Fig. 4

Heatmaps reflecting repeatability of standard unlabeled analytes from plasma (left) and serum (right). Heatmap, arranged by five extraction methods (MeOH, MeOH:ACN, ACN, Hybrid-SPE/ACN, and Hybrid-SPE/MeOH) within four LC–MS analysis (+ ESI HILIC, − ESI HILIC, + ESI RP C18, and − ESI RP C18), comparing the mean value (n = 3) repeatability (RSD) for each analytes. RSD value of analyte over 0.3 is considered as no-interpretable. Colour coding: RSD increases from dark green to dark red; White box: unidentified standard. MeOH methanol, MeOH:ACN methanol combined with acetonitrile, ACN acetonitrile, SPE solid preparation extraction, ESI electrospray ionisation, RP reverse phase, RSD relative standard deviation