Calculation of the confidence range in order to obtain a linear calibration graph in stable isotope dilution mass spectrometry: application to reference methods and pharmacological studies

Abstract

Isotope dilution mass spectrometry is a technique that has been shown to be capable of producing the accuracy and precision required of a reference method. The use of a labelled standard, selected ion monitoring and isotope ratio measurements has been applied to the determination of many compounds. In pharmacology, drug metabolism can be studied using ‘inverse dilution’. The calibration graph is obtained by plotting peak-area ratio against the ratio of the amount of unlabelled compound to that of labelled compound. The graph is of hyperbolic form, part of which can be linearized. To define the limits of this linear confidence range, calculation procedures are described. The equations given are clearly useful in determining the limits within which the operator can control the relative error. These equations are defined for isotope dilution whatever the ratio of the amount of unlabelled to labelled compound. Examples are discussed.