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:The method takes advantage of the reaction of GSH with o-phthalaldehyde at pH 8.0 and of GSSG with o-phthalaldehyde at pH 12.0; GSH can be complexed to N-ethylmaleimide to prevent interference of GSH with the measurement of GSSG (17).
Category: