An important factor that can affect the intensity ratio of chemical components, e.g. Al₂O₃, is chromatic aberration of the objective lens [1]:
          i) zero-loss electrons are normally focused at the TEM specimen, and thus different energy loss will produce discs of confusion at different sizes.
          ii) these discs, when projected onto the entrance aperture of spectrometer, will be cut off by a different amount, and thus the relative intensity will be affected. [2]

To eliminate chromatic aberration at an energy loss [1]:
          i) the specimen is first focused to give minimum contrast.
          ii) the accelerating voltage is increased by an amount equal to the energy loss so that the disc of confusion at that energy loss is completely eliminated.

Measuring the intensity this way described above minimizes the effects of chromatic aberration on the measured ratio.

[1] K. Wong and R. F. Egerton, Correction for the effects of elastic scattering in core-loss quantification, Journal of Microscopy, 178(3), (1995) pp. 198-207.
[2] Yang. Y.-Y. & Egerton, R.F. (1992) The influence of lens aberration ' on electron energy-loss spectroscopy quantitative measurements. Microsc. Res. Techn. 21, 361-367.