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:However, when testing the effect of S-nitroso- L-cysteine a different approach was used since this compound has a half-life of less than 30 s (Myers, Minor, Guerra, Bates & Harrison, 1990; Bates, Aldape & Baker, 1991).
Category: