X-ray emission spectrometry is of great practical interest because the method is rapid, non destructive and quantitative. However, the analysis of light elements which emit highly absorbed long wavelength radiations is always difficult. Although recent development of multilayered microstructures as dispersive devices has brought higher count rates for light elements, the improvement of performance in the relevant energy range depends, largely, on the improvement of the excitation. For this purpose, an electronic source providing a non-focused low energy electron beam constitutes a means of obtaining a very efficient excitation. As a result, we have used such an excitation to compare the performances of some layered synthetic microstructures (LSM) and conventional crystals for the detection of soft and ultra-soft xrays, to show up a few spectral interferences and to solve them by choosing the best dispersive device. Among the cases studied, we have shown how to carry out an accurate analysis of boron in borophosphosilicate glass (BPSG) samples. Finally a survey of the lower detection limits obtained for different elements using different dispersive devices was done.