= Mass spectrum
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= Mass spectra
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A <mass spectrum> displays detected signal against <mass-to-charge ratio>. Peak positions identify candidate <ions>; peak areas give <isotopic abundances> only after the response and overlapping contributions have been accounted for.
\Image[/neon-synthetic-mass-spectrum-weighted-mean.png]
{title=Synthetic <neon> <mass spectra> and the <weighted mean> of neutral <atomic masses>}
{description=This is a model, not a measured <mass spectrum> or a reconstruction of Aston's plate. All <ions> have $z=+1$; neutral <atomic masses> approximate <ion> <masses>. Chosen <Gaussian functions> have unit area and widths $\sigma=0.04$ and $0.70$ in the <dimensionless> coordinate $x$. Equal responses make component areas equal the displayed reference <isotopic abundances>. The inset reveals the small <neon-21> peak. The bottom diagram places the <weighted mean> between the component <atomic masses>; it is not an additional <isotope>.}
Broadening can hide a weak component without changing its area in this ideal model. An actual <mass spectrum> can also contain molecular <ions>, different <electric charge> states, background, and unequal detection responses. A missing visible peak therefore cannot by itself prove the absence of an <isotope>. Conversely, a fractional <atomic weight> is a property of a population: there is no requirement for an <ion> peak at the <weighted mean>.
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