Due to the high fat and protein content, a different sample preparation protocol was used for the breast milk samples (accredited to Braun et al. 2018 ). 250 μL aliquots were fortified with 25 μL of the internal standard mixture in ACN/H2O (10/90, v/v), followed by addition of 25 μL ACN/H2O (10/90, v/v; 25 µL of a solvent standard in ACN/H2O (10/90, v/v) were used in spiking experiments for method validation).