The selection of an ion pairing reagent for developing and validating a stability-indicating HPLC method for cromolyn sodium and its known impurities

A robust, stability-indicating isocratic HPLC method for the quantitation of cromolyn sodium and its related substances was developed and validated. The two known synthetic impurities likely to be present in cromolyn drug substance, designated as impurity 1 and impurity 2, are non-ionic compounds while cromolyn is a weak acid. The described HPLC method exploits the additional selectivity offered by using an ion pairing reagent. Several quaternary amine ion pairing reagents were evaluated for their chromatographic effects on cromolyn and, its impurities, i.e., resolution and retention time. The retention time of cromolyn and the two known impurities varies greatly depending on the type of ion pairing reagent present in the mobile phase. Myristyltrimethylammonium bromide provided baseline resolution between the impurities and cromolyn within the shortest run time. The chromatographic conditions, such as the organic modifier, pH, temperature, and flow rate, were investigated and adjusted to provide a robust method. Forced degradation studies demonstrated that the method is selective for CROMOLYN, providing a spectrally pure peak that is resolved from all detectable degradation products. The method is, thereby, stability-indicating. Cromolyn sodium and its related compounds can be quantitated in a single chromatographic run of about 30 min. The limit of quantitation (LOQ) was determined to be 0.05% for cromolyn and the two known impurities. The method was shown to be precise, accurate, and linear in the assay range (i.e., 75-125%; 100%= 0.5 mg/mL) and in the related substances range (i.e., 0.05-1.0%). The validation data is summarized below: Assay Range: 75-125% (100%=0.5 mg/mL of Cromolyn Sodium, anhydrous basis) Validation Parameter result Repeatability (n = 6) RSD = 0.3% Intermediate Precision (n = 18) RSD = 0.3% Accuracy (n = 12) Recovery = 99.5% Linearity r = 1.0000; bias = 1.4% Related Substance Range: 0.05-1.0% Validation Parameter Result Accuracy - Cromolyn (n = 10) Recovery = 10 1.2%; RSD = 9.6% Accuracy - Impurity 2 (n = 9) Recovery = 100.2%; RSD = 2.0% Linearity - Cromolyn r = 0.998; bias = 1.7% Linearity - Impurity 2 r = 1.000; bias = 1.4%.