Ultra Trace Element Analysis

The ability to measure a zero concentration in high purity materials (e.g., trace Ti in synthetic quartz) is the most robust method to determine your trace element accuracy.

Using this blank measurement we can quantitatively apply a correction to our unknown samples which ensures that our trace accuracy is equal to our measurement precision.

The blank correction can even be applied using a non-zero blank, because the method is completely quantitative as it applies the matrix correction based on the compositional differences between the blank material and the unknown sample.

• Modern EPMA instruments equipped with low noise detectors, counting electronics and large area analyzing crystals can now routinely achieve sensitivities for many elements in the 3 to 100 PPM levels. However, because of various sample and instrumental artifacts in the x-ray continuum, absolute accuracy is more often the limiting factor for trace element quantification as opposed to sensitivity.
• This “blank” correction developed for Probe for EPMA can be automatically applied to x-ray intensities during the matrix iteration process to correct for these systematic accuracy errors that are measurable at levels up to 50 PPM or higher depending on particular spectrometer and crystal configurations.
• Trace concentration accuracy even at 500 PPM levels are improved significantly as the figure above demonstrates .

High accuracy trace element determinations generally require high precision spectrometer scans to optimize placement of the high and low side background positions in order to avoid possible interferences on the off-peak intensity measurement. In high sensitivity measurements, the presence of minor and even trace levels of unexpected elements may cause significant interferences with the nominal high and low off-peak positions of the analytical peak in question.

Therefore, in materials where the composition is variable or several different phases are present, it is usually necessary to perform many time-consuming spectrometer scans at sufficiently high precision levels to avoid these interferences and other continuum artifacts.

To handle these situations automatically and accurately, Probe Software in conjunction with researchers at the University of Massachusetts have developed new background acquisition and optimization methods, collectively known as the “multi-point background” (MPB) feature. This MPB acquisition will automatically acquire a number of off-peak intensities distributed on each side of the analytical peak (which can be specified precisely by the user) so that at least a few of the background measurements will be unaffected by the unexpected presence of other elements or continuum artifacts that could lead to systematic errors. The background intensity is calculated automatically by iteratively looping on the measured multi-point intensities and optimizing on the best fit of the relative lowest variances until the specified number of usable background positions is achieved.

On the lower left is a screen shot showing this multi-point background calculation for one data point. As can be seen, the off-peak positions closer to the Pb Ma analytical line were interfered by the tails of the Th Mz1 and Mz2 lines (there is no Pb in this ThSiO4 sample).

The optimized background positions selected are automatically shown circled in red for ease of interpretation by the user.

The program correctly iterates the multi-point off-peak backgrounds to find the best fit to remove the problematic background measurements automatically.

When one has acquired two or more elements on a single spectrometer-crystal combination (e.g., Spc1 and LiF) using normal off-peak backgrounds, one now has the option in post-processing, to apply the multi-point background method to all elements acquired on that spectrometer. This is accomplished by “sharing” the background intensities between the elements on that spectrometer.

Using this “shared” method, the backgrounds from multiple elements on a specific spectrometer-crystal can be exploited as multi-point backgrounds for improved trace element analytical accuracy.

The mean atomic number (MAN) background correction method can routinely perform background corrections on points and X-ray map pixels with an accuracy of approximately 100 to 200 PPM in silicates and oxides, which is sufficient accuracy for most major and minor element concentrations.

Without actually measuring any off-peak backgrounds!

In addition, by combining this time saving background correction technique with the “blank” correction method described above, we can further improve the MAN background correction accuracy to the level of the measurement precision.

And in half the acquisition time of using off-peak background corrections!

Correction of secondary fluorescence boundary effects from nearby phases containing the element of interest from WDS and EDS spectrometers is now possible.

This boundary fluorescence effect is generally of concern for trace element analysis, although at distances less than tens of microns from phase boundaries, these artifacts can reach percent level concentrations for some element couples, such as Cu-Co, Ni-Fe, Cr-W, etc.

Correction of these boundary fluorescence effects is available through recent efforts by Francesco Salvat and Xavier Llovet at the University of Barcelona, Spain and are implemented in an easy to GUI in our Probe Software applications.

Starting with the development of sophisticated Monte Carlo models for electron-photon fluorescence effects, followed by new work implementing these insights into analytical expressions, means that these time consuming and tedious operations can now be performed in a fraction of the time that was once required for high precision modeling calculations.

Probe Software has implemented these newly developed boundary fluorescence calculations in our Probe for EPMA acquisition and analysis software for the automatic correction of boundary effects in real-time based on the actual spatial distances and analyzed compositions.