However, corrections when effects are subtle are also important if spectroscopy is to be a quantitative tool. Fig. shows an example of subtle self-absorption effects for a concentrated sample, Pb foil; while the lack of intense features in the spectrum means that the correction is subtle, the distortion is far from negligible. and As(V) compounds. In a mixed oxidation state sample, measuring at the emission energy peak of one component means that the other is collected off-peak, and its HERFD-XAS spectrum will be both distorted and shifted in energy , complicating the analysis. With heavier elements such as mercury or lead, the additional shells of electrons provide sufficient shielding that chemical shifts are not observable.