Examination of the Fluorine K Line and Iron L Line Overlap

When an iron rich material is analyzed, an analyst may often observe the peak identification of elements such as fluorine (F) even though they are unlikely to be present. This article discusses the reason of that problem.

X-ray Lines for F-K and Fe-L

The reason of the problem is that the difference in energy of the X-ray lines for F-K (F-Kα 677eV) and Fe-L (Fe-Lα 704eV) is only 27eV. Although, this value is very close, it is an addressable challenge for today’s EDS detectors and peak detection algorithms.

In this region, the peak widths are approximately 60 or 70eV based on the type of the detector and the amplifier settings.

Visual confirmation of the presence or absence of fluorine in an iron sample may be a difficult task. However, differentiating these peaks is comparatively easy for the peak fitting algorithms, considering adequate accuracy in the data. This consideration contributes to the likely reason that leads to the misidentification of ‘F’ in a sample.

EDS Spectra

The construction of EDS spectra involves counting and addition of individual photons one by one to the spectrum. An attribute of this type of data is that the accuracy of the peak centroid and peak width is related to the number of counts acquired. This is also applicable to the overall shape of the peak. If the number of counts in the peak is very small, then the software can easily identify the presence of ‘F’ in the absence of other elements.

The acquisition of a lot more data has simplified the identification problem, and as a result the software will typically correct itself and eliminate the F label- if, in fact, in the absence of ‘F’.

Figure 1 presents a collection of EDS spectra from samples of fluorite, zinnwaldite, triplite, bixbyite, iron, and manganese, illustrating this peak overlap. The following table presents the combinations of Mn, F and Fe in these samples:

This type of samples offers all combinations of ‘Fe’ and ‘F’ present or not with the added difficulty of the possible presence of the Mn-L line, which is 41eV lower in energy at 636eV when compared to the F-K line. This is far enough away that it does not result in any problem with misidentifying ‘F’ when ‘Mn’ is present.

From the images presented in Figure 1, one can conclude that it is virtually not possible to visually identify the presence or absence of an element in this region of the spectrum.

Figure 1. Collection of EDS spectra

WDS Spectra

The use of a Thermo Scientific™ MagnaRay™ WDS spectrometer (Figure 2) can readily confirm whether F is present or not in these samples. The resolution of the MagnaRay spectrometer is much higher than that of an EDS detector.

Figure 2. The Thermo Scientific™ MagnaRay™ WDS spectrometer

As can be seen in Figure 3, the peak widths of the MagnaRay in this region of the spectrum are five folds narrower than the EDS peaks, thus allowing to observe all of the lines of interest without any overlap. Here, a magnetite sample was used instead of the pure iron employed in the collection of the EDS spectrum of iron.

Figure 3. Collection of WDS spectra

The following table summarizes the nominal compositions of the mineral samples:

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.