What it does
In electrospray mass spectrometry a molecule rarely flies as itself. It picks up a proton, a sodium or an ammonium ion in positive mode, or loses a proton or picks up chloride or formate in negative mode, and it often sheds a small neutral piece such as water on the way. One compound can therefore show up as many peaks. This calculator lists them all and tells you which of your peaks they explain.
- Adducts: the m/z of 25 common adducts, including [M+H]⁺, [M+Na]⁺, [M+NH₄]⁺, [M+K]⁺, [M+2H]²⁺, [2M+H]⁺, [M−H]⁻, [M+Cl]⁻, [M+HCOO]⁻ and [M−2H]²⁻, with the electron mass handled correctly.
- Neutral losses: the ions left after losing water, ammonia, CO, CO₂, formic acid, methanol, acetic acid, phosphoric acid, SO₃, a hexose, a pentose or glucuronic acid, or any formula you type.
- Matching: paste or open a peak list and see which ions each peak could be, with the error in ppm.
- No molecule? Choose “Nothing” and each observed m/z is turned into the neutral mass it would have as each common adduct, with pairs of peaks that agree on one mass pointed out.
How to use it
- Type the formula of the neutral molecule (or its monoisotopic mass) and pick the ion mode.
- Tick the neutral losses to consider and add any others as formulas.
- Paste observed m/z values (one per line; extra columns are ignored) or open a CSV or Excel file, and set the tolerance in ppm (and an optional mDa window).
- Read the matches, adducts and loss ions. Download the ions or the matches as CSV, or the report as an image, PDF or Word file.
Worked example
The example molecule is a hexose, C6H12O6: the page shows M = 180.06339 Da, [M+H]⁺ 181.07066 and [M−H]⁻ 179.05611. Against the made-up peaks, 203.0530 is [M+Na]⁺ at +1.9 ppm, 181.0710 is [M+H]⁺ at +1.9 ppm, 163.0605 is [M+H−H₂O]⁺, the water loss, at +2.5 ppm, and 179.0560 is [M−H]⁻ at −0.6 ppm: all four peaks matched.
Switch “Give it as” to Nothing and the same peaks become candidate neutral masses; the page notices that 203.0530 as [M+Na]⁺ and 181.0710 as [M+H]⁺ agree on M ≈ 180.06375, a strong hint that both come from one compound.
Tips
- Sodium and potassium adducts are common with glass vials and buffers; ammonium adducts with ammonium salts in the mobile phase.
- A peak 21.98 above [M+H]⁺ is usually [M+Na]⁺; 17.03 above is [M+NH₄]⁺.
- Use a tolerance that suits your instrument: about 5 ppm for Orbitrap and modern TOF, wider for older instruments.
FAQ
Why do two ions have the same m/z? [M+H−H₂O]⁺ and “[M+H]⁺ minus water” are the same ion, listed once as an adduct and once as a loss.
Does a match identify my compound? No. Many formulas share a mass. Check the isotope pattern, retention time and fragmentation against a standard. For study and planning, not a substitute for professional judgement.
Where the numbers come from
- Monoisotopic masses of the elements from the fibub element data (isotope masses from NIST/IUPAC via chemical-elements, MIT licence; see NOTICE) and the CODATA 2018 electron mass.
- Adduct and neutral loss formulas are standard electrospray chemistry, written by fibub; no adduct tables were copied.
- Checked against an independent Python calculation from the same atomic masses (for example glucose [M+Na]+ 203.05261, [M−H]− 179.05611). The example peaks are made up.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.