EDS Measurement of Nitrogen
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Light elements such as nitrogen (N K) and oxygen (O K) are detectable with different modern EDS detectors, e.g. ultra-thin window X-ray detectors (see page4589). However, an absorption correction will be needed even for the thinnest TEM specimens. Note that absorption does not only happen in the specimen itself but also in surface layers, e.g. contamination, and intentionally coated carbon and metal conductive layers, and in the detector window. The X-ray transmission of Be window is close to 100% for energies down to 2 keV, while it drops rapidly to about zero at 0.5 keV. For instance, nitrogen X-ray line is almost fully absorbed by such a window. Significant decrease of detection efficiency for N Kα can be induced by carbon contaminants due to the high mass absorption coefficient (25500 cm2g-1) of this X-ray line.

Figure 1738a shows the deconvolution of an EDX spectrum with peak overlaps, taken from a material that contains O, Ti, N, and C elements.

Deconvolution of an EDX spectrum with peak overlaps

Figure 1738a. Deconvolution of an EDX spectrum with peak overlaps. [1]

For most common detector designs, nitrogen produces a very weak response, resulting in its unreliable detection for most materials.

As discussed on page4650, X-ray absorption is a function of the energy of X-rays. Low energy peaks will be more strongly absorbed than high energies ones. For thick TEM samples, k-factor correction due to X-ray absorption is needed in order to accurately quantify EDS measurements. Table 1738 lists N-examples of thicknesses at which the thin-film approximation is no longer valid due to X-ray absorption effects in specific materials.

Table 1738. Examples of limits to the thin-film approximation caused by X-ray absorption: Maximum thicknesses of thin specimens for which the absorption correction (or error) is less than ±10% and ±3%.

Material
10% error in kAB
3% error in kAB
Absorbed X-ray lines
Primary X-ray lines
Thickness (nm)
Si3N4
 413 6 Si Kα and N Kα Si Kα (1.739 keV) and N Kα (0.392 keV)

Figure 1738b shows the percentage of x-ray transmitted through an H2O-ice contamination layer depending on the thickness of the H2O-ice layer up to 1 μm. As expected, the absorption effect of ice layer is greatest for the low-energy boron x-rays and the least for the silicon signal. Such H2O-ice layer is normally formed in cryo-TEM measurements.

Calculated percentage absorption of characteristic x- rays in an H2O-ice contamination layer

Figure 1738b. Calculated percentage absorption of characteristic x- rays in an H2O-ice contamination layer. [2]. X-ray energies: B-K = 183 eV, C-K = 277 eV, O-K = 525 eV, N-K = 392 eV and Si-K = 1.739 keV.

 

 

 

 

 

 

 

 

 


         

 

 

 

 

 

 

 


[1] J. Berlin, T. Salge, M. Falke, and D. Goran, Recent Advances in EDS and EBSD Technology: Revolutionizing the Chemical Analysis of Chondritic Meteorites at the Micro- and Nanometer Scale, 42nd Lunar and Planetary Science Conference, 2723, (2011).
[2] M. Malac and R.F. Egerton, Calibration Specimens for Determining Energy-Dispersive X-ray k-Factors of Boron, Nitrogen, Oxygen, and Fluorine, Microsc. Microanal. 5, 29–38, (1999).


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