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Checking the performance of energy dispersive X-ray spectrometers as well as validation of the results obtained with energy dispersive X-ray spectrometry (EDX) at a scanning electron microscope (SEM) involve the use of (certified) reference and dedicated test materials. This paper gives an overview on the test materials mostly employed by SEM/EDX users and accredited laboratories as well as on those recommended in international standards. The new BAM reference material EDS-CRM, which is currently in the process of certification, is specifically designed for the characterization of EDS systems at a SEM through calibration of the spectrometer efficiency in analytical laboratories in a simple manner. The certification of the spectra by means of a reference EDS is described. The focus is on the traceability of EDS efficiency which is ensured by measurements of the absolute detection efficiency of silicon drift detectors (SDD) and Si(Li) detectors at the laboratory of the PTB using the electron storage ring BESSY II as a primary X-ray source standard. A new test material in development at BAM for testing the performance of an EDS in the energy range below 1 keV is also briefly presented.
An inter-laboratory comparison organized as a pilot study (PS) has been performed under the umbrella of the Consultative Committee for Amount of Substance – Metrology in Chemistry (CCQM) / Surface Analysis Working Group (SAWG) with the objective to compare k-values measured with Electron Probe X-ray Microanalysis (EPMA) for copper-gold alloys. The k-value, defined as the ratio of the Xray photon intensity of an X-ray characteristic line of the unknown sample to the corresponding one in the reference material, is a rough estimate of the mass fraction of the respective element. Basically, the mass fractions can be calculated from the k-values after applying a matrix correction procedure, which includes empirical approaches, so that traceability gets lost. Due to its physical, measurable nature, the k-value has been selected in the present PS as the primary measurand. The mass fraction has been set as the secondary (derived) measurand. Both EDX and WDX have been employed. The WDX results and the overall comparability between the k-values obtained by EDX and WDX are still in progress.
The high specificity of the coherent (Rayleigh), as well as incoherent (Compton) X-ray scattering to the mean atomic number of a specimen to be analyzed by X-ray fluorescence (XRF), is exploited to gain more information on the chemical composition. Concretely, the evaluation of the Compton-to-Rayleigh intensity ratio from XRF spectra and its relation to the average atomic number of reference materials via a calibration curve can reveal valuable information on the elemental composition complementary to that obtained from the reference-free XRF analysis. Particularly for matrices of lower mean atomic numbers, the sensitivity of the approach is so high that it can be easily distinguished between specimens of mean atomic numbers differing from each other by 0.1. Hence, the content of light elements which are invisible for XRF, particularly hydrogen, or of heavier impurities/additives in light materials can be calculated 'by difference' from the scattering calibration curve. The excellent agreement between such an experimental, empirical calibration curve and a synthetically generated one, on the basis of a reliable physical model for the X-ray scattering, is also demonstrated. Thus, the feasibility of the approach for given experimental conditions and particular analytical questions can be tested prior to experiments with reference materials. For the present work a microfocus X-ray source attached on an SEM/EDX (scanning electron microscopy/energy dispersive X-ray spectroscopy) system was used so that the Compton-to-Rayleigh intensity ratio could be acquired with EDX spectral data for improved analysis of the elemental composition.
For ten years µ-XRF (micro-focus X-ray fluorescence) analysis has been performed with SEM/EDS (scanning electron microscope with an energy dispersive X-ray detector) so that non-destructive analysis of elements at trace level concentrations below 100 µg g-1 becomes possible. This can be considered as a valuable completion of the classical electron probe microanalysis by EDS, an analytical method 'suffering' from rather poor limits of detection in the range of one to two orders of magnitude higher than those of µ-XRF. Based on a representative actual application, namely analysis of RoHS relevant elements at trace concentration levels, the performance of the rather new analytical method with respect to its limits of detection is systematically evaluated. CRMs (certified reference materials) specially prepared to support the quantitative XRF analysis of RoHS relevant elements were employed. On the other side, based on calculations of µ-XRF spectra according to a recently developed physical model the optimization of the analytical performance is also successfully undertaken.
It is a latent wish of any SEM/EDS (scanning electron microscope with an energy dispersive spectrometer) analyst to “see more” of the analyzed specimen, i.e. to improve the existing analytical figures of merit.
One key issue are the relatively poor limits of detection (not below 0.1 mass-%) provided by energy dispersive X-ray spectrometry (EDX) with the conventional electron excitation (ED-EPMA). This is a consequence of relatively low peak-to-background ratios and reduced energy resolution when compared to wavelength dispersive spectrometry (WD-EPMA). Recent technological developments make possible to equip the SEM with a wavelength dispersive spectrometer (WDS), so that
significantly better energy resolution can be attained. Also a relative new product that can be easily attached to a SEM/EDS system is a micro-focus X-ray source. Hence, it is possible to perform (micro-focus) X-ray fluorescence spectrometry (μ-XRF) and take advantage of the enhanced peak-to-background ratios (well suited for trace analysis). However, there are also some disadvantages: an increased measurement time and excitation with a high current in the 10s of nA range are usually required for WDS. μ-XRF provides more bulk information and poor limits of detection for light elements. By combining the advantages of these analytical techniques “seeing more” becomes possible.
A microfocus X-ray source mounted on the analysis chamber of an SEM was used to excite X-ray fluorescence spectra and to detect chemical elements with concentrations below the detection limit of the electron-excited X-ray spectra conventionally measured with SEM/EDS. An aluminium alloy and a hard material ceramics were analysed as representative examples. It is demonstrated that the combination of the three analytical methods: (1) SEM imaging for surface morphology characterisation; (2) electron-excited X-ray spectroscopy with its high spatial resolution for element analysis of inclusions or precipitates; and (3) X-ray fluorescence for the detection of elements with concentrations below 0.1 mass% considerably improves the performance of SEM/EDX analyses.
The accurate knowledge of the properties of polycapillary X-ray semi-lenses has a significant influence on the quantitative results in a widespread field of applications involving microfocus X-ray beams. A routine procedure for the characterisation of a polycapillary X-ray semi-lens with a scanning electron microscope (SEM) having attached an energy dispersive spectrometer (EDS) is presented in this paper. A key issue of the procedure consists of fitting the semi-lens in front of the EDS for spectra acquisition. Relevant semi-lens parameters such as focal distance, full width at half maximum (FWHM) of the acceptance area, and transmission are determined in parallelising mode of the semi-lens. Special attention has been paid to the calculation of the transmission.