Srinivasu, K., et al. “Simultaneous RP-HPLC Method for the Estimation of Formoterol Fumarate and Tiotropium Bromide in Pharmaceutical Dosage Forms”. Asian Journal of Chemistry, vol. 22, no. 5, Mar. 2010, pp. 3943-8, https://asianpubs.org/index.php/ajchem/article/view/11633.