Calibration for isotope dilution mass spectrometry - Description of an alternative to the bracketing procedure

A calibration method for isotope dilution mass spectrometry is presented that fully accounts for non-linearity of calibration functions, caused by the interference of the analyte on the m/z used for measurement of the internal standard and vice versa. In this way, it is also possible to use incompletely labelled molecules or analogues with a mass increment of only 1 u for isotope dilution, on condition that certain restrictions are respected. In addition, the proposed method is less time consuming than bracketing. The method works with the computer-stored full calibration curve and a single calibration point that is measured daily. The calibration curve is constructed from the experimentally determined mass spectral overlap between the unlabelled analyte and the labelled internal standard at the mit values chosen for measurement. Measurement results for samples with unknown analyte concentration are calculated on the basis of a linear relationship between their ion abundance ratio and that of the daily single calibration point, but corrected by a factor derived from the theoretical calibration function. All calculations are performed with commonly available spreadsheet software. An application is presented far determining serum uric acid with a candidate gas chromatographic/isotope dilution mass spectrometric reference method.