Rizk, M.S. et al. 2017. Simultaneous determination of levonorgestrel and ethinyl estradiol in combined dosage form utilizing spectrophotometric methods and high performance thin layer chromatographic method on nanosilica gel plates. European Journal of Chemistry. 8, 1 (Mar. 2017), 52–59. DOI:https://doi.org/10.5155/eurjchem.8.1.52-59.1523.