Direct quantification in bioanalytical LC–MS/MS using internal calibration via analyte/stable isotope ratio

The possibility to rationalize and simplify bioanalysis, without compromising the analytical quality, by omitting the calibration curves was studied. Using mass spectrometry (MS) and a stable isotope labeled internal standard it was possible to get equally good results by calculating the results directly from the analyte/internal standard area ratio and a predetermined response factor as by the traditional way, using a calibration curve run at the same occasion. To be able to use this simplified quantification method, that we call internal calibration, in its most simple form there are some prerequisites that must be considered: (1) The relative response should not be concentration dependent. (2) The relative response should be constant between batches/days. (3) The level of analyte in the internal standard should not be detectable. (4) There should be no influence from naturally occurring isotopes of the analyte on the internal standard peak area. A bioanalytical LC–MS/MS method for a research compound was validated both with and without calibration curves and no significant differences were found regarding precision and accuracy. It was shown that all four prerequisites above were fulfilled. Validation data were very good for the whole concentration range, 0.010–30 μmol/L. Long-term data for QC samples showed excellent precision and accuracy.