HRMS Exact Mass & m/z Calculator
Calculate monoisotopic mass and theoretical HRMS m/z from a molecular formula. Optionally add your observed m/z to get ppm error.
What is an HRMS calculator?
An HRMS calculator helps you predict the exact monoisotopic mass and expected m/z values for compounds in high-resolution mass spectrometry (HRMS). In practical terms, you enter a molecular formula, choose an ion/adduct type, and the calculator gives a theoretical value you can compare against your instrument result.
This is useful for confirming identity, checking data quality, and quickly spotting formula mismatches before deeper structural analysis.
How this calculator works
1) Formula parsing
The calculator reads your molecular formula and counts atoms for each element. It supports:
- Standard formulas: C20H25N3O
- Parentheses: C6H4(OH)2
- Hydrate separators: CuSO4·5H2O
2) Monoisotopic mass
It sums monoisotopic atomic masses (the lightest naturally abundant isotopes) to compute neutral mass M. HRMS assignments are usually based on monoisotopic values, not average masses.
3) Adduct and charge handling
Different ionization conditions create different adducts. For example, [M+H]+ and [M+Na]+ for the same molecule will appear at different m/z values. This page includes common positive and negative mode ions.
Why adduct selection matters
A frequent interpretation mistake is matching a peak to the wrong adduct. If your mobile phase contains sodium, [M+Na]+ can dominate in ESI positive mode. In negative mode, formate or chloride adducts may appear depending on solvents and buffers.
- [M+H]+: most common in positive ESI
- [M+Na]+ / [M+K]+: common alkali adducts
- [M-H]-: common deprotonated ion in negative ESI
- [M+HCOO]-: formate adduct from formic acid systems
Understanding ppm error
PPM (parts per million) error tells you how close the observed value is to theoretical m/z.
Formula: ppm = ((observed − theoretical) / theoretical) × 1,000,000
As a rough guide:
- < 2 ppm: excellent agreement
- 2–5 ppm: usually acceptable depending on instrument/method
- > 5 ppm: check calibration, adduct assumptions, and formula
Best practices for accurate HRMS matching
Use the correct formula form
Input the neutral formula of your target analyte. Do not include the adduct in the formula itself if you're selecting it from the dropdown.
Confirm isotope pattern and retention behavior
Exact mass alone is powerful, but not absolute proof. For robust identification, combine HRMS exact mass with isotope pattern, retention time, fragmentation data, and standards when possible.
Watch for matrix effects
Complex samples can suppress signals or produce unexpected adducts. If a peak is close but not convincing, verify source conditions and sample cleanup.
Limitations
This calculator is designed for fast screening and educational use. It does not replace full vendor software workflows, isotope fine structure tools, or formula generation engines using elemental constraints and isotopic fitting.
Quick FAQ
Does this use monoisotopic masses?
Yes. Results are based on monoisotopic atomic masses commonly used for HRMS formula confirmation.
Can I enter charged formulas directly?
Use the neutral molecular formula and choose the correct adduct/charge state from the list for best results.
What if I get an element error?
Check capitalization and symbols (e.g., Cl not CL). The parser is case-sensitive and expects valid chemical notation.