A certificate says "identity: LC-MS/MS, 3108.3 observed". A buyer who reads carefully asks what that proves about the order of the residues, and the honest answer is: more than a single mass, less than a full sequencing run, and the difference depends on the method. This post explains the three methods that establish a sequence, what each one proves, and how to read the identity line on a certificate against them. The sequence post covers how to read a sequence and check a mass by hand; the LC-MS post covers what the mass measurement itself does. This one is about order.
What "sequence verified" would require
A method that reads position. The molecular mass of a peptide is the sum of its residue masses, which is why two peptides with the same residues in a different order weigh exactly the same. Pinealon, Glu-Asp-Arg, and the hypothetical Arg-Asp-Glu are both 418.4. A mass alone cannot tell them apart, and neither can HPLC retention with any confidence. Verifying a sequence means breaking the chain in a controlled way and finding out which residue was where.
Three methods do that, or part of it.
Method 1: tandem mass spectrometry
The peptide is ionised, the molecular ion is selected, and it is broken along its backbone by collision with a gas. Each break produces two fragments, one carrying the N-terminus and one the C-terminus, and the instrument measures the masses of all of them. Roepstorff and Fohlman's 1984 nomenclature names the common fragments b-ions, which carry the N-terminal piece, and y-ions, which carry the C-terminal piece. The result is a ladder: the b-ion series climbs by one residue mass at each rung, and the difference between adjacent rungs is the mass of the residue at that position. Reading the differences in order reads the sequence. Steen and Mann's 2004 review is the standard introduction.
What it proves: that a molecule of the measured mass fragments in the pattern the stated sequence predicts. For a synthetic peptide of known intended sequence, a matching ladder is strong confirmation of order as well as composition. What it does not prove: anything the mass cannot see, which is the subject of the last section.
Every certificate on this site issued for the August 2026 lots reports identity by LC-MS/MS: the peptide is run through the chromatograph, the main peak's ion is selected and fragmented. The MOTS-c post and the thymosin alpha-1 post read two of those certificates.
Method 2: Edman degradation
Pehr Edman's 1950 method labels the N-terminal residue with a reagent, cleaves it off under conditions that leave the rest of the chain intact, identifies the released derivative by chromatography, and repeats. Each cycle reads one residue, from the N-terminus inward. It is slow, consumes material, and loses efficiency with each cycle so that long peptides become unreadable after thirty to fifty residues, but it reads sequence directly rather than inferring it from masses, and it distinguishes leucine from isoleucine, which mass spectrometry cannot.
Its decisive limitation for this catalogue: it requires a free N-terminal amine. A peptide whose N-terminus is acetylated, like thymosin alpha-1, or cyclised to pyroglutamate, like ARA-290 and thymulin, gives nothing in the first cycle. For those, tandem MS is the only sequencing option without chemically deblocking the terminus first.
Method 3: amino acid analysis
The peptide is hydrolysed completely to its free amino acids, usually in hot acid, and the amino acids are separated and quantified. The result is a composition: so many glutamates, so many lysines, per mole of peptide. It confirms that the right residues are present in the right proportions and, done quantitatively against a standard, gives the most accurate measurement of how much peptide is in a sample, which is why it is the reference method behind net peptide content. It says nothing about order, because hydrolysis destroys it. Tryptophan is destroyed by acid hydrolysis and asparagine and glutamine are converted to their acids, so those residues are under-reported or merged.
What a single mass on a certificate does and does not prove
Certificates on this site issued before August 2026 report identity by MALDI-MS, which measures the molecular ion without fragmenting it. The vilon post and the melanotan-1 post read two of those. What a single mass proves:
- the composition, to within the method's tolerance;
- that the material is not a deletion sequence, an oxidised form, a dimer or the wrong salt, each of which has a different mass;
- for a defined synthetic product, that the intended molecule was made, since the alternative is a coincidental isomer the synthesis would not produce.
What it does not prove is the order of the residues. For a dipeptide the point is nearly moot; for a 29-residue peptide like CJC-1295 it is the reason tandem MS is the better method, and the reason the recent certificates use it.
What no mass-based method can see
Three limits apply to every method above except, for the second, Edman.
- D versus L residues. Enantiomers have identical masses and identical fragment ladders. SS-31 has a D-arginine and CJC-1295 a D-alanine; no mass method confirms the configuration. A chiral amino acid analysis, which hydrolyses the peptide and separates the enantiomers, does.
- Leucine versus isoleucine. Same mass, same formula. Edman distinguishes them; standard tandem MS does not without specialised fragmentation.
- Co-eluting isomers. If an isomer with the same mass is present and separates poorly by HPLC, the fragment ladder is a mixture. For a synthetic product this is rare, because the isomer would have to be made.
The sequence tells you which limits apply before you read the certificate. A peptide with no D-residue, no leucine-isoleucine ambiguity and no plausible isomer is fully characterised by a matching tandem-MS ladder. One with a D-residue is characterised by the ladder plus the manufacturer's synthesis record, which is the honest statement of what the certificate can carry.
When to commission sequencing
Rarely, for a research reagent with a tandem-MS certificate. The cases that justify it: a result that depends on configuration at a D-residue, a dispute between two certificates, or a lot with only a single-mass identity and a long sequence. The independent testing post covers which laboratories accept single vials; ask specifically for MS/MS with a fragment assignment, or for chiral amino acid analysis if configuration is the question.
Frequently asked questions
Does a certificate mass verify the sequence?
A single mass verifies composition and rules out the wrong compound, deletion products, oxidation and dimers. It does not verify order. Tandem MS, which reads a fragment ladder, does.
What does LC-MS/MS on a certificate mean?
That the main HPLC peak's ion was selected and fragmented, and the fragment masses matched the stated sequence. It is a sequence-level identity, within the limits above.
Why can Edman degradation not be used on thymosin alpha-1?
Because its N-terminus is acetylated and Edman chemistry needs a free N-terminal amine. The same applies to pyroglutamate peptides such as ARA-290.
Can any method tell a D-arginine from an L-arginine?
Not by mass. Chiral amino acid analysis after hydrolysis can. Where configuration matters, that is the test to ask for.
What does amino acid analysis add?
The most accurate measure of how much peptide is present, and confirmation of composition. Nothing about order.
References
- Steen H, Mann M. The ABC's (and XYZ's) of peptide sequencing. Nature Reviews Molecular Cell Biology 2004;5(9):699-711. doi.org/10.1038/nrm1468
- Edman P. Method for determination of the amino acid sequence in peptides. Acta Chemica Scandinavica 1950;4:283-293. doi.org/10.3891/acta.chem.scand.04-0283
- Roepstorff P, Fohlman J. Proposal for a common nomenclature for sequence ions in mass spectra of peptides. Biomedical Mass Spectrometry 1984;11(11):601. doi.org/10.1002/bms.1200111109
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