A commonly used approach to deal with the influence of matrix effects in quantitative LC–MS is the addition of isotopically labelled standards. The use of these substances is useful for the correction of the signal deviation because they have the same chemical properties and the same retention times as the non-labelled substances. However, isotopically labelled internal standards were not available for the analytes studied. Therefore, the matrix-matched calibration approach was applied to perform correct quantitative analysis. Furthermore, by spiking the standards in the matrix prior to the extraction and clean-up steps, the resulting data were automatically corrected for recovery.