Accurate EPMA Quantification of the First Series Transition Metals using Ll Lines

Abstract

In conventional EPMA, X-ray K lines are used for accurate quantification of the transition metal elements. However, at low beam accelerating voltages (i.e., < 5 keV), only low-energy X-ray lines are emitted, including K lines for Z ≤ 22 and L and M lines for Z > 22. Low beam energy operation offers several advantages: improvement of analytical spatial resolution and reduction of both secondary fluorescence and sample charging effects. The use of low-energy X-ray lines for quantitative analysis does present new analytical challenges, though, because these lines are subject to larger peak shifts, more line overlaps and lower fluorescence yields, as compared to higher-energy K lines. The low yields also reduce the intensity of certain lines, as does the low overvoltage, U (defined as the ratio of beam energy to ionization energy for a given line), which lowers the ionization probability for the X-ray line. If we consider the example of Fe (Z=26) analyzed at 15 keV, many characteristic X-ray lines (K-and L-series) are produced from the atom. The X-ray lines traditionally used for quantification are the Kα line (transition from L 3 sub-shell to K sub-shell) and Lα line (transition from M 5 sub-shell to L 3 sub-shell). Generation of the Fe Lα line involves valence electrons, which are affected by the chemical bonding of Fe in the target sample. Wavelength shifts, peak shape modifications, and increases or decreases in the relative intensities of the characteristic lines can be readily observed between different chemical types. In general, for elements of the first transition metal series (Sc to Zn), the pattern of L emission spectra varies according to the energy of the incident electrons (E 0), as follows: • E 0 between the L 3 and L 2 threshold energies: intensity of the high energy satellite lines is reduced. • E 0 from the L 3 sub-shell threshold energy up to 3 times this energy: excitation and