Different from EDS collection which has smaller collection efficiency, the EEL spectrometer is introduced in the path of the electron beam that has gone through the TEM sample. Therefore, an extremely high percentage of electrons carrying the elemental information
from the area of interest can be directed into the spectrometer, and thus EELS provides better signal collection efficiency to detect the elements from a very limited volume of material at the area of interest.
Generally EELS (Electron Energy Loss Spectroscopy) has better sensitivity than EDS (Energy-dispersive X-ray spectroscopy), due to the potential of utilizing signals
generated from larger ionization cross sections (L rather than K) and the much greater collection
efficiency. [1 - 2] The EELS collection efficiency is in the range of 20 to 50 % in normal measurement conditions. However, some TEM instruments and conditions can obtain about 90% collection efficiency.
Note that the collection efficiency of EELS is a function of the collection angle, e.g. shown in Figure 4790 for a primary electron beam of 200 keV. For instance, with the optimized collection semi-angle of 4.7 mrad for Cu L3 (931 eV), only ~30 % of the total available signal is collected by the EELS detector. However, as discussed on page2400, in practice, it is not desirable to collect all of the scattered electrons.
|Figure 4790. The collection efficiency of EELS as a function of the collection angle for a primary electron beam of 200 keV.
 M. Isaacson, D. Johnson, Ultramicroscopy 1 (1975) 33.
 H. Shuman, P. Kruit, Rev. Sci. Instrument 56 (1985) 231.