Using Zetium for Practical Element Mapping and Small Spot Analysis

Conventional X-ray fluorescence spectrometry is a commonly used analytical method to establish the bulk elemental composition of a wide range of materials in both manufacturing and R&D. Process monitoring and final QC in a number of industries like mining, petrochemical, building construction and metals is carried out using XRF spectrometry.

This method is capable of analyzing bulk areas of homogenous samples only, since the spectrometer configuration is not suitable for the analysis of minute areas of the sample and heterogeneous samples.

However, the latest developments in the XRF technology allow bulk analysis combined with small spot analysis and elemental mapping on a single device. The added capability of small area analysis and elemental mapping on a fine scale extends the scope of possible applications for basic investigations in materials research and production control.

Wavelength dispersive (WD) XRF and energy dispersive (ED) XRF are incorporated into the SumXcore technology in the Zetium XRF spectrometer (Figure 1). This combination expands the scope of applications of the instrument. In addition to the accelerated data acquisition, using the ED core for small spot analysis provides other benefits like allowing the WD core to exclusively perform high accuracy and precision bulk analysis​.

Unlike other elemental mapping techniques such as electron microprobe analysis or scanning electron microscopy which require elaborate pre- treatment of samples the XRF method requires minimal sample preparation. This simplifies the task of elemental mapping for operators of varying skill levels and also makes XRF a valuable tool in process trouble shooting and scientific research.

Figure 1. The Zetium spectrometer

Advantages of Small Spot Analysis and Mapping on ED Core

In contrast to spectrometers that perform small spot analysis based on a sequential wavelength dispersive optical path, the ED core enables a much faster and simultaneous multi-element analysis per analysis spot. Another advantage is unparalleled sensitivity and improved measurement throughput due to the close coupling of the ED core and the sample.

In element mapping and inclusion studies it is desirable to gather spectra and identify the elements across the entire periodic table in a single point measurement, especially in scenarios of unknown sample compositions.

Instrumentation

The experimental set up consisted of a Zetium spectrometer equipped with a 4 kW SST R-mAX tube. For accelerated sample throughput an improved X-Y sample handler is used. Data recording was done with the help of advanced analytical software.

The ED core in the spectrometer enabled small spot analysis and mapping capabilities. High-precision turret translation mechanics was incorporated for positioning of sample and a high-resolution camera was used to capture sample images.

The latest version of the SuperQ software simplified the application set up and the generation of 2D images. Contoured 3D images were produced by simply exporting data to third-party image manipulation software.

Innovative Silicon Drift Detector Technology

A 4th generation silicon drift detector (SDD) technology is provided in the Zetium spectrometer which can be customized for high X-ray flux environments. The advantages of ED technology and enhanced performance of combined WD/ED allow the measurement speed to be increased by up to 50%.

The flexibility of performance is improved by a count rate capability of 1M cps, which is enabled using fast digital pulse processing (DPP). The fast DPP also enables the improved handling of spectral artifacts when compared to conventional DPPs.

Set up of Small Spot Analysis and Mapping

Samples with diameters up to 35 mm can be handled by the small spot analysis and mapping module. The diameter of the measurement spots is 500 µm (FWHM), and the size of the minimum intervening step is less than 100 µm, as shown in Figure 2, which results in a maximum of 24,000 spots per sample.

Figure 2. Schematic illustrating the small spot mapping accuracy/resolution (not to scale)

The sample preparation process is straightforward; the samples are mounted in a dedicated holder that has been exclusively designed for samples of irregular shapes and different sizes, as shown in Figure 3. A small clamping device is available in the holder for accurate positioning of small samples and to avoid damage to samples.

Figure 3. (a) Small spot mapping loading position on the Zetium sample changer bed, (b) sample holders and clamping tools (c )

Omnian

A number of spectrometers such as the Axios, Axios^mAX, Zetium, MagiX and benchtop EDXRF spectrometers can be suppliedwith the Omnian, which is particularly useful for standardless quantitative analysis of unknown powders, solids and liquids.

Omnian is designed for easy use with a default analytical setting and an exclusively manufactured set of reference samples that can be used for the initial set up. It uses complete fundamental parameter calculations and numerous advanced algorithms to deliver superior standardless accuracy. The key program features are:

• Fundamental parameter (FP) calculations
• Flexible sample description
• Adaptive sample characterization (ASC)
• Line-overlap corrections
• Finite thickness (FT) corrections
• Dark matrix (DM) characterization
• Fluorescence volume geometry (FVG) corrections
• Wavelength scan/EDXRF spectrum and channel measurements (WDXRF only)

Application Examples

Investigations of a variety of materials can be performed based on small spot analysis and mapping. The performance of the Zetium XRF spectrometer is demonstrated via the range of applications presented in this article.

Quantitative applications that are based on concentration and qualitative applications that are based on intensity can be set up on the spectrometer.

Quantitative applications may either be traditionally calibrated for simple inclusion analysis or may involve complex multi-element distribution analyses for numerous sample types. Semi-quantitative Omnian applications that include the advantages of standardless analysis are possible in the multi-element acquisition mode of the ED core.

Quantitative Applications - 1

Stainless Steel Calibration

A conventional calibration using eight internationally certified reference materials (CRMs) for stainless steel (series SS 461-468) was built based on small spot analysis. Table 1 presents the measurement conditions and Figure 4 represents the calibration graphs for Mn, Si, Ni and Cr.

Table 1. Conditions used for small spot analysis using the ED core

Figure 4. Calibration graphs for Ni, Cr, Mn and Si

A certified reference material was taken as the unknown and measured against the calibration. It was seen that after 100 seconds of measurement the results were in line with the certified values, as shown in Table 2.

Table 2. Comparison between certified and measured values including RMS for ten consecutive measurements of a certified reference material measured as unknown

Quantitative applications - 2

Compositional Mapping of a Meteoritic Sample

In this example, a combination of various elements was mapped in a chondritic meteorite sample (type CV3) which has calcium-aluminium-rich inclusions. The sample was first mounted onto the special sample holder and images were captured with the high resolution camera (Figure 5a). Later the sample was positioned for measurement, and using the SuperQ software, its features and analysis areas were identified (Figure 5b).

Figure 5. Setup procedure for small spot analysis (a and b) and results from the distribution mapping (c)

The chosen analysis area for the meteorite sample measured 5 X 7.5 mm. This area was mapped with a total of 600 spots. Each of these spots was measured for 60 seconds, resulting in a total measurement time of 10 hours.

The relative concentration and the distribution of 15 elements present in the analysis area are depicted in the images. Compositional variations between host matrix of the sample and the Ca-Al-rich inclusions can be observed in the images (Figure 5c). The distributions of Cr, Ni, Ca and Zn are presented as 3D contour plots based on the Gnuplot software (Figure 6).

Figure 6. 3D contour plots for Cr, Ni, Ca and Zn

Figure 7 shows the selection of six different spots across the sample surface. The quantitative capabilities of small spot analysis were demonstrated by measuring these spots.

Figure 7. Selection of six measurement spots located in different positions of the meteoritic surface area

Table 3 shows the results of these measurements. The results show the expected variations in elemental composition with Al, Mg, Ca and Fe, with large variations corresponding to different mineral phases.

Table 3. Element composition correspondent to 6 different measurement spots shown in Figure 9.

A norm factor of 1 roughly was used to normalize the results, which validates the capability of the FP algorithm in efficiently handling matrix variations within the sample applications for providing compositional values that match real concentrations.

Compositional mapping of a steel sample

In this application, the distribution of the nine constituent elements of a steel sample were mapped for the quality control. First, the steel sample was mounted onto the special mapping sample holder and the high-resolution camera was then used to capture its images (Figure 3).

Through these images the features and the analysis areas were identified (Figure 5). No heterogeneities on the sample surface were observed.

The chosen analysis area on the steel sample measured 5.75 X 8.75 mm². This area was mapped with a total of 864 spots and each spot was measured for 100 seconds, resulting in a total measurement time of 27.4 hours.

The results showed the elemental distribution of only five important elements along with their relative concentration. Figure 8 shows that compositional variations were unexpected at this scale.

Figure 8. 2D and 3D contour plots for Mn, Mo, Fe, Ni and Cr in a steel sample and high resolution image indicating sample area mapped.

It was observed that the elemental distribution of Fe was uniform over the entire analysis area, with the exception of the edges where the concentration of Fe decreases and that of Mn, Mo and Cr increases. This suggests a coupled substitution between Fe and these elements.

The result of performing practical compositional mapping at production control is demonstrated in this example. Based on the results, it can be concluded that compositional mapping helps improve manufacturing processes significantly.

Qualitative applications

Analysis of an Archeological Coin

In this example analysis of a coin found in Overijssel region (the Netherlands) region, which dated back to the 19th century, was performed based on qualitative intensity-based element mapping.

First the coin was carefully clamped and mounted onto the sample holder. An analysis area measuring 10 X 3.75 mm was mapped with a total of 600 spots and an intervening step size of 250 µm. The total measurement time was 13.3 hours, with each spot being measured for 80 seconds. The relative sensitivities and distribution of 15 elements present in the analysis area are shown in Figure 9.

Figure 9. 2D contour plots showing the distribution and relative abundances for 15 elements detected in the archeological coin.

Distinct variations in composition were observed between the inner and the outer regions of the coin, implying that the coin surface was affected by its interaction with the earth in which it was buried. According to the 2D images, the substrate is abundant in Ni and Cu, while the outer surface is abundant in Ti and Fe. In addition to this, high concentrations of heavy metals such as As and Pb are also seen which suggest that these metals are inseparable during metal production.

Based on the elemental composition of the coin, the origin of the metal that was used to forge it can be traced back and the provenance if the ores used in the smelting of the metal can also be found. Such reconstructions are useful in the identification of trade routes.

Conclusion

Performing small spot analysis and element distribution based on the XRF method is a useful tool for trouble shooting production processes and materials research. The XRF adds value to the bulk sample analysis spectrometer.

The ability of Zetium equipped with SumXcore technology in performing quick, precise and accurate small spot analysis is validated by the data presented in this article. This capability helps gain new insights into a material's compositional characteristics.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.