---
title: "Charge states and adducts in peptide mass spectra: reading the extra peaks | Sable Peptides"
description: "Why an electrospray mass spectrum of one peptide shows several peaks: multiple charge states, sodium and potassium adducts, TFA clusters and in-source fragments, and how deconvolution turns them into one mass. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/esi-charge-states-and-adducts-in-peptide-spectra
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# Charge states and adducts in peptide mass spectra: reading the extra peaks

One peptide gives several peaks in an electrospray mass spectrum because it carries different numbers of protons (charge states) and sometimes sodium, potassium, ammonium or TFA adducts. Each extra peak sits at a predictable m/z. Deconvolution converts the charge ladder to one neutral mass, which is the figure that should match the theoretical mass.

## Why does one peptide show several charge states?

A researcher opening the mass spectrum on a certificate often sees a cluster of peaks rather than one. That is normal for electrospray ionisation (ESI). Most of the extra peaks are the same molecule carrying different charges or a small passenger ion, and each has a predictable m/z.

ESI adds protons, so a peptide of mass M appears at m/z = (M + n×1.007)/n for each charge n. A 3,000 Da peptide may show ions at about 1001 (3+), 751 (4+) and 601 (5+). Larger and more basic peptides carry more charges. Software 'deconvolutes' this charge ladder back to a single neutral mass; published algorithms describe how charge ladders are clustered and decharged.

Source: [Optimal decharging and clustering of charge ladders generated in ESI-MS, J Proteome Res 2010 (PubMed 20201597)](https://pubmed.ncbi.nlm.nih.gov/20201597/)

## Which adducts are common, and what mass shift does each cause?

| Species | Shift vs [M+H]+ (Da) | Usual source |
| --- | --- | --- |
| [M+Na]+ | +21.98 | Glassware, salts |
| [M+K]+ | +37.96 | Salts, buffers |
| [M+NH4]+ | +17.03 | Ammonium buffers |
| +TFA | +114.0 per TFA | Mobile phase, counter-ion |
| −H2O | −18.01 | In-source loss or cyclisation |
| −NH3 | −17.03 | In-source loss or pyroglutamate |

-   \[M+Na\]+: +21.98 Da relative to \[M+H\]+ (sodium replaces a proton).
-   \[M+K\]+: +37.96 Da relative to \[M+H\]+.
-   \[M+NH4\]+: +17.03 Da, from ammonium buffers.
-   TFA adducts or clusters: +114 Da steps, from trifluoroacetic acid in the sample or mobile phase.
-   Loss of water (−18.01 Da) or ammonia (−17.03 Da): in-source fragments rather than impurities.

## How do you tell an adduct from an impurity?

-   Adduct peaks co-elute exactly with the main peak in LC-MS; a true impurity usually has its own retention time.
-   Adduct intensity changes with salt content and source conditions; impurity ratios are stable for a given lot.
-   An unexplained +16 Da is more likely oxidation than an adduct; a +1 Da shift may be deamidation; −18 Da can be a cyclised impurity such as pyroglutamate.
-   Reviews of LC-MS characterisation of synthetic peptides list the impurity classes to rule out.

Source: [Characterization of synthetic peptide therapeutics using LC-MS, J Am Soc Mass Spectrom 2021 (PubMed 34110145)](https://pubmed.ncbi.nlm.nih.gov/34110145/)

## What should you check on a CoA mass spectrum?

-   Which ion is labelled as the observed mass (\[M+H\]+, a higher charge state, or deconvoluted M).
-   Whether the theoretical value is monoisotopic or average mass, and that the observed value uses the same convention.
-   That the spectrum is for the lot printed on your vial.

## How does Sable report identity?

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

### Why does one peptide give several peaks in ESI-MS?

It picks up different numbers of protons (charge states) and sometimes sodium, potassium or other adduct ions.

### What mass shift does a sodium adduct cause?

About +22 Da relative to the protonated ion.

### How do I know if an extra peak is an impurity?

Check whether it co-elutes with the main peak and whether its mass shift matches a known adduct.

## Related

-   [b and y ions explained](/guides/tandem-ms-b-and-y-ions-explained)
-   [Monoisotopic vs average mass](/learn/monoisotopic-vs-average-mass)
-   [Measuring residual TFA](/guides/measuring-residual-tfa-in-peptides)
-   [HPLC and MS on a CoA](/guides/hplc-and-mass-spectrometry-on-a-peptide-coa)
-   [What does 'mass accuracy 2 ppm' mean? High-resolution MS for peptide identity](/guides/high-resolution-ms-mass-accuracy-and-isotope-patterns)

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)

-   m/z = (M + n×1.007)/n
-   Na adduct +21.98 Da; K +37.96 Da; NH4 +17.03 Da vs \[M+H\]+
-   TFA clusters +114 Da
-   References: PubMed 20201597, 34110145
-   Updated 2026-10-16