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:Periplasmic, cytoplasmic and membrane fractions from E. coli cells were prepared as described (Neu & Heppel, 1965), using ice-cold 1 mM MgCl2 instead of water to reduce the amount of shock-released cytoplasmic proteins (Hantash & Earhart, 2000).
Category: