Sources of Error in Mass Spectrometric Isotopic Analysis and Possibilities for Their Minimization

This paper presents a comprehensive methodological framework based on 55 years of practical experience in high-precision mass-spectrometric isotopic analysis. The scope of the recommendations encompasses magnetic sector mass spectrometers equipped with standard and modified electron ionization (EI) sources, applicable to gaseous compounds, solid-phase inorganic substances (via thermal evaporation), and various classes of organic molecules. Rather than validating a single analytical protocol, this work outlines general empirical guidelines to identify, quantify, and minimize key systematic and dynamic error sources encountered during the development of diverse isotopic methods. Special emphasis is placed on controlling operational parameters such as maintaining ionization chamber pressures below 10^？3 Pa to prevent secondary ion-molecule reactions, managing instrumental drift, and accounting for memory effects when consecutive sample compositions vary significantly. Furthermore, during method development, mass discrimination effects and potential isobaric interferences across molecular and fragment ions are evaluated using the isotope dilution technique and multi-line congruence testing, directly incorporating residual variances into the expanded measurement uncertainty budget. Acceptance criteria for measurement repetition are framed within target repeatability limits rather than absolute identity. The outlined practical criteria provide analytical chemists with robust strategies for optimizing high-accuracy isotopic measurement methods across varied mass-spectrometric applications.