Principal component 1 (PC1) represents the maximal variation of data. Principal component 2 (PC2) is orthogonal to the PC1 axis and accounts for the second highest variation. a) Principle component analysis for ESI positive mode b) Principle component analysis for ESI negative mode. Each point represents an individual LC-MS injection. c) Heat map of 35 features with the highest fold change of F3/F5 observed in ESI positive mode and d) in ESI negative mode.