Isotope Inversion Experiment evaluating the suitability of calibration in surrogate matrix for quantification via LC–MS/MS—Exemplary application for a steroid multi-method

• Semi-automated on-line SPE UHPLC–MS/MS method addressing six corticosteroids.
• Surrogate matrix calibration for quantification of endogenous biomarkers in serum.
• Isotope Inversion Experiment as novel validation experiment for endogenous analytes.
• Isotope Inversion Experiment evaluating suitability of surrogate matrix calibration.
• Successful implementation of novel experiment in validation protocol based on EMA.

For quotable quantitative analysis of endogenous analytes in complex biological samples by isotope dilution LC–MS/MS, the creation of appropriate calibrators is a challenge, since analyte-free authentic material is in general not available. Thus, surrogate matrices are often used to prepare calibrators and controls. However, currently employed validation protocols do not include specific experiments to verify the suitability of a surrogate matrix calibration for quantification of authentic matrix samples.The aim of the study was the development of a novel validation experiment to test whether surrogate matrix based calibrators enable correct quantification of authentic matrix samples.The key element of the novel validation experiment is the inversion of nonlabelled analytes and their stable isotope labelled (SIL) counterparts in respect to their functions, i.e. SIL compound is the analyte and nonlabelled substance is employed as internal standard. As a consequence, both surrogate and authentic matrix are analyte-free regarding SIL analytes, which allows a comparison of both matrices. We called this approach Isotope Inversion Experiment. As figure of merit we defined the accuracy of inverse quality controls in authentic matrix quantified by means of a surrogate matrix calibration curve. As a proof-of-concept application a LC–MS/MS assay addressing six corticosteroids (cortisol, cortisone, corticosterone, 11‐deoxycortisol, 11‐deoxycorticosterone, and 17‐OH‐progesterone) was chosen.The integration of the Isotope Inversion Experiment in the validation protocol for the steroid assay was successfully realized. The accuracy results of the inverse quality controls were all in all very satisfying. As a consequence the suitability of a surrogate matrix calibration for quantification of the targeted steroids in human serum as authentic matrix could be successfully demonstrated.The Isotope Inversion Experiment fills a gap in the validation process for LC–MS/MS assays quantifying endogenous analytes. We consider it a valuable and convenient tool to evaluate the correct quantification of authentic matrix samples based on a calibration curve in surrogate matrix.

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