its simple analytical form that described plasma emission spectra to its current numerical form that describes plasma dynamics, chemistry, and interaction with a substrate surface. Several examples will be given for the application of the model to practical problems such as spectroscopic chemical analysis, plasma enhanced chemical vapor deposition, and surface modification by laser ablation.
The amount of data generated worldwide is constantly increasing. These data come from a wide variety of sources and systems, are processed differently, have a multitude of formats, and are stored in an untraceable and unstructured manner, predominantly in natural language in data silos. This problem can be equally applied to the heterogeneous research data from materials science and engineering. In this domain, ways and solutions are increasingly being generated to smartly link material data together with their contextual information in a uniform and well-structured manner on platforms, thus making them discoverable, retrievable, and reusable for research and industry. Ontologies play a key role in this context. They enable the sustainable representation of expert knowledge and the semantically structured filling of databases with computer-processable data triples.
In this perspective article, we present the project initiative Materials-open-Laboratory (Mat-o-Lab) that aims to provide a collaborative environment for domain experts to digitize their research results and processes and make them fit for data-driven materials research and development. The overarching challenge is to generate connection points to further link data from other domains to harness the promised potential of big materials data and harvest new knowledge.
Microscope slide collections represent extremely valuable depositories of research material in a natural history, forensic, veterinary, and medical context. Unfortunately, most mounting media of these slides deteriorate over time, with the reason for this not yet understood at all. In this study, Raman spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, and different types of light microscopy were used to investigate the ageing behaviour of naturally aged slides from museum collections and the experimentally aged media of Canada balsam and Permount™, representing a natural and a synthetic resin, respectively, with both being based on mixtures of various terpenes. Whereas Canada balsam clearly revealed chemical ageing processes, visible as increasing colouration, Permount™ showed physical deterioration recognisable by the increasing number of cracks, which even often impacted a mounted specimen. Noticeable changes to the chemical and physical properties of these mounting media take decades in the case of Canada balsam but just a few years in the case of Permount™. Our results question whether or not Canada balsam should really be regarded as a mounting medium that lasts for centuries, if its increasing degree of polymerisation can lead to a mount which is no longer restorable.
Laser-induced plasma (LIP) has drawn significant amount of attentions in the past decades, particular in elemental analyses for solid or liquid samples. Through proper focusing of the highly energetic laser beam, the plasma can also be ignited in the ambient air, where airborne analytes can be ionized. Such an effect enabled the use of airborne LIP as an ambient ionization source for mass spectrometric analyses. In contrast to other ambient desorption/ionization sources, airborne LIP does not require a specific discharge medium or expensive gas stream. Meanwhile, the airborne LIP produces reagent ion species for both proton-transfer and charge-transfer reactions in addition to the vacuum ultraviolent photons that are capable of promoting single photon ionization, which can be utilized to ionize polar and non-polar analytes. In order to gauge the analytical performance of airborne LIP, it is critical to understand the undergoing chemistry and physics during and after the plasma formation.
Due to the ambient nature of airborne LIP, the variations of air composition and flow strongly affect the plasma behaviors. Preliminary result suggested the addition of a laminar flow of nitrogen gas favored the formation of protonated species (MH+) against the molecular ones (M+). Although the gas addition approach cannot fully tune the ionization process towards the specific production of pseudo-molecular species versus molecular ones, the alternation of molecular ion formation can be used for analyte recognitions through post processing of the ion patterns. The pulsed character of the used lasers makes the reagent ion equilibrium both transient- and highly fluid-dynamically controlled. The acoustic shock-waves induced by the airborne LIP get affected by an applied gas streams towards the plasma center, influencing the molecular-ion and ion-ion interactions in the near proximity of the plasma.
To understand the airborne LIP formation, the temporally and spatially resolved optical emission spectra were recorded. The results will be correlated to time-resolved mass-spectrometric investigations of the ion profile during different stages of the plasma formation. As one example, the formation of pyrylium ion originating from aromatic compounds will be highlighted.
The development and enhancement of new ionization techniques for mass spectrometry often needs to be custom-tailored for specific sampling approaches. Here, a direct sampling ionization technique is presented for ambient mass spectrometry. Ambient mass spectrometry based techniques are typically used to analyze samples in their native states without sample pretreatment. This new design is based on a quasi-continuous airborne plasma which is ignited inside the particulate air via a focused laser irradiation. Desorption and ionization of the analyte molecules are achieved by the laser plasma without reaching the plasma. The ionization process is induced by interaction with nascent ionic fragments, electrons and ultraviolet photons in the plasma vicinity. Previously, this method was solely used for the characterization of solid and gaseous analytes. The sample introduction was occurred via thermal desorption and headspace analysis. This study focuses on the potential applicability of liquid samples. In comparison to previous approaches, the usage of liquid samples has an impact on the stability of typically used plasma of 532 nm. It was necessary to realize an alternative plasma using light of the fundamental wavelength of 1064 nm. That new plasma resulted in a significant more stable and bright plasma and the first laser plasma ionization spectrum was recorded for an analyte in the condensed phase with a mass spectrometer of type LCQ DecaXP.
The development and improvement of new ionization techniques for mass spectrometry often requires dedicated, specific sampling approaches. Recently, a novel ionization scheme for ambient MS has been introduced based on a quasi-continuous laser-induced plasma, which was ignited directly before the MS inlet. This setup combines the general advantages of ambient ionization, provides electro neutrality, sufficient duty cycle and a ubiquitous plasma medium.
A high repetition rate DPSS laser (Conqueror 3-LAMBDA, Nd:YVO4, 1 - 500 kHz, average output power: 12 W at 50 kHz, Compact Laser Solutions GmbH, Germany) and the corresponding optomechanical system were installed on an optical breadboard above the inlet of a LCQ DecaXP ion trap mass spectrometer. The quasi-continuous airborne plasma was ignited inside the sprayed sample in front of the inlet via focused laser irradiation.
The introduction of liquid samples into laser-induced plasmas requires higher plasma power during solvent evaporation as compared to gaseous samples. This increased demand was approached via a two-fold strategy: Firstly, an alternative, more powerful, laser plasma driven by the fundamental instead of the second harmonic wavelength was implemented, which provided a 10-fold increase of signal intensity, while maintaining the same reagent ion pattern as the previous plasma. Protonated water clusters [(H2O]nH]+, NH4+ as well as charge transfer promoting ion O2+, dominated the reagent ion mass spectrum. Secondly, a miniaturized nebulizer was used to minimize the size of the plasma quenching solvent droplets. The result of these improvements was a new and very stable ion source for direct microfluidic coupling. A variety of samples demonstrated the performance of the ion source.
A laser-driven plasma was shown to be a powerful ion source for gaseous and solid samples. For the first time, liquid samples were examined using the novel source. In addition to demonstrating an improved strategy for igniting the laser plasma, this contribution also covers the miniaturization of the spray source for enhanced ionization, while minimizing sample consumption via a microfluidic spray systems.
Electrochemical methods make great promise to meet the demand for user-friendly on-site devices for monitoring important parameters. Food industry often runs own lab procedures, e.g., for mycotoxin analysis, but it is a major goal to simplify analysis, linking analytical methods with miniaturized technologies. Enzyme-linked immunosorbent assays, with photometric detection of the horseradish peroxidase (HRP) substrate 3,3’,5,5’-tetramethylbenzidine (TMB), form a good basis for sensitive detection. To provide a straight-forward approach for the miniaturization of the detection step, we have studied the pitfalls of the electrochemical TMB detection. By cyclic voltammetry, it could be shown that TMB electrochemistry is strongly dependent on the pH and the electrode material.
It was found that screen-printed gold electrodes and a very low pH value (pH 1) are well-suited to perform the electrochemical detection of TMB, due to the reversible character of the redox reaction under these conditions. Under these conditions, a good signal stability over several measuring cycles is achieved, providing the basis for analyzing multiple samples. In contrast to this, for carbon screen-printed electrodes, it was found that the signal response has changed after the electrochemical reaction with TMB at pH 1. At moderately acidic conditions (pH 4), neither with carbon nor with gold electrodes a reproducible electrochemical detection of TMB could be achieved.
Based on these findings, we created a smartphone-based, electrochemical, immunomagnetic assay for the detection of ochratoxin A (OTA) and ergometrine in food samples. A competitive assay is performed on magnetic beads using HRP and TMB/H2O2 to generate the signal. Enzymatically oxidized TMB is quantified after addition of H2SO4 by amperometry with screen-printed gold electrodes in a custom-made wall-jet flow cell. The results are in good correlation with the established photometric detection method, providing a solid basis for sensing of further analytes in HRP-based assays using the newly developed miniaturized smartphone-based, electrochemical, immunomagnetic assay.
An alternative method for lithium isotope analysis by using high-resolution atomic absorption spectrometry (HR-CS-AAS) is proposed herein. This method is based on monitoring the isotope shift of approximately 15 pm for the electronic transition 22P←22S at around the wavelength of 670.8 nm, which can be measured by state-of-the-art HR-CS-AAS. Isotope analysis can be used for (i) the traceable determination of Li concentration and (ii) isotope amount ratio analysis based on a combination of HR-CS-AAS and spectral data analysis by machine learning (ML).
In the first case, the Li spectra are described as the linear superposition of the contributions of the respective isotopes, each consisting of a spin-orbit doublet, which can be expressed as Gaussian components with constant spectral position and width and different relative intensity, reflecting the isotope ratio in the sample. Precision was further improved by using lanthanum as internal spectral standard. The procedure has been validated using human serum-certified reference materials. The results are metrologically comparable and compatible with the certified values.
In the second case, for isotope amount ratio analysis, a scalable tree boosting ML algorithm (XGBoost) was employed and calibrated using a set of samples with 6Li isotope amount fractions ranging from 0.06 to 0.99 mol mol−1. The training ML model was validated with certified reference materials. The procedure was applied to the isotope amount ratio determination of a set of stock chemicals and a BAM candidate reference material NMC111 (LiNi1/3Mn1/3Co1/3O2), a Li-battery cathode material. These determinations were compared with those obtained by MC-ICP-MS and found to be metrologically comparable and compatible. The residual bias was −1.8‰, and the precision obtained ranged from 1.9‰ to 6.2‰. This precision was sufficient to resolve naturally occurring variations. The NMC111 cathode candidate reference material was analyzed using high-resolution continuum source atomic absorption spectrometry with and without matrix purification to assess its suitability for technical applications. The results obtained were metrologically compatible with each other.