Although applied to proteomics, a related approach consists of "spectral archives," which include MS/MS spectra that have been repeatedly observed, irrespective of whether they could be annotated . Spectral archives can be built by large-scale clustering of MS/MS spectra across multiple datasets or in an entire repository . In this fashion, commonly observed spectra can be grouped and unannotated spectra can be linked across multiple experiments to find correlations with identified compounds .