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:Given that the determination of phenolic OH groups of lignins with 1H NMR has also been recommended (Faix et al., 1994), we calculated functional groups (phenolic hydroxyl (ArOH), aliphatic hydroxyl (AlkOH), and methoxyl (MeO) groups) with 1H NMR (Pan et al., 2006b).
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