Fig. 5

Heatmaps reflecting linearity (r²) 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) linearity (r²) for each analytes. r² (or coefficient of determination) value should be as close to 1 as possible. Colour coding: r² increases from light to dark green; White box: unidentified standard. MeOH methanol, MeOH:ACN methanol combined with acetonitrile, ACN acetonitrile, SPE solid preparation extraction, ESI electrospray ionisation, RP reverse phase