Stock solutions of 2 mg/mL of standard PAs were prepared by
dissolving the appropriate amount of each in 50 mL methanol. A
methanolic stock solution of the IS (paracetamol) was also prepared
at 2 mg/mL. All solutions were kept at 20 C. The working
standard solutions were subsequently prepared by dilution (in
the concentration range of 0.1–100 lg/mL) with HPLC-water, each
containing 10 lg/mL of paracetamol.