Comparison of EDS and WDS

Using EDS, all of the energies of the characteristic X-rays incident on the detector are measured simultaneously and data acquisition is therefore very rapid across the entire spectrum. However, the resolution of an EDS detector is considerably worse than that of a WDS spectrometer.

The WDS spectrometer can acquire the high count rate of X-rays produced at high beam currents, because it measures a single wavelength at a time. This is important for trace element analysis. The resolution of the EDS detector is such that situations may arise in which overlap of adjacent peaks becomes a problem. Many of the overlaps can be handled through deconvolution of the peaks. Others, however, are more difficult, particularly if there is only a small amount of one of the overlapped elements. Examples of these more difficult overlap situations are listed in Table 2.

Common Peak Overlaps in EDS Microanalysis

S Kα - Mo Lα − Pb Mα

Na Kα - Zn Lα

Ni Lα - La Mα

Zr Lα - Pt Mα - P Kα - Ir Mα

Nb Lα - Hg Mα

Si Kα - W Mα − Ta Mα - Rb Lα

Al Kα - Br Lα

Y Lα- Os Mα

O Kα - V Lα

Mn Lα - Fe Lα - F Kα

In practice it is advantageous to use the speed of EDS for an initial survey of an unknown sample because major elements will be rapidly identified. However, if trace elements are present they will not be identified, and it may be difficult to interpret complex overlaps. Following the initial ED survey, WD can be used to check for overlaps and to increase sensitivity for trace elements.

The following two examples show how the improved resolution of WDS make peak identification easy.

The WD spectrum for MoS₂ has been acquired from the INCAWave spectrometer using INCAEnergy+ software. Using this application, the WD spectrum acquisition is initiated by highlighting part of the ED spectrum. The two spectra are then superimposed automatically to illustrate the difference in resolution. In the yellow ED spectrum (fig. 4), the molybdenum Lα line at 2.293 keV is severely overlapped by sulfur Kα at 2.307 keV, but the WD spectrum (in blue) clearly resolves these lines and SKβ and MoLβ.

Sub-micron precipitates in an alloy sample require analysis at relatively low beam voltages to eliminate possible X-ray contribution from surrounding phases. This means that L lines for tungsten (8.3977 keV) which are clearly visible in the ED spectrum, are not efficiently excited and so are unsuitable for characterizing composition. In such circumstances, the M-lines of W must be used. Since the M lines for W are not resolvable from Si K lines by EDS in this case (WMα at 1.774 keV and Si Kα at 1.740 keV), the use of WDS is preferred. The spectra in Fig. 5 illustrate this point. Again, INCAEnergy+ was used to obtain a WD spectrum overlaid on the ED spectrum (scaled to the WD spectrum). The presence of tungsten in the ED spectrum is masked by the Si K lines while the two are clearly distinguishable in the WD spectrum.