masstocharge

Masstocharge, commonly written as mass-to-charge ratio and denoted m/z, is a fundamental concept in mass spectrometry. It describes the ratio of an ion’s mass to its electric charge and is the primary quantity measured by most mass analyzers. The value is defined as m/z = M/z, where M is the mass of the ion and z is its charge (in units of the elementary charge).

The unit of m/z is daltons per elementary charge (Da/e), also referred to as Thomson (Th). Because ions can carry more than one positive or negative charge, the same neutral molecule can produce multiple m/z values corresponding to different charge states. For ions formed by adding n protons, m/z equals (M + n×H)/n, where H is the proton mass, so for large molecules the m/z is approximately M/n with a small offset.

In practice, electrospray ionization (ESI) often generates multiply charged ions, enabling the analysis of large biomolecules such as proteins within the mass range of many spectrometers. Mass spectrometers report spectra as peaks at various m/z values; the charge state and isotopic pattern help determine the neutral mass and identity of the analyte.

Applications of masstocharge include molecular weight determination, compound identification, proteomics, metabolomics, and polymer analysis. The concept underpins data interpretation across many instrument types, including time-of-flight, Orbitrap, and Fourier transform ion cyclotron resonance analyzers. While MALDI commonly yields singly charged ions, ESI routinely produces multiple charge states, illustrating the practical importance of m/z in analyzing high-mass species.

The term masstocharge is an informal variant of mass-to-charge ratio and is not a distinct physical quantity. See also mass spectrometry, m/z, ionization, and isotopic pattern.