4.3.4. Quality Control Samples and Blanks Preparation

Two different types of quality control (QC) samples were prepared: pooled QCs made by mixing equal volumes (5 μL) from each sample previously normalized for the specific gravity and dilution QCs prepared by 2, 4, and 8-fold diluting the pooled QCs with LC– MS-grade water. All QCs were further diluted by 3-fold with ISWS-A for RP analysis or ISWS-B for HILIC analysis. Pooled QC samples were injected first (*n* = 20) to condition the LC–MS system and obtain stable retention times and MS response. Subsequently, pooled QCs were injected every six true samples (*n* = 8 in total) to perform intra-batch signal drift corrections. Dilution QCs were analyzed four times and were regularly incorporated along the sample list to verify the linear response of the MS signal. Blanks consisted of LC–MS-grade water for RP analysis and ACN: H2O 80:20 (*v*/*v*) for HILIC analysis. Blank injection (*n* = 3) was performed at the beginning of the batch to collect a background signal excluded from the dataset.
