K. Tekade, Rakesh, et al. “Simultaneous Spectrophotometric Estimation of Methotrexate and All-Trans Retinoic Acid in Mixture”. Asian Journal of Chemistry, vol. 21, no. 7, Mar. 2010, pp. 5145-50, https://asianpubs.org/index.php/ajchem/article/view/18284.