Advancements in Energy Dispersive Spectroscopy (EDS) with Wavelength Dispersive Spectroscopy (WDS)

The latest user-friendly EDAX APEX 3.0 software adds wavelength dispersive spectroscopy (WDS) capabilities, enabling those who use it to seamlessly integrate the new functionalities and use WDS if energy dispersive X-ray spectroscopy (EDS) maxes out. WDS greatly increases the accuracy of the results through the resolution of EDS peak overlaps, which improves the minimum detection limit by 10x and provides precise quantification.

Results and Discussion

EDS is prone to insufficient energy resolution to differentiate energy lines near each other. For example, Si K, W M, and Ta M lines have only 30 eV energy differences. When these elements occur in the same area of interest, they present as an amalgamated peak in the EDS spectrum (Figure 1 red outline). As seen in the WDS spectrum (Figure 1 cyan color), EDAX Lambda WDS systems can essentially resolve these ambiguities in analysis intrinsically with up to 15x better energy resolution than common EDS systems.

Figure 1. An overlay of EDS (red outline) and WDS (cyan color) spectra of a Si-W-Ta sample. Image Credit: Gatan, Inc.

In a sample of Si-W-Ta, EDS has difficulty differentiating the peaks and presents them as identical distributions in the EDS maps (Figure 2 top). In comparison, WDS can generate distinct visualizations of the individual distributions of each element (Figure 2 bottom).

Figure 2. EDS (top) and WDS (bottom) maps of the Si-W-Ta sample. The WDS maps resolve the artifacts due to Ta M, Si K, and W M peak overlaps in the EDS maps. Image Credit: Gatan, Inc.

WDS is reliable in detecting trace and minor elements with improved peak-to-background ratios (P/B) and can achieve detection limits up to 10x lower. For instance, a borosilicate glass that contains 2 wt% of boron has a boron peak that is barely visible within the EDS spectrum. A clear boron peak is present in the WDS spectrum (Figure 3) due to the substantially lower detection limits. The Lambda WDS system, which features a parallel beam design, provides up to 8x higher P/B versus Rowland’s Circle WDS systems.

Figure 3. The boron peak in the WDS spectrum of a borosilicate glass containing 2 wt% boron. Image Credit: Gatan, Inc.

The greater energy resolution and increased P/B make WDS an exceptional tool for resolving the transition metal L lines. Alloy surface and inclusion analyses usually require decreased accelerating voltages to reduce the excitation volume. However, transition metal L line peaks have numerous overlaps that EDS cannot resolve, and the P/B can be low (Figure 4 red outline) at 5 kV. WDS can deliver data with enhanced peak separation and P/B to discover transition metal L line peaks that are challenging to see through EDS alone (Figure 4 cyan color) at the same accelerating voltage.

Figure 4. An overlay of EDS (red outline) and WDS (cyan color) spectra of an alloy sample at 5 kV. Image Credit: Gatan, Inc.

The combination of EDS and WDS quantitative analysis utilizes a robust analytical algorithm, which ensures reliable elemental quantification. Quantitative results from standardless EDS analysis of a Ni monocrystal show significant discrepancies in Ta, W, and Re caused by extensive peak overlaps (Table 1). The combined EDS/WDS quantification allows Al, Ta, W, and Re to be quantified with WDS, at the same time that other elements are quantified with EDS, so the result of concentrations is consistent with the actual values.

Table 1. Quantitative results of a Ni monocrystal. Source: Gatan, Inc.

Conclusion

The improved energy resolution, sensitivity, precession, and P/B position WDS are essential supplements to EDS detectors. The new EDAX APEX 3.0 is the ideal materials characterization software, with integrated EDS, WDS, and EBSD capable of delivering solutions that were previously unattainable.

This information has been sourced, reviewed and adapted from materials provided by Gatan, Inc.

For more information on this source, please visit Gatan, Inc.