Improvement of sensitivity for the determination of propylene glycol in rat plasma and lung tissue using HPLC/tandem MS and derivatization with benzoyl chloride

Propylene glycol (PG), a simple alcohol, is a commonly used vehicle for aerosol dosage formulations. Quantification of PG in plasma and lung tissue is, therefore, important for new drug development. We describe a highly sensitive and selective method for the quantitative determination of PG in rat plasma and lung tissue, using liquid chromatography (LC) with positive atmospheric pressure chemical ionization (APCI) tandem mass spectrometry (MS) detection. Propylene glycol and the internal standard (IS) 1,4-butanediol were derivatized with benzoyl chloride under alkaline conditions to enhance the sensitivity of detection. Ionization efficiency was improved following derivatization. The limits of detection (LOD) for rat plasma and rat lung tissue were 0.269 mug/mL and 1.12 mug/g, respectively. The LOQs for rat plasma and lung tissue were 0.448 mug/mL and 1.62 mug/g, respectively. Calibration curves were linear from 2 to 4000 mug/mL for rat plasma (r = 0.9990) and from 2 to 2400 mug/g for rat lung tissue (r = 0.9985). The assay showed intra-assay and inter-assay precision (%RSD) of 3.8-4.8% (n = 6) and 4.2-6.6% (n = 18) for rat plasma, 3.8-4.0% (n = 6) and 6.5-7.4% (n = 18) for rat lung tissue, respectively. The percent inaccuracy for inter-assay results in rat plasma were 2.5-4.7% and in rat lung tissue were 1.3-6.8% for different concentrations. All plasma and lung tissue samples demonstrated acceptable freeze/thaw stability, bench stability, and prepared sample stability over a 24 hours period. An alternative derivatizing method using perfluorooctanoyl chloride with negative APCI/MS detection was investigated. Although strong fragment ions of the derivatives could be detected, the feasibility of this method was limited by sample preparation. Additionally, fragment ions produced from the perfluorooctanoyl moiety lacked selectivity. The benzoyl chloride method is proved to be more sensitive, selective, and robust.