X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg Flock

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СКАЧАТЬ monitor samples

       Qualitative and quantitative evaluation of results

       Estimation of measurement uncertainties

       Preparation of a report of the analytical results

      In the case of unknown samples and high accuracy requirements this process can take up a considerable amount of time; in the case of repetitive measurements on known sample material it is possible to obtain an analysis result in a very short time even with low measurement uncertainties.

      The individual steps can be summarized as follows:

       The actual measurement includes the determination of the test conditions as well as the measurement of the test sample itself and, if necessary, of the calibration samples with the available analytical technique.Figure 2.11 Steps for an analytical procedure.

       The analytical method includes the measurement, sample preparation, and evaluation of the measured data. The analytical method can be used in an identical way for comparable analytical questions.

       The analytical procedure includes additionally the sampling procedure and the data processing, namely, the preparation of the analysis report including a possible statistical evaluation of the results.

This situation is demonstrated in Figure 2.11.

The individual steps of an analytical method and their influence on the analysis results are discussed in more detail in the following sections. In addition to the various possibilities of sample preparation, the influence of the analytical technique must be discussed as well. In both cases, the analytical problem itself, as well as the analytical accuracy that is required, has a great influence on their selection.

      When performing the measurement itself, the selection of the optimal measuring conditions as well as the processing and evaluation of the spectra, including quantification, is important. Finally, the estimation of the analysis errors and the uncertainties is an important part of an analysis procedure in order to assess the quality of the results. These considerations essentially depend on the respective analytical task.

      2.4.2 Sequence of an Analysis

      The first step in an analysis is to define its goal. Several separate problems have to be considered.

      2.4.2.1 Quality of the Sample Material

      The laboratory sample, i.e. the sample material that is available in the laboratory, can have different forms: it can be compact, it can be small-particle size granule or powder-like, or it can be a paste or even a liquid. Increasingly, finished products have to be analyzed for quality assurance or fault identification purposes. As far as possible, they should not be modified for the analysis. Knowing the sample material, important information about the sample matrix can be obtained for the evaluation, such as information about non-measurable light elements.

      2.4.2.2 Sample Preparation

      For the various material qualities, the possibility of different sample preparation techniques is required. They also depend on the type of the spectrometer as well as on the desired analytical accuracy.

The simplest method of sample preparation is to cut the laboratory sample into a shape appropriate for the spectrometer. Nonetheless, it may also be necessary to homogenize, compress, or clean the sample surface so that the analyzed sample volume represents the material to be characterized. In special cases, it can be advantageous to change the state of the sample by dissolution, digestion, or deposition to achieve better analytical results.

      These options are discussed in detail in Chapter 3.

      2.4.2.3 Analysis Task

      Analytical tasks for element analyses can vary broadly. They can be

       a simple determination of the elements present in the laboratory sample (qualitative analysis);

       a monitoring of the content of one or more elements with respect to a predetermined threshold value (semiquantitative analysis);

       a comparison of the measured intensities of one or several elements with those of reference samples for identifying a material class (positive material identification – PMI);

       the quantitative determination of the mass fractions of some or all elements in the sample material with a predetermined uncertainty level as an overview analysis, as well as for an accurate or even highly accurate analysis (quantitative analysis);

       the characterization of layer systems on the sample with respect to layer thickness and layer composition, again with different accuracy requirements;

       the determination of element distributions in inhomogeneous samples.

      Starting from these very different objectives, the individual steps for the analytical method are to be defined, such as the selection of

       the equipment to be used for the measurement

       the reference samples to be used to calibrate the method

       the selection of appropriate measuring conditions as well as

       the procedures for data processing and evaluation.

      2.4.2.4 Measurement and Evaluation of the Measurement Data

      In accordance with these definitions, the measurements shall be carried out both on the unknown sample and, if necessary, on reference samples with the selected measuring conditions.

      Thereafter, a processing of the raw data is required, i.e. the measured intensities must be corrected for overlap with other lines, for detector artifacts as well as for the spectral background. The resulting net intensities can then be converted into mass fractions or layer thicknesses using appropriate quantification models. By comparison with reference samples, the analytical result can be improved, or the measurement uncertainty can be reduced.

Finally, before using the test method it can be essential to validate the entire analytical process. According to ISO EN DIN 17025 (DIN-EN-ISO-17025 2005) the validation is defined as “the confirmation by examining and providing proof that the special requirements for a specific intended use are met.” For that purpose, the required statistical parameters can be determined with the help of suitable reference samples or by comparing the analyses results of different samples with those of other independent analytical methods. These include the validity range of the method, and parameters such as trueness, repeatability and comparability, specificity and selectivity, as well as in the case of the detection of low mass fractions, the detection and determination limits. Validation is necessary to demonstrate the quality of an analytical method and to determine the achievable uncertainty of the analytical results.

      2.4.2.5 Creation of an Analysis Report

      Often, in addition to the СКАЧАТЬ