XMM-Newton Users Handbook Detectability of weak absorption lines

Long integrations should in principle allow the detection of weak absorption lines, but do expose systematic instrumental features that can be confused with real astrophysical lines. The ultimate sensitivity for detection of weak absorption lines is estimated at $\approx $1.5% of the continuum. This limit is reached at signal-to-noise ratios of around 70, when the statistical and the systematic uncertainties are of the same order.

European Space Agency - XMM-Newton Science Operations Centre