A narrow Gaussian was simulated at redshifts a) z = 0, b) z = 0.5, c) z = 1 and d) z = 2, as observed with the EPIC pn, and then converted to a rest-frame spectrum after correcting for detector response and redshift for individual sources in the stacking sample. The rest-frame line energy is accurately recovered. The line profiles of higher redshift objects broaden because the energy resolution, (ΔE/E), of the detector becomes worse at lower energies. For this reason the line peak is lowered while the integrated line flux is recovered.