Importance of using highly pure internal standards for successful liquid chromatography/tandem mass spectrometric bioanalytical assays.

Internal standards (IS) with similar physicochemical properties to the analyte provide multiple advantages in liquid chromatography/tandem mass spectrometric (LC/MS/MS) bioanalytical methods such as: reduction of the analysis run time, improvement in the intra-injection reproducibility, impact reduction of matrix and ionization effects. However, it is important to evaluate the purity of the IS prior to their use. Indeed, a minor impurity in the IS may lead to an important issue during bioanalytical method development. Stable labelled internal standards are usually appropriate IS for bioanalysis. The use of oxycodone-D3, ursodiol-D5 and atovaquone-D4 as internal standards in three different bioanalytical methods was evaluated. During oxycodone, oxymorphone and noroxycodone simultaneous quantification method development, oxymorphone was identified as a contaminant in oxycodone-D3. Since the limit of quantification for oxymorphone was very low (10 pg/mL), the presence of an even low percentage of oxymorphone in oxycodone-D3 leads to the change of the stable labelled IS for an analogue, ethylmorphine. 23-Nordeoxycholic acid was preferred to ursodiol-D5 as internal standard for the ursodiol, tauroursodiol and glycoursodiol simultaneous quantification method. Indeed, more than 7% of ursodiol was identified in the ursodiol-D5 which could not be bypassed by decreasing the IS concentration without compromising the linearity. An atovaquone-D4 reference standard revealed the non-negligible presence of atovaquone-D5 to atovaquone-D8 that has a large impact on the method validation. Therefore, atovaquone-D4 was sent for recertification since its isotopic purity was found to be much less than the isotopic purity mentioned on its certificate of analysis. Consequently, during bioanalytical method development, the purity of the IS should be scrutinized.