Peptide Molecular Weight and Mass Spec: Average Mass, Monoisotopic Mass and Charge States
For in vitro laboratory research use only. This article explains analytical chemistry terms. It makes no claims about the effects of any material.
Look up a peptide’s molecular weight in two places and you may find two slightly different numbers. Look at its mass spectrum and you may see it at several values that don’t match either. None of these are errors. They come from different ways of stating mass, and from how a mass spectrometer measures it. This guide explains the three ideas you need: average mass, monoisotopic mass and charge state.
Molecular weight and its units
Molecular weight is the mass of one molecule. For peptides it is usually given in grams per mole (g/mol) or in daltons (Da). The two are numerically the same: a peptide of 1,500 g/mol has a mass of 1,500 Da. Larger peptides are often written in kilodaltons (kDa), so 5,000 Da is 5 kDa.
For a linear peptide, molecular weight is the sum of its amino acid residue masses plus one molecule of water for the free ends. Our guide to peptide sequence notation shows that sum worked out for a short example.
Average mass vs monoisotopic mass
Most elements come in more than one natural isotope. Carbon is mostly carbon-12, but about 1.1% is the heavier carbon-13. Hydrogen, nitrogen and oxygen have heavier isotopes too. That gives two common ways to state a molecule’s mass:
- Average mass uses each element’s average atomic weight, weighted by natural isotope abundance. It is the figure usually listed as “molecular weight” in reference databases and specification tables.
- Monoisotopic mass uses only the most abundant isotope of each element (carbon-12, hydrogen-1, nitrogen-14, oxygen-16). It is the mass of the lightest common form of the molecule, and it is the figure mass spectrometrists often compare against.
Using the short Gly-Ala-Ser example from our sequence notation guide:
| Average mass | Monoisotopic mass | |
|---|---|---|
| Gly-Ala-Ser | about 233.2 Da | about 233.10 Da |
The gap is small for a short peptide and grows with size, because a bigger molecule has more atoms that can be a heavier isotope. For a peptide of several thousand daltons, the average and monoisotopic masses can differ by a few daltons.
Why a mass spectrum shows a cluster of peaks
A mass spectrometer doesn’t see an average. It sees individual molecules, and in any sample some molecules contain one or more heavier isotopes. So a single peptide appears as a small cluster of peaks, each about 1 Da apart (for a singly charged ion), called the isotope pattern.
For a small peptide, the first peak in the cluster (the monoisotopic peak) is usually the tallest. For larger peptides, molecules carrying at least one carbon-13 become more common than molecules with none, so the tallest peak shifts to the right of the monoisotopic peak. That is one reason larger peptides are often reported by average mass or by a calculated mass from the whole cluster.
m/z and charge states
A mass spectrometer measures mass-to-charge ratio, written m/z, not mass directly. In the electrospray sources commonly used in LC-MS, peptides pick up one or more protons and so carry one or more positive charges. Each added proton adds about 1.007 Da of mass and one unit of charge.
That means one peptide can appear at several m/z values at once. A made-up example for a peptide with a mass of 1,500 Da:
| Ion | Charge (z) | Approximate m/z |
|---|---|---|
| [M+H]+ | 1 | 1,501.0 |
| [M+2H]2+ | 2 | 751.0 |
| [M+3H]3+ | 3 | 501.0 |
The same molecule produces all three peaks. Larger peptides tend to carry more charges, so their main peaks can appear at m/z values far below their actual mass. The spacing inside an isotope cluster also tells you the charge: about 1 m/z apart for a singly charged ion, about 0.5 apart for a doubly charged ion, and so on.
Software works backward from these peaks to the neutral mass of the molecule, a step called deconvolution. Peaks can also appear from adducts, where an ion such as sodium attaches instead of a proton, which shifts the m/z by a predictable amount.
Molecular weight vs the weight of the powder
Molecular weight describes one molecule of the peptide. It does not describe the powder in a vial. A lyophilized (freeze-dried) powder can also contain water and counter-ions such as trifluoroacetate (TFA) or acetate, which add weight without changing the peptide’s molecular weight. Our explainer on net peptide content vs purity covers that difference.
How this connects to a COA
On a certificate of analysis, identity (LC-MS) is liquid chromatography with mass spectrometry, used to check that the measured mass matches the expected compound. The expected mass is worked out from the sequence and its end groups. The result line names what was identified. Where a mass spectrum is printed, it shows the measured masses used for the identity check. Our LC-MS identity explainer covers what that result confirms and what it doesn’t.
Published molecular weights in databases such as PubChem are reference figures for the compound, not measurements of a specific lot. Our specifications comparison lists the reference values for BPC-157 (about 1,419.5 g/mol) and thymosin beta-4 (about 4,963 g/mol). For other terms, see the research peptide glossary.
Peptix certificates
The peptide lots with a published certificate on our peptide COA lookup page each have a third-party Certificate of Analysis showing the lot number, HPLC-UV purity and LC-MS identity. A certificate reports results for the sample tested. It is not a guarantee for other lots. For example, you can view the TB-500 10 mg product with its lot certificate, or browse all research peptides.
For research use only. Not for human or animal use.
