If you've ever stared at a CoA and wondered what '4731.4 Da observed vs 4731.2 Da theoretical' means, this guide is for you.
What a mass spectrometer does
A mass spectrometer ionises molecules and measures their mass-to-charge ratio. For peptide identity, the relevant output is the observed mass of the molecule, which should match the theoretical mass calculated from the peptide's amino-acid sequence.
Monoisotopic vs average mass
Two mass values appear on CoAs:
Monoisotopic mass: sum of the lightest isotope of each atom. What modern high-resolution mass spectrometers actually measure.
Average mass: natural abundance weighted average. Older instruments report this.
For a peptide around 4000-5000 Da, the two values differ by 2-3 Da, so make sure the CoA states which it's reporting.
Acceptable tolerance
A modern LC-MS instrument should match the theoretical mass to within 0.5 Da or better. If a CoA reports the observed mass 5+ Da away from theoretical, the molecule is not what the label says.
Common peptide masses
BPC-157-1419.5 Da
TB-500-4963.5 Da
Semaglutide: 4113.6 Da
Tirzepatide: 4813.5 Da
Retatrutide: 4731.4 Da
Ipamorelin: 711.9 Da
CJC-1295 (no DAC): 3367.8 Da
What if the CoA only has a purity number?
Then you have no identity proof, only a purity number for "something". Purity without identity is the single most common gap in vendor CoAs.
Frequently asked questions
Is LC-MS the same as MALDI-TOF?
Both are mass spectrometry, but different ionisation methods. Either confirms identity if run properly.
Can mass spec detect a fake peptide?
Yes: if the vial contains a different molecule than the label, observed mass will not match theoretical mass.
Why do some CoAs skip identity testing?
Cost. A purity-only HPLC run is cheaper than HPLC plus LC-MS, so cut-rate labs skip it.