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

The quantification of compounds using isotope dilution mass spectrometry requires the establishment of calibration curves prior to determination of any unknown sample. When calibration over a wide concentration range is required and/or when an overlap exists between internal standard and analyte ions (if mono- or di-isotopically-labelled internal standards are used), second-order calibration curves are obtained. In this paper we have compared several calculation methods to linearize such calibration curves. We found that the method published by Bush and Trager(6) gives a satisfactory linear relationship between the corrected amount ratio y = Q(1)(Q(u) + tQ(1)) (the value Q(u) being the amount of unlabelled analyte, Q(1) the amount of labelled internal standard and t, the fixed fraction of the internal standard, which 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 curve build-up over a wide concentration range. Copyright (C) 2001 John Wiley & Sons, Ltd.