As visible in Fig. and Figure , the reduction in the average number of gold atoms in the protein is correlated to an increase of the intensity at the mass of bare BSA (m/z 66 500). The same trend is observed in both the positive and the negative ion mode. We tentatively interpret this correlation as the result of fragmentation of part of the protein-cluster complexes, leading to the loss of several Au atoms, and eventually to the observation of bare BSA ions.