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:Aliquots (5 mL) of the cell suspension were collected from the mixture at various times after mixing, fixed in 4% paraformaldehyde in phosphate-buffered saline (PBS) (Iwatani et al., 2005) for 10 min, and then subjected to IIFM without air-drying.
Category: