The chief advantages of WDS are better peak resolution and decreased noise from electronics or stray radiation. WDS provides better peak separation and increased peak-to-background ratios. WDS is most useful when:

• Compared with EDS, resolution is typically an order of magnitude better; peak overlaps are virtually eliminated.
• The high resolution leads to order-of-magnitude better sensitivity than EDS. The sensitivity improvement is often most dramatic in the light element region.
• The better peak-to-background ratios and elimination of overlaps yield more accurate quantitative analysis for those elements in low concentration or involved in the overlap.
• Because the WD spectrometer can be positioned at the center of the peak, where peak-to-background is maximized, WDS provides superior x-ray maps, especially for low concentrations.
• The serial nature of spectrum acquisition means that qualitative analysis is slow compared to EDS.
• Because of the relatively inefficient diffraction process and the long L distances involved, considerably higher specimen currents (1 to 100 nA, typically) are needed for WDS compared to EDS.
• WDS is often operated by driving the crystal directly to the peak, with or without peak searches. In this manner, the counting time is devoted only to the points of real interest. For minor elements, WDS then becomes fast relative to EDS, which spends most of its time counting x-rays from the major elements.

Parallel Beam Spectrometry

Thermo Scientific's exclusive MAXray parallel beam spectrometer (PBS) combines the resolving properties of WDS with the sensitivity of a larger area detector. While WDS has excellent resolution, it shows poor intensity at low beam currents. Other WDS limitations are a small solid angle, long working distances, and SEM chamber limitations.

Designed for Field Emission Scanning Electron Microscopes, or other low-voltage applications, the PBS uses a grazing incident x-ray reflector to create a collimated parallel beam of x-rays, which are sent through diffractor to proportional x-ray counter.

Because the parallel beam does not need to be focused by the diffractor, the PBS uses a flat diffractor (reducing intensity loss due to aberrations). A large area window is fitted to the proportional counter to match the diameter of the back of the collimating optic. The large solid angle subtended by the optic, the larger active surface area of the diffractor, and the proximity of the detector yield a significantly higher detection sensitivity over other WDS systems (two to three times more sensitive to light elements then traditional WDS).

Extended Range Parallel Beam Spectrometry

Using collimating x-ray optics alone, the PBS is analyzes elements with K, L, or M lines in the range of 100 eV to 1.8 keV. Thermo also offers an extended range parallel beam spectrometer, which combines a polycapillary x-ray optic with the grazing incident x-ray reflector.

This hybrid optic extends the range of microanalysis applications to 10 keV and generates a higher analytical spatial resolution.

The hybrid optic PBS provides solid angles of 0.184 Steradian (SR) below 2 keV and 0.012 SR above 2 keV. This compares to typical values of 0.0006 to 0.002 SR for traditional WDS. The improved solid angle provided by the hybrid optic, enables PBS analyses with cold or thermal field emission SEMs, as well as with traditional tungsten and LaB6 emitter microscopes.