Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:If the purified proteins are used for mass spectrometric analyses, elution from streptavidin beads is not required because trypsin digestion can be performed efficiently on bead-bound proteins and the released peptides can be analysed by mass spectrometry (Guerrero et al., 2005; Kho et al., 2004; Rybak et al., 2005; Tagwerker et al., 2006).
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