Model for determining the influence of instrumental variations on the long-term precision of isotope dilution analyses

Abstract

Isotope dilution calibration curves for the quantitative analysis of organic compounds are determined at one point in time. Future analyses of unknowns are then referred to that single calibration. However, between the time that the calibration was performed and the time that the unknowns are analyzed, numerous changes in mass spectrometric operating conditions have often occurred. These include changes in resolution, mass discrimination, fragmentation patterns due to temperature changes, and electrometer offset. These changes will alter the mass spectrometric response and may reduce the accuracy and precision of the analysis. In order to investigate the effect of the above variations, a mathematical model has been developed which permits the influence of the operating conditions to be quantitated. The mass spectrometric response—i.e. isotope ratio—was very sensitive to changes in operating conditions. Changes in the isotope ratios of mixtures that were comprised predominantly of either the natural abundance compound or the labeled compound varied by up to 50%. However, the use of data reduction procedures that include a correction term for the isotope ratio of either or both of the natural abundance or labeled compound reduced the errors to 5% or less.