Wavelength-dispersive spectrometry (WDS) is an advanced method of microanalysis, which offers high-quality peak separation, element detection and quantification results. Previous spectrometers were horizontal designs that needed high beam currents and high kV to attain these benefits. Modern parallel beam spectroscopy (PBS) designs include focusing optics that improve X-ray signal efficiency and are effective at low beam currents and low kV.
With these designs more focus on sample positioning is required and in the past, the quality of the results was based on the operator skill. Now a smart routine has been formulated that fully optimizes and evaluates the sample position doing away with the user setup demands that challenged data collection quality previously.
Corrie Van Hoek and Max Koolwijk listed the WDS spectrometer types in 2007, which highlighted the significance of accurate sample height in obtaining the highest intensity X-ray signal when using PBS. A manual method has been described by them for sample height adjustment and then turn to more recent stage Z axis control routines.
According to them, “due to the automatic alignment procedures reproducible quantification is now routinely better than 1% (relative error).” It is therefore possible to perform a quality analysis, but there are still tedious routines involved.
Early routine limitations include the following:
- Choosing the best element for setting the sample height – In this step, the operator must understand the key elements in their sample or must use EDS for identifying the best element to be used for the adjustment
- For obtaining the highest potential signal, highest intensity elements are best
- Adjusting the spectrometer to the selected element – The spectrometer must be configured or the right WDS crystal should be configured for the selected element. On setting completion, a continuous and constant collection must be done only for that element
- Sample Z height movement – The sample height or the Z-axis of a stage are physically moved via settings in a motorized stage control panel, while only the selected element is collected by the spectrometer
- With a variation in the Z height, the increase or decrease in signal is monitored until a maximum intensity is reached. The stage position achieving maximum signal intensity is registered
- Return to Z height of maximum intensity – The Z-axis is finally moved to the setting that reached the maximum number of X-ray counts
- Requirement for repeated set up – This routine must be performed any time the sample height is different, such as for uneven surfaces or in cases where a different area of the sample is analyzed
The EDAX TEAM™ EDS/WDS Solution
The previous routine is completely automated with the new EDAX TEAM™ Smart Focus and enables the right decisions for the user.
The key features of the new routine are:
- Smart use of EDS spectrum - For the WDS setup, the EDS spectrum determines the highest intensity element for the optimization. In case two elements have almost the same intensity, the higher energy peak is chosen as this will offer accurate results
- Smart Autofocus settings – Default settings are used by the autofocus routine to move the sample through several physical height positions. Both coarse and fine controls are available. While the stage is being moved, an algorithm keeps track of the change of the intensity. Based on the projected area of maximum counts, the software will change the stage movement to more effectively travel to the stage height where maximum signal intensity is collected
- User selectable checkbox – There is a checkbox in the WDS spectrum collection area of the software, which the user can select. If this checkbox is selected, the routine will automatically run at the beginning of any collection to repeat the optimization
Figure 1. TEAM™WDS User interface.
Figure 2. Check Height box which starts the routine and can be left activated for repeated activation as the sample areas change.
All spectrometer settings are then automatically set for this element, including WDS crystal and element energy line.
Testing and Analysis
Using a carbide sample, a WDS analysis was done. The initial step in the analysis was collecting an EDS spectrum and then selecting the Check Height checkbox in the WDS collection area. Carbon was automatically chosen and the crystal was set to collect for this element.
In order to shift the stage through several heights, the coarse setting was used and the area of maximum count intensity was found to be 15.575mm and the stage then moved back to this position. The sample height is presently optimized.
Figure 3. Stage Z locations and corresponding signal intensity as the stage is moved through the coarse or fine travel range.
Figure 4. WDS overlay with (red) and without (blue) Smart Focus.
PBS WDS offers advantages in WDS analysis for element detection and quantification even at low beam currents and low kV. Prior sample height routines were successful in obtaining quality data but required user knowledge to set the collection sweet spot.
Now with TEAM™ EDS/WDS Smart Focus, optimization is possible with the help of fully automated logical hardware and software controls. Maximum WDS signal with Smart Focus obtains maximum quality data.
This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.
For more information on this source, please visit EDAX Inc.