Peptide Modification Mass Table

Written and reviewed by LifeTein’s peptide chemistry team, PhD-level scientists working in our Somerset, New Jersey laboratory · Last reviewed 4 November 2026

When you check a modified peptide by mass spectrometry, the number you are looking for is the mass of the unmodified peptide plus the shift contributed by the modification. This table lists the shifts for the modifications we are most often asked about, with both monoisotopic and average values, and the elemental composition so you can verify any of them yourself.

Mass shifts

Pairs that are easy to confuse

Several common modifications sit close enough together in mass to be mistaken for one another on an instrument of modest resolution. These are worth knowing before you interpret a spectrum.

Pair	Difference	What it takes to separate them
Phosphorylation (+79.9663) and sulfation (+79.9568)	About 0.0095 Da	High-resolution MS. Sulfation is also far more labile and is often lost in positive-ion mode.
Trimethylation (+42.0470) and acetylation (+42.0106)	About 0.0364 Da	High-resolution MS, or fragmentation and diagnostic ions.
Deamidation (+0.98) and the natural 13C isotope peak	About 0.019 Da from the +1 isotope	Good resolution and careful isotope-pattern interpretation; chromatographic separation of the Asp and isoAsp products helps.
Dimethylation (+28.0313) and formylation (+27.9949)	About 0.0364 Da	High-resolution MS.

Halogenated groups give a distinctive isotope pattern

Bromine and chlorine each have two abundant stable isotopes, which produces a characteristic signature that is useful for confirming that a halogenated group is present.

• Bromine gives two peaks separated by 2 Da of roughly equal height, because ⁷⁹Br and ⁸¹Br occur in almost equal natural abundance. A bromoacetyl peptide therefore shows an unmistakable doublet. See bromoacetyl peptides.
• Chlorine gives an M+2 peak about one third the height of the main peak, from ³⁵Cl and ³⁷Cl.
• Sulfur contributes a small M+2 contribution of a few percent per sulfur atom, which is worth remembering when interpreting cysteine-rich or biotinylated peptides.

Working out an expected mass

1. Calculate the mass of the unmodified peptide from its sequence.
2. Add the shift for each modification, multiplied by the number of occurrences.
3. For disulfide-containing peptides, subtract 2.0157 Da per bond formed.
4. Remember that the measured value in positive-ion mode is the protonated species, so compare against [M+H]⁺, [M+2H]²⁺ and so on rather than the neutral mass.
5. Counter-ions such as trifluoroacetate are not covalently attached and do not appear in the peptide mass, although they do affect the weight of the lyophilized powder. See net peptide content.

Related pages

• Peptide modifications and labeling
• Phosphorylated peptides
• Biotinylated peptides
• Fluorescent dye labeling
• Acetylation and amidation
• Bromoacetyl peptides
• QC standards for peptide synthesis

Quotation

If the modification you need is not listed here, ask: the list above covers the common requests, not the limit of what we can make. Email sales@lifetein.com or use our quotation form.