Category: Formulation and Quality
Purpose: Polygycols are used in a variety of pharmaceutical formulations . They are non-irritating to the skin and can be used as solubilizing agents or to adjust the viscosity of the formulation. Given all these attributes, it suffers from being thermally labile in the presence of oxygen generating reactive impurities such as peroxides, carboxylic acids and aldehydes. These reactive impurities unfortunately have the tendency to lower pH and degrade drug molecules most commonly via peroxide oxidations, and reaction with formaldehyde, ultimately resulting in a loss of potency of the drug product formulation . Current analytical techniques for the quantitative analyses of aldehydes involve derivatization with DNPH (Brady’s test) followed by UV analysis . A novel approach for aldehyde analysis in polyglycols is described herein which involves in-situ oxidation of aldehydes to carboxylic acids using the Tollens’ reagent (also known as Sliver-Mirror test) shown in Equation 1. The resulting carboxylic acids are then separated by HPLC and monitored at 210 nm.
Equation 1. Tollens test for oxidation of aldehydes
2 AgNO3 + 2 NaOH → Ag2O (s) + 2 NaNO3 + H2O
Ag2O (s) + 4 NH3 + 2 NaNO3 + H2O → 2 [Ag (NH3)2] NO3 + 2 NaOH
2 [Ag (NH3)2]+ + RCHO + H2O → 2 Ag(s) + 4 NH3 + RCOOH + 2 H+
Methods: The Tollens’ reagent in the sample preparation was composed of 1.25% silver nitrate, concentrated ammonium hydroxide and 0.8 M sodium hydroxide which generated the in-situ complex for oxidation of aldehydes to carboxylic acids. The separation employs 5-100 mM phosphate concentration gradient of a pH 6 buffer with acetonitrile as a mobile phase on a Weak Anion Exchange (WAX) and Reversed Phase (RP) mixed mode column. This methodology has demonstrated specificity for glycolic acid (GA), acetic acid (AA) and formic acid (FA), all of which are by products from the thermal degradation of polyglycols. The dynamic range for quantitation of these acids is 2-100 µg/mL and suitable recovery has been demonstrated in a formulation containing polygycol as an excipient.
Results: An optimized concentration of AgNO3 was used in the sample preparation based on the stoichiometry of the Tollens reaction. Linear response for the oxidation of formaldehyde and acetaldehyde to their corresponding carboxylic acids was established. For GA, AA and FA, the linear range is 2-100 µg/mL with coefficient of linear regression (R2) > 0.999. These carboxylic acids are well resolved with each other (Resolution > 2.0) within 9 minutes of phosphate concentration gradient. Injection precision (n=5) of GA, AA and FA at Limit of Quantitation is ≤ 5.9%. The AA and FA products formed as a result of oxidation demonstrated linear correlation to their corresponding aldehyde concentrations.
Conclusion: Successful analysis of formaldehyde and acetaldehyde in polygycols can be obtained using oxidative approach with Tollens’ reagent followed by chromatographic separation and quantitation.