Spectrophotometric resolution of ternary mixtures of Dexamethasone, Polymyxin B and Trimethoprim in synthetic and pharmaceutical formulations

Four spectrophotometric methods are described and applied to resolve ternary mixtures of the corticosteroid Dexamethasone (DEX), the antibiotic Polymyxin B (PLX) and its encouraging Trimethoprim (TMP). The simultaneous determination of these three compounds was firstly accomplished by a derivative method using the "ratio spectrum-zero crossing derivative" (satisfactory results in synthetic mixtures in different ratios and pharmaceutical formulations for DEX and TMP) and then by multivariate methods (satisfactory results in synthetic mixtures in different ratios and pharmaceutical formulations for DEX, PLX and TMP). Absorption spectra of compounds were used to optimise the spectral data set performs the calibration by partial least squares (PLS) 1 and 2 and principal component regression (PCR) algorithms. These calibration models were evaluated by internal validation (prediction of compounds in its own designed training set of calibration), by cross-validation (obtaining statistical parameters that show the efficiency for a calibration fit model) and by external validation over synthetic and pharmaceutical mixtures. The four described procedures do not require any separation step. Repeatability and reproducibility studies were achieved over two series of 10 standards for each compound showing no significant differences at 95% confidence level in the four spectrophotometric methods. A comparison of the derivative and multivariate calibration results obtained in pharmaceutical formulations was performed resulting in agreement of the values obtained.