Mass spectrometry for peptide identity.
HPLC says how pure a peptide is; mass spectrometry says what it is. The identity line on a Certificate of Analysis rests on one measurement, the mass of the molecule, compared with the mass the sequence should have. Here is how that measurement is made, why the number on the report rarely matches the molecular weight on the label to the decimal, and what counts as a pass.
Mass to charge.
A mass spectrometer turns molecules into ions, sorts them by mass divided by charge, written m/z, and counts them. For peptides two ionization methods dominate. Electrospray ionization, ESI, sprays the peptide from solution and tends to put several protons on each molecule, so one peptide shows up as a series of peaks at (M + nH)/n for n of 1, 2, 3 and up. Matrix-assisted laser desorption, MALDI, fires a laser at the dried sample in a matrix and mostly produces singly charged ions, one peak near M + 1. Either way the lab works back to M, the mass of the neutral peptide.
Two right answers.
Every element has isotopes, so a peptide is a cluster of masses rather than one. The monoisotopic mass counts only the lightest isotope of each atom and is what a high-resolution instrument resolves as the first peak of the cluster. The average mass weights every isotope by its abundance and is the figure usually printed as molecular weight. For BPC-157 the two are 1418.70 and 1419.5, almost a dalton apart; for a 5000 Da peptide they differ by about three. A report that lists the found mass next to the expected one should say which kind it compares. The molecular weight calculator gives both for any sequence.
Reading an ESI spectrum.
An ESI spectrum of a 2174.6 Da peptide such as MOTS-c shows peaks near 2175.6 (one proton), 1088.3 (two) and 725.9 (three). The spacing between charge states is what lets software deconvolute the series back to a single neutral mass. A report may print the deconvoluted mass, a single charge-state peak, or both; a peak labeled [M+2H]2+ at 1088.3 is the same evidence as a deconvoluted 2174.6.
What counts as a match.
No instrument is exact. A low-resolution instrument is typically trusted to within about one dalton or 0.05 percent of the mass; a high-resolution one to a few parts per million. A found mass within the instrument's stated tolerance of the expected mass is a pass. A found mass that matches only after assuming a different salt, a lost water or an extra oxygen is a question for the lab, not a pass. How to verify a COA covers taking that question to the lab that issued the report.
Purity and amount are separate.
A matching mass shows that the main component of the vial is the expected molecule. It does not say how much of the powder is that molecule, which is HPLC purity, nor how much is in the vial, which is content. It also cannot distinguish two peptides of the same formula, such as a sequence and its scrambled version or an L-amino acid and its D form, so identity testing of a D-amino-acid peptide leans on the manufacturer's synthesis record and on chromatographic behavior as well. Peptide testing covers how the three tests fit together and Edman degradation covers sequencing, the test that does read the order of residues.
Four lines.
Look for the method (ESI or MALDI, with the instrument), the expected mass with its kind stated (monoisotopic or average), the found mass with the charge state or the word deconvoluted, and the tolerance or the word pass. A panel with a spectrum image lets you see the series yourself. How to read a peptide COA shows where the panel sits on a typical report.
Identity on every report.
Each Helion lot's third-party report in the COA Portal includes identity by mass spectrometry next to HPLC purity, and every compound page lists the formula and both masses the result should match.
Specs and storage by compound.
Questions (6)
How does mass spectrometry confirm a peptide's identity?
It measures the mass of the molecule and compares it with the mass calculated from the sequence; a match within the instrument's tolerance confirms the main component is the expected peptide.
What is the difference between ESI and MALDI?
ESI sprays the peptide from solution and gives several charge states; MALDI uses a laser on a dried sample and gives mostly singly charged ions. Both lead back to the same neutral mass.
Why is the found mass different from the molecular weight on the label?
Usually because one is monoisotopic and the other average, or because a charge-state peak is quoted instead of the neutral mass. Both are correct once the kind is stated.
What does [M+2H]2+ mean?
The peptide carrying two extra protons, so the peak appears at half its mass plus one: a 2174.6 Da peptide shows at about 1088.3.
Can mass spectrometry tell purity?
No. Purity comes from HPLC; mass spectrometry answers identity. Content, how much is in the vial, is a third measurement.
Can mass spectrometry tell a D-amino acid from an L one?
Not by mass alone; the formulas are identical. That identity rests on the synthesis record and chromatography.
Published studies (2)
- Electrospray ionization for mass spectrometry of large biomolecules
- Laser desorption ionization of proteins with molecular masses exceeding 10,000 daltons
Citations from PubMed for reference. They describe the compound in the literature, not any Helion lot, and nothing here is guidance on use.