Standard reference materials (SRM 1950 and 1958 from NIST) were purchased from NIST and stored at -80°C. In total, 9 replicates of each SRMs were prepared following the protocol previously described by Jamnik et al (2022) . For extraction, 30 µL of both SRM were mixed with 120 µL of extraction solution (ACN/MeOH 50:50, v/v) in Eppendorf tubes and sonicated for 10min on ice. The extraction solution contained labeled IS (Table ) for accounting for the total variation (sample preparation + analytical). A 2 h protein precipitation step was performed at -20°C afterwards, followed by centrifugation for 10 min at 18,000 x g at 4°C. The supernatant (120 µL) was transferred to a new Eppendorf tube and dried under vacuum at 4°C overnight. From the nine replicates, three were reconstituted using 120 µL of ACN/H2O 10:90, v/v). The additional 6 replicates were used to build a matrixmatched calibration curve with the same solvent composition and spiked at the same levels as the standard solutions in solvent. Samples were vortexed for 10min after reconstitution and centrifuged at 18,000g and 4°C. Finally, 100 µL of the supernatant was transferred to an amber glass vial for data acquisition. Sample preparation protocol was chosen to maximize chemical coverage .