7.1 Selectivity
Selectivity is the property of a measuring system, used with a specified measurement procedure, whereby it provides measured quantity values for one or more measurands such that the values of each measurand are independent of other measurands or other quantities in the phenomenon, body, or substance being investigated.
In EDXRF, the selectivity is attained by the identification of the elements based on the measurement of their characteristic x-ray emission lines. A proper energy calibration of the spectrometer under each of the measurement conditions provides the means to establish a dependence of energy with the spectrum channel numbers. The energy calibration is made by measuring multi-element reference samples with no overlapping emission lines and establishing the dependence of energy with the channel number i:
The selectivity of the instrument is only limited by the energy resolution, which can be determined by a resolution calibration based on the measurement described above. If the shape for a peak with characteristic energy Ech and its centroid located in the spectrum in the channel i with energy Ei corresponding from the calibration to is assumed to be a Gaussian described as:
then the resolution σi is calculated as
where Noise takes into account the electronic noise of the spectrometer, ε the charge required to form one electron-hole pair in the detector crystal (for Si detectors ε = 3.6 eV), and Fano is a factor that accounts for the spread in the charge generated in the detector (e.g. detector intrinsic resolution). Typical values for these two variables are Noise ≈ 100 eV and Fano ≈ 0.1.
Therefore, selectivity is conditioned to the spectrometer energy resolution capabilities. Typical cases of non-resolvable peaks in EDXRF are the determination of Co in soil samples (with a large presence of Fe), the overlap of As-Kα and Pb-Lα, among other spectral interference cases. Extreme care has to be taken as to define the capability of determining each element in case of spectral interferences.