Isotope Abundance
Natural-abundance distributions for mass-spec deconvolution.
How to use this tool
See the natural isotope pattern of any element, every stable isotope, its exact mass, and how common it is, so you can read or predict the isotope clusters that show up in a mass spectrum.
What to enter
- Element: a chemical symbol (Cl, Br, Sn) or full name (chlorine). One element at a time.
Reading the result
You get a table of each stable isotope with its exact mass and natural abundance (bars scaled to abundance), plus the average atomic mass Ā. Use the ratios to recognise tell-tale signatures, chlorine and bromine give distinctive M+2 peaks, or to count carbons from the size of the M+1 peak. Masses shown are isotopic masses, not the average atomic weight.
Worked example
Chlorine returns ³⁵Cl 75.76% and ³⁷Cl 24.24%, about a 3:1 ratio, average ≈ 35.45 u, which is why chlorinated compounds show an M+2 peak roughly one-third the height of M.
Bars scale to natural abundance. The M+2 elements (Cl, Br, S) and the ¹³C M+1 marker (~1.1% per carbon) are what let you read isotope clusters in a mass spectrum. Masses shown are isotopic, not the average atomic weight.
Reading isotope patterns
Natural abundances drive the isotope clusters you see in mass spectra. Chlorine (³⁵Cl:³⁷Cl ≈ 3:1) gives an M+2 peak ~33% of M; bromine (⁷⁹Br:⁸¹Br ≈ 1:1) gives M and M+2 near-equal. Carbon's 1.1% ¹³C is the basis of the M+1 peak used to count carbons. Masses are isotopic (not average atomic) masses.
Sources
- IUPAC isotopic compositions (CIAAW 2021); NIST Atomic Weights and Isotopic Compositions.