A solution of the FLP (450 µg/mL) with the N-1, N+1, PO, and DA-NC each at 10 µg/mL was made by mixing the 1-mg/mL standard solutions in the appropriate ratios. Additionally, a mixture of the NC (450 µg/mL) and DA-NC (10 µg/mL) was made. For IP-RPLC measurements, the FLP and each impurity in the mixtures was 900 µg/mL and 20 µg/mL, respectively.