Analytische Chemie
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The progress in X-ray detector electronics (sensitivity and speed) allows meanwhile fast single photon detection by a matrix detector. Combined photon counting and energy discrimination is implemented in the electronic circuit of each detector pixel. The company XCounter developed detectors based on CdTe single crystals, which can be tiled to larger areas and have a pixel size of 100ìm. The largest area available in beginning of 2014 is 50x75 mm². These detectors have very promising properties, which make them very suitable for NDT applications:
1. A CdTe attenuation layer of 750 µm thickness allows efficient X-ray detection up to ca. 300 keV. In counting mode only photon noise is important; no other detector noise sources need to be considered. There is no Offset signal without radiation.
2. Each of the detector pixels has two energy thresholds. These can be used for dual energy imaging for materials separation. Also the suppression of scattered radiation by energy thresholding will improve the image contrast sensitivity. First experiments will be presented which demonstrate the advantages of this new detector technology over the conventional charge integrating detectors.
A challenge is the development of a modified detector calibration procedure, which becomes critical at longer exposure times.
Luminescence-based detection methods, ranging from fluorescence spectroscopy for photophysical and mechanistic studies over sensing applications, chromatographic separation techniques and the microarray technology with fluorescence detection to fluorescence microscopy, flow cytometry, single molecule spectroscopy, and molecular imaging to integrating sphere spectroscopy, are among the most widely used methods in the life and material sciences. This is due to e.g., their unique sensitivity enabling the detection of single molecules, potential for multiplexing, ease of combination with spatial resolution, and suitability for remote sensing. Many of these advantages are closely linked to the choice of suitable molecular and nanoscale fluorescent reporters, typically required for signal generation. This includes organic dyes without and with sensor function, fluorophore-encoded polymeric and silica nanoparticles as well as nanocrystalline systems like semiconductor quantum dots and upconversion phosphors, emitting in the visible (vis), near-infrared (NIR), and IR (infrared). Current challenges present the environment sensitivity of most fluorophores, rendering fluorescence spectra, measured intensities/fluorescence quantum yields, and fluorescence decay kinetics matrix-dependent, and instrument-specific distortions of measured fluorescence signals that need to be considered for quantification and comparability of data, particularly fluorescence spectra.
Here, current applications of luminescence-based methods and different types of reporters will be presented. In this context, suitable spectroscopic tools for the characteri-zation of the optical properties of fluorescent reporters and fluorophore-encoded microparticles, analytical tools for the determination of the surface chemistry of different types of particles, and different multiplexing strategies will be discussed.
The calibration of fluorescence signals in assays as well as in biological systems is a key requirement. Especially for quantitative studies of living cells, e.g. expression of biomolecules the generation of concentration-proportional analytical statements obtained by the fluorescence intensity plays a significant role. This also applies for cell assays like immunofluorescence assays. Also, the determination of local concentrations of target proteins within cells or within their environment is a major challenge in modern biology.
Proper calibration of the fluorescence imaging systems is the prerequisite to ensure that test results from different instruments and different test environments are comparable. The existing calibration solutions for fluorescence imaging systems, however, mostly depend on simple instrument calibration without reference to absolute physical standards or with insufficient stability for multiple measurements. In order to obtain the necessary quantitative information, it is required to calibrate both instruments and the corresponding assays, ideally with one calibrating system.
We present several new calibration tools for the most commonly used platforms for cytometry, i.e. fluorescence microscope based systems and flow cytometry based systems. They are comprised of ultra-stable fluorophores that are encapsulated in bio- and cell-compatible polymer matrices. Each tool is tailored to the specific requirements of the different read-out platforms. Both re-usable single-color calibration slides for fluorescence microscopes as well as new multi-color calibration beads are presented.
The traceability of the calibration tools to international standards and the wide calibration range of the fluorescence intensity are illustrated. They are thus suitable for calibration of the signal over concentration and quantum yield to that of the targets.
Production of soil reference materials (for project partners) is integrated into the project „X-ray fluorescence, laser-induced breakdown spectroscopy (and Raman spectroscopy) as tools for a site-specific management of soil fertility“. BAM provides good analytics which is necessary for measuring the right values. Why is it important to have reference values? They enable the connection of own measuring results to the SI (international system of units) and the comparison with results from other laboratories. The soil reference materials are also needed for the calibration of the matrix dependent methods laser-induced breakdown spectroscopy (LIBS) and X-ray fluorescence analysis (XRF). For the production of soil reference materials different steps are required, for example drying, sieving and homogenisation.
Gas sensors in linear form based on the measuring principle of gas selective permeability through a membrane were developed and introduced for the detection and quantification of gas concentrations. A current field study focuses on measuring CO2 concentrations for a spatially distributed monitoring of subsurface CO2 gas storage sites in near real time. A 400 m² test site and a corresponding laboratory system were built up to characterize, validate, and optimize the sensor. A calibration routine was developed, which can be applied subsequently to underground installation. First measurement results indicate the potential of the method.
Direct current (DC) fast flow glow discharge mass spectrometry is an important and versatile tool for multielemental trace analysis of conductive solid materials down to the µg/kg level. Special benefits are minimum demands on sample preparation, avoiding losses and contamination, and only short analysis time.
For fast flow GD sources, the quantification strategy based on relative standard sensitivity factors (Standard RSFs) which are independent of matrix is not sufficiently satisfying regarding accuracy for each matrix and element [1]. Therefore matrix-matched calibration samples (MMCS) are required to achieve reliable quantification. In fact, the list of existing certified reference materials (CRM) appropri-ate for calibration in trace analysis is rather short.
Convenient synthesis of homogeneous MMCS, as e.g. easily obtained in liquid sampling spectrometry, can facilitate the application of fast flow GD-MS for quantification of impurities in a variety of matrices.
Pressing of metal powders for the preparation of MMCS for GD-MS was suggested earlier [2], the method was further modified by use of analyte solutions for doping of rather pure metal powders such as Cu and Zn [3], but has not yet been applied Ni matrix.
In the present work we describe the determination of matrix-dependent relative sensitivity factors (RSFs) for Mg, Al, Cr, Mn, Fe, Co, Cu, Zn, Ag, Cd, Tl, Pb and Bi in pure Ni by using the liquid doping approach for the preparation of synthetic pressed Ni-powder samples. A four point-calibration was used applying the fast flow glow discharge mass spectrometer, Element GD (Thermo Fisher). The determined RSF were verified against suitable CRM and compared with the standard RSF given by the supplier of the instrument.
The obtained results demonstrate a satisfying Agreement with the certified values of the CRM and a significant improvement for the quantification of most of the determined elements in comparison with standard RSF.
Elemental imaging of biological samples (bio-imaging) using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) provides spatially resolved information on element distribution (qualitative and quantitative) in thin sections of biological samples. By rastering with a laser across the sample, a two-dimensional image of the elemental distribution can be reconstructed that shows the relative intensities of the respective elements. However the method is hampered by a lack of internal standards and quantification concepts, which will be discussed in this lecture in more detail.
In liquid analysis the internal standard is used for drift correction and calibration and thus it is required that the standard should have similar physical and chemical properties similar to the analyte element during the pneumatic nebulization process, the transport, ionization and transmission into the ICP-MS. In laser ablation it should correct additionally for differences in the ablation process by laser instabilities or changes of sample properties to compensate variations or drift effects during the LA process.
X-ray fluorescence (XRF) analysis is a well suited methodology for thin film analysis. With XRF the mass deposition of elements can be non-destructively determined. By knowledge of the density of the thin film as well the thickness can be given. In the field of industrial thin film manufacturing various material systems and layer structures are used. the analysis relies on well-known calibration samples or even reference materials to determine absolute analytical results from the measurement values recorded. Within this concept of chemical traceability the calibration sample has to be as similar as possible to the industrial thin film sample with respect to the spatial distribution of elemental composition in order to minimize matrix effects or analysis related uncertainties. The procurement of such certified similar calibration samples or reference materials including their required traceability is a challenge due to limited number of available calibration samples or reference materials, in particular at the nanoscale. There are only few providers of calibration standards for layer or coating thicknesses which are suitable for XRF analysis. The limited number of available certified reference materials (CRMs) for XRF thin film analysis and in parallel the growing market of novel thin film materials induces a growing gap of required calibration samples for XRF analysis.
The exploitation of process-near samples benefits from a leverage effect: on one hand reference materials are needed for calibration and alignment procedures for X-ray fluorescence devices. They are customized for the special need of the end-user, e.g. a company producing thin film solar cells. On the other hand the market for EDXRF devices develops positively by providing novel thin film calibration samples for industry and end user-related production processes or application. Two aspects are hereby being addressed; the improvement of product quality which in certain sectors like aerospace and automotive industry directly translates to product safety as well as the expansion of the sales potential of EDXRF measuring devices.