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Isotope distribution calculator

Natural isotope envelope · M+1 / M+2 intensities · monoisotopic and average m/z

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Isotope distribution calculator

Per-element binomial/multinomial expansion convolved across elements, binned by neutron number (M, M+1, M+2 …) with peak positions from exact masses; Gaussian broadening mimics the instrument response. The input can be a molecular formula or an amino acid composition (A3R2K1, termini included); pick a charge z and the peak table is reported on the m/z scale, with monoisotopic and average m/z on the cards.

Per-element binomial/multinomial expansion convolved across elements, binned by neutron number (M, M+1, M+2 …) with peak positions from exact masses; Gaussian broadening mimics the instrument response. The input can be a molecular formula or an amino acid composition (A3R2K1, termini included); pick a charge z and the peak table is reported on the m/z scale, with monoisotopic and average m/z on the cards.

Every calculation runs locally in your browser. Nothing you type is ever uploaded.

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Each element is expanded into its isotope polynomial and the elements are then convolved, giving the relative and normalised intensity of every peak at M, M+1, M+2 and beyond, with peak positions placed at exact masses - the isotope envelope you see in a real spectrum. Br, Cl, S and Fe have very distinctive distributions, which is why envelope matching is a standard way to confirm a formula.

Input is not limited to a formula: switch to Amino acid composition (for example A3R2K1, with N-term and C-term groups) to model the isotope envelope of a peptide or protein. Choose a charge z and the peak table is listed on an m/z scale, while the cards report the monoisotopic and average m/z; the peak shape can be drawn as sticks or as a Gaussian profile set by FWHM or resolution.

How to use it

  1. Pick the input type: molecular formula, or amino acid composition (one-letter counts such as A3R2K1).
  2. Enter the formula, or the residue counts plus N-term and C-term groups.
  3. Choose the charge z; for an instrument-like profile switch to Gaussian broadening and set the width by FWHM or resolution.
  4. Read the monoisotopic and average m/z on the cards, then the peak table on the m/z scale.

Frequently asked questions

Why are the M+1 and M+2 peaks so high?
M+1 comes mainly from 13C (about 1.1% natural abundance), so the more carbon the taller the M+1 peak. M+2 comes from 34S, 37Cl, 81Br and similar isotopes, which is why chlorine- or bromine-containing molecules show a striking M+2 peak - a handy halogen fingerprint.
Is the isotope distribution from a composition as accurate as from a formula?
The tool first assembles the full peptide formula from the residues plus the terminal groups and then runs the same NIST-abundance convolution, so the result is the same number you get by typing the peptide formula directly.
Is anything I type uploaded?
No. There is no back end: the algorithms, the NIST isotope data and every dictionary ship with the page, and all calculations run in your browser.
How accurate is the data?
Isotope masses and abundances come from a NIST snapshot (94 elements, 298 isotopes). Isotope distributions are exact polynomial convolutions and agree with mainstream simulators to several decimal places.
Is it free? Do I need an account?
It is completely free, with no usage limit and no sign-up or login.
Can I send a calculation to someone else?
Yes. Click Copy link and the input state is encoded into the URL hash, so opening that link restores exactly what you had.

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