A least-squares approach to the quantitation of stable isotopes in mass spectrometry

Abstract

A least-squares method has been adapted for the quantitation of stable isotopes in mass spectrometry. The method uses the normalized spectra of the two isotope species and the observed intensities to calculate the isotope ratio by a series of matrix manipulations; it is applicable to systems with multiple overlapping ions. The method is used to establish the expected sensitivity of measurement of 15N enrichment of an amino acid by a focal plane mass spectrometer equipped with an electro-optical ion detector. Since multiple spectral peaks are used, the relationship between additional spectral information and the reduction in variance in ion-current statistics can be demonstrated. The example suggests a method for optimization of simultaneous ion monitoring, or limited scanning methods, by monitoring the number of ions which would contribute to the reduction of variance of the ion current of the compound of interest.