Linearization of second-order calibration curves in stable isotope dilution-mass spectrometry

The quantification of compounds using isotope dilution mass spectrometry re quires the establishment of calibration curves prior to determination of an y unknown sample. When calibration over a wide concentration range is requi red and/or when an overlap exists between internal standard and analyte ion s (if mono- or di-isotopically-labelled internal standards are used), secon d-order calibration curves are obtained. In this paper we have compared sev eral calculation methods to linearize such calibration curves. We found tha t the method published by Bush and Trager(6) gives a satisfactory linear re lationship between the corrected amount ratio y = Q(1)(Q(u) + tQ(1)) (the v alue Q(u) being the amount of unlabelled analyte, Q(1) the amount of labell ed internal standard and t, the fixed fraction of the internal standard, wh ich is identical to the unlabelled analyte) and the ratio of unlabelled to labelled ion intensities. All the other calculation methods that have been published so far have failed to linearize the second-order calibration curv e build-up over a wide concentration range. Copyright (C) 2001 John Wiley & Sons, Ltd.