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:Samples previously analyzed by de Stephanis et al. (2008b) were analyzed at the isotopic mass relationship spectrometry laboratory of the Autonomous University of Madrid (LIM-UAM, Spain) using a Micromass Cf-Isochrom of magnetic sector that analyzes the isotopic relationship through the continuous…
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