CONCEPTIONS AND MISCONCEPTIONS IN STABLE-ISOTOPE DILUTION - FUNDAMENTAL MATHEMATICAL CONSIDERATIONS

The combination of gas chromatography and mass spectrometry enables the detection and the quantitative analysis of various compounds. Isotope dilution mass spectrometry has proved to be an accurate and precise technique meeting the requirements of absolute or reference methods. Selected ion monitoring is used to monitor the gas effluents. The calibration graph indicates the ratio of peak-intensities to defined masses against the ratio of the quantities of labelled and unlabelled compounds. When a labelled molecule is used as an internal standard in a reference method, or as a tracer iii pharmacology or in biochemistry, two different cases of isotope dilution are encountered. Corresponding characteristic equations are dissected and explained. The graphs have a hyperbolic form in which the lower parts can be linearized. This paper describes the mathematical approach of present data acquisition and assesses the extent to which accuracy can be obtained in practice. Calculation procedures are described and the limits of linearization are defined.