The correct use of the term isobar for molecules

Abstract

In analytical chemistry literature, the term isobar is frequently used to describe small molecules with identical molecular formulas, often interchangeably used with isomer. This usage conflicts with the historical and International Union of Pure and Applied Chemistry (IUPAC) definition of isobar, which refers to nuclides of different elements with equal mass numbers but different proton-to-neutron ratios. In small-molecule LC-MS/MS analysis, this terminological overlap leads to confusion, particularly when distinguishing between compounds with identical exact masses (isomers) and those with different elemental compositions but nearly identical masses (molecular isobars). This article explores the origin and definitions of both terms and provides examples highlighting that the term isobar can be used for molecules but not for isomers, thereby emphasizing the importance of precise nomenclature for accurate molecular identification and interpretation in mass spectrometry.