Target factor analysis (TFA) of a series of angle-resolved reflection electron energy loss spectra (REELS) was recently demonstrated to be a useful method to determine bulk energy loss functions (ELFs), which by the TFA are separated from the surface-loss structures of REELS. The dielectric function is then readily derived by Kramers–Kronig analysis of the ELF. The advantage of the method compared with other methods, which are also based on the analysis of REELS, is that the condition of the outermost surface region is unimportant because the excitations that occur there are removed by the TFA and ideally a pure bulk component is determined. Our method is thus particularly useful for determining the ELF from compound materials that are hard to clean without modifying the outermost atomic layers. In this paper, the robustness of the method was studied by applying it to three GaAs samples with different surface compositions caused by different surface cleaning methods. The results showed that when electrons of energy 3000–4500 eV were used, the resulting bulk ELFs were essentially identical except for small differences for the sample that had the largest thickness of the modified surface layer. It is concluded that this is a useful method, provided that the thickness of the modified layer is kept to a minimum by using shallow angle sputtering and by using REELS electrons at a sufficiently high energy that a major part of the electron trajectories are at a depth larger than the thickness of the modified surface layer. Copyright © 2013 John Wiley & Sons, Ltd.