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:Isotopic enrichment of glucose and glycerol was determined using gas chromatography-mass spectrometry (GC model 6890 and MS model 5973, Hewlett-Packard, Fullerton, CA) of the pentaacetate and tris-trimethylsilyl derivatives, respectively, as previously described (4).
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