Question 1

What is WDS and how does it work?

Accepted Answer

Wavelength dispersive spectroscopy (WDS) separates x-rays according to their wavelength using Bragg diffraction on specialized analyzing crystals. These crystals are mounted on a rotating turret, allowing precise selection of the x-ray energy by adjusting the turret’s angle. Once the x-rays are diffracted, they are directed to either a gas-flow or sealed proportional counter, which converts the x-rays into an electrical voltage and then into a measurable count. This process enables highly accurate identification and quantification of elements in a sample.

Question 2

What elements can WDS detect?

Accepted Answer

Wavelength dispersive spectroscopy (WDS) can detect the same broad range of elements as energy dispersive spectroscopy (EDS). This means WDS is capable of identifying and quantifying nearly all elements in the periodic table, from light to heavy elements, depending on the system configuration and crystal selection.

Question 3

What's the difference between WDS and EDS?

Accepted Answer

WDS offers significantly better energy resolution than EDS, typically 5 – 20 eV for WDS compared to about 123 eV for EDS. This higher resolution allows WDS to distinguish closely spaced x-ray peaks, resulting in improved peak-to-background ratios and superior detection of light elements. However, WDS analyzes elements sequentially, making it slower than EDS, and it requires calibration standards for accurate quantitative analysis.

Question 4

How accurate is WDS for quantification?

Accepted Answer

WDS is recognized for its high accuracy and precision in elemental quantification. When using high-quality standards and maintaining stable analytical conditions, WDS typically achieves relative errors in the range of 0.1 – 1%. This level of accuracy makes it a reliable technique for quantitative analysis in demanding applications.

Question 5

Why do I need multiple crystals in WDS?

Accepted Answer

WDS uses different types of analyzing crystals, such as TAP, PET, and Lif, because each crystal is optimized for a specific range of x-ray wavelengths. This allows for efficient diffraction of both light and heavy elements. Some crystals provide higher resolution for certain elements, while others offer greater sensitivity. Selecting the appropriate crystals is essential to achieve the best analytical performance for your specific application.

Question 6

What’s the spatial resolution of WDS?

Accepted Answer

The spatial resolution in WDS depends on several factors, including the density of the material, the accelerating voltage (kV) of the electron beam, and the energy of the x-ray being analyzed relative to the sample matrix. Complex sample geometry and material composition can also influence resolution. For standard conditions and typical bulk samples, spatial resolution is generally in the range of approximately 0.5 – 3 µm. Modeling techniques, such as Monte Carlo simulations, can be useful for predicting and optimizing spatial resolution in more complex scenarios.

Question 7

Can WDS do mapping?

Accepted Answer

WDS is capable of elemental mapping; however, the process is typically much slower than with EDS. WDS mapping requires longer dwell times, often ranging from seconds to minutes per point, to achieve accurate results. For rapid mapping applications, EDS is generally preferred due to its faster data acquisition.

Question 8

Why does WDS need a higher beam current than EDS?

Accepted Answer

WDS systems experience efficiency losses at multiple stages,including the optics, analyzing crystals, and detection counters. To compensate for these losses and ensure sufficient x-ray yield for reasonable collection times, WDS requires a higher beam current compared to EDS. This increased current boosts the number of x-rays generated, enabling accurate measurement despite the lower overall efficiency of the system.

Question 9

Can WDS quantify trace elements?

Accepted Answer

Yes, quantifying trace elements is one of the primary applications of WDS. For most elements, WDS achieves detection limits in the hundreds of parts per million (ppm), compared to approximately 0.1 – 1% for EDS. This enhanced sensitivity is due to the higher peak-to-background ratio inherent to the WDS technique, making it especially effective for detecting and measuring low concentrations of elements in a sample.

Question 10

Is sample polish critical for WDS?

Accepted Answer

Yes, proper sample polishing is essential for high-quality quantification in WDS. Achieving a submicron polish ensures a smooth, even surface, which minimizes measurement errors and enhances analytical accuracy. Well-polished samples help produce reliable results, especially when precise elemental quantification is required.

Question 11

Can WDS and EDS be used together?

Accepted Answer

Yes, modern analytical systems can operate both EDS and WDS simultaneously. This approach allows you to quickly quantify most elements using EDS, while reserving WDS for elements that require higher resolution or sensitivity. By combining both techniques, you can achieve comprehensive and efficient elemental analysis, leveraging the strengths of each method for optimal results.

Wavelength dispersive x-ray spectroscopy FAQs