---
title: "What does 'mass accuracy 2 ppm' mean? High-resolution MS for peptide identity | Sable Peptides"
description: "How high-resolution mass spectrometry confirms a peptide's elemental formula: ppm mass error, resolving power, isotope patterns and the monoisotopic peak, and how to read an observed-vs-theoretical mass line on a CoA. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/high-resolution-ms-mass-accuracy-and-isotope-patterns
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# 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)](https://pubmed.ncbi.nlm.nih.gov/16931035/); [Accurate chemical formula prediction using HRMS, MS/MS and isotope patterns, Anal Chem 2012 (PubMed 22497521)](https://pubmed.ncbi.nlm.nih.gov/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

-   [Monoisotopic vs average mass](/learn/monoisotopic-vs-average-mass)
-   [Charge states and adducts](/guides/esi-charge-states-and-adducts-in-peptide-spectra)
-   [b and y ions](/guides/tandem-ms-b-and-y-ions-explained)
-   [HPLC and MS on a CoA](/guides/hplc-and-mass-spectrometry-on-a-peptide-coa)

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