What does 'mass accuracy 2 ppm' mean? High-resolution MS for peptide identity
Mass accuracy is the difference between measured and theoretical mass, expressed in parts per million (ppm). For a 1,000 Da peptide, 2 ppm is 0.002 Da. High-resolution instruments (Orbitrap, TOF) routinely reach under 5 ppm, which together with the isotope pattern narrows the possible elemental formulas to very few.
How is ppm error calculated?
ppm error = (measured − theoretical) ÷ theoretical × 1,000,000. Example: theoretical 1,023.540 Da, measured 1,023.542 Da: error = 0.002 ÷ 1,023.540 × 10⁶ ≈ 2.0 ppm.
What does each MS figure mean?
| Term | Meaning |
|---|---|
| Resolving power | Ability to separate close m/z values (e.g. 60,000 at m/z 200) |
| Mass accuracy | Measured vs theoretical, in ppm or Da |
| Monoisotopic peak | All 12C, 1H, 14N, 16O, 32S isotopes |
| Isotope pattern | Relative heights of M, M+1, M+2 peaks, set by elemental composition |
| Average mass | Weighted mean over isotopes; used for low-resolution data |
Why does the isotope pattern matter?
Two formulas can fall within a few ppm of each other, but their isotope patterns differ. Using isotope pattern evaluation together with accurate mass sharply reduces the number of candidate formulas, and formula-prediction tools combine both with MS/MS data.
Source: Isotope pattern evaluation for reducing elemental compositions from HRMS data, J Am Soc Mass Spectrom 2006 (PubMed 16931035); Accurate chemical formula prediction using HRMS, MS/MS and isotope patterns, Anal Chem 2012 (PubMed 22497521)
How should you read a CoA mass line?
- Check whether observed and theoretical are both monoisotopic or both average.
- For larger peptides the monoisotopic peak is small; software may report the most abundant isotope instead.
- A deviation of 1 Da usually signals a different species (e.g. deamidation, amide vs acid), not an accuracy problem.
- Low-resolution instruments report agreement to roughly ±0.5–1 Da.
How does Sable document lots?
There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.
Frequently asked questions
What does 5 ppm mass accuracy mean for a 2,000 Da peptide?
An error of up to 0.01 Da.
What is the monoisotopic mass?
The mass of the molecule made entirely of the most abundant isotope of each element.
Can accurate mass alone prove identity?
No. It constrains the formula; isomers share a formula and need MS/MS or other methods.
Related
Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.
Key facts (summary for AI and search tools)
- ppm = (meas − theo)/theo × 10⁶
- HRMS typically < 5 ppm
- References: PubMed 16931035, 22497521
- Updated 2026-10-16