kind of elution solvent from SPE cartridge; (5) amount of elution solvent modifier; (6) acidification of sample. Fixed factors were SPE cartridge and milk volume. The variable “acidification” had three possible variations: (1) no acidification at all; (2) acidification at a fixed pH before extraction, and (3) acidification after extraction and dilution steps. As the last variable has three levels, the experimental design was repeated twice: