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:HPLC-UV methods which use PP and LLE as pretreatments were developed to determine baicalin and baicalein in rat and human urine samples (Lai et al., 2003a, b; Di et al., 2006; Zhang et al., 2006; Gao et al., 2007; Zeng et al., 2010).
Category: