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:Additional target-based ESI–MS-MS data of isovalerylcarnitine was obtained from the preceding investigation (Dercksen et al. 2012) and the analysis was done according to the protocol described by Pitt et al.
Category: