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:HPLC analysis of phosphoinositides was performed by 32P labeling of cells, extraction of lipids, deacylation of lipids and HPLC analysis of glycero-phosphoinositides as previously described (Serunian et al., 1991).
Category: