Selank identity is confirmed with mass spectrometry, read against an average mass near 751.9. The separation must account for proline shapes that split the peak, and composition evidence backs it up. The impurity to rule out is the tuftsin fragment Selank is built from. It is far lighter and easy to see.
- Selank is C33H57N11O9, average mass near 751.9, PubChem CID 11765600.
- Tuftsin is the fragment to rule out, and it is far lighter so it is easy to see.
- Proline creates cis-trans conformers: same mass, different shape, broadened or split peak.
- Mass and column temperature both distinguish a conformer from a real impurity.
- No aromatic residues means no meaningful absorbance at 280 nm; 214 is the wavelength.
- No sulfur in the formula rules out methionine and cysteine by composition alone.
What does identity mean for a synthetic peptide?
In plain terms, identity testing is like checking a name badge against a photo. The mass is the photo, and a close match says the molecule is the one the label names. The lighter tuftsin fragment is an easy mismatch to catch.
That the molecule in the vial is the one the label names, established by evidence rather than assertion. Selank is a heptapeptide, molecular formula C33H57N11O9, average mass near 751.9, cataloged as PubChem CID 11765600.
Identity and purity answer different questions. A vial can hold the right molecule at poor purity or the wrong molecule at excellent purity, and a certificate reporting one has answered half of what you asked.
Strong identity evidence is layered. A single instrument agreeing with itself is one result wearing two hats, which is why good reports stack methods that look redundant until you ask what each one could miss.
- Observed mass against an average near 751.9
- The tuftsin fragment ruled out, and it is far lighter so it shows clearly
- Purity read at 214 nm, because there is no chromophore at 280
- A broadened or split peak checked by mass before it is called an impurity
- Column temperature raised to merge a conformational doublet
- No sulfur in the composition rules out methionine and cysteine
- Accession number confirmed at the issuing laboratory
What does the mass result confirm?
That the assembled molecule weighs what Selank weighs. Electrospray ionisation charges the molecule, the analyser reports mass-to-charge, and the result is compared with the expected value.
At 752 daltons this is a comfortable measurement. The molecule is small enough that the instrument sees a simple charge pattern rather than the broad envelope a large peptide produces, and small enough that a one-dalton difference is an obvious signal rather than something lost in scatter.
So mass carries more of the identity weight here than it does for a long peptide. It is still not proof of sequence, because mass is a sum and different arrangements of the same residues weigh the same.
Which related molecule matters most?
Tuftsin. Selank is that tetrapeptide with a Pro-Gly-Pro tail attached, so the fragment it is built from is the most plausible related species to find in a synthesis that did not go to completion.
Fortunately it is easy to see. Removing three residues takes a large amount of mass off a 752-dalton molecule, so the fragment and the finished peptide are nowhere near each other on a mass spectrum.
This is a genuine advantage of a short peptide. On a 29-mer, a deletion impurity differs by one residue on a large total and can hide inside measurement scatter. On a heptapeptide, the failures that matter are proportionally enormous and the instrument resolves them without difficulty.
Why does the peak broaden or split?
Proline. The peptide bond preceding a proline interconverts between two forms, and Selank carries several prolines, so the effect is pronounced rather than marginal.
Both forms are the same molecule. They weigh the same and are chemically identical, but they differ in shape, and reversed-phase separation responds to shape. The two can therefore move through the column at slightly different speeds.
The result is a broader peak than expected, or under some conditions an apparently split one. Reading that as an impurity is the standard mistake with proline-rich peptides, and it leads people to reject perfectly good material or to demand explanations a supplier cannot give.
How do you distinguish a conformer from an impurity?
Two tests, both straightforward. The first is mass: conformational isomers of one molecule weigh exactly the same, and a genuine impurity almost never does. A mass detector on the same run settles the question immediately.
The second is temperature. Interconversion between the two forms speeds up as the column warms, so raising column temperature tends to collapse a conformational doublet into a single peak. A real impurity does not helpfully vanish when the column gets hotter.
A certificate reporting a purity number without the chromatogram gives a reader no way to tell which they are looking at. For a proline-rich peptide that omission costs more than it does elsewhere.
Why does detection wavelength matter here?
Because Selank has no aromatic residues at all. There is no tryptophan, no tyrosine and no phenylalanine in the sequence, so the molecule has essentially nothing absorbing at 280 nanometres.
The workable wavelength is 214, which responds to the peptide bond itself and therefore detects any peptide regardless of side chains.
That gives a reader a free cross-check on a certificate. A Selank purity figure reported at 280 nanometres is not a stricter measurement, it is a measurement of a signal the molecule does not meaningfully produce, and it is worth asking about rather than accepting.
How does the basic character affect the separation?
Lysine and arginine carry positive charges, and those interact with residual acidic sites on silica-based columns. The consequence is peak tailing and retention that shifts with conditions.
The usual remedy is an ion-pairing additive in the mobile phase, commonly trifluoroacetic acid, which pairs with the positive charges and restores peak shape.
The practical point for a reader is that retention time for this peptide depends strongly on which additive was used. Two laboratories running different mobile phases will report different retention times for identical material, so retention time is weak identity evidence here in a specific, demonstrable way rather than as a general caution.
What does composition analysis add?
Amino acid analysis hydrolyses the peptide into its constituent residues and quantifies each, giving the ratio of amino acids present.
For Selank there is a useful shortcut buried in the formula. C33H57N11O9 contains no sulfur, so any material genuinely containing methionine or cysteine is not this peptide. That is a crude check and a real one, available from elemental composition without any separation at all.
Amino acid analysis also yields a peptide content figure by weight that does not depend on chromatographic assumptions, which is the honest answer to how much peptide is in the vial and why net peptide content is reported separately from area purity.
Why does the counterion matter more for a small peptide?
Proportion. A counterion has a fixed mass, and the smaller the peptide it is paired with, the larger a share of the powder it represents.
Selank is basic, carrying multiple positive charges, so it pairs with multiple counterion molecules. On a 752-dalton peptide that adds up to a meaningful fraction of the total weight before any water is counted.
The identity result is unaffected, because a mass spectrometer reports the peptide rather than the salt. The quantity result is affected considerably, which is why net peptide content exists as a number separate from purity and why the two are not interchangeable.
How do you check the report describes your vial?
Confirm the accession or verification number at the issuing laboratory rather than with the seller. The laboratory holds the record; a seller holds a copy of a document. If the number resolves to a different lot, a different product, or nothing, the analysis is not evidence about your material.
Then match the lot number on the report to the vial. Rigorous analysis attached to the wrong batch is not rigour, and that mismatch is far more common than falsified results.
Reports for material supplied here resolve through the certificate verification page, and the sizes carried appear on the Selank product record.
What should you ask a supplier about identity?
Three questions. What was the observed mass, as a number rather than a verdict. At what wavelength was purity measured. And does the accession number resolve at the issuing laboratory.
The middle question is the one specific to this compound. A molecule with no chromophore at 280 nanometres cannot be sensibly quantified there, so the answer tells you quickly whether the laboratory understood what it was measuring.
How to read the rest of the document is covered in the certificate guide.
What is the regulatory position?
International guidance on analytical validation defines what makes an identity method fit for purpose: specificity, accuracy, precision, and a demonstration that the method distinguishes the target from what else might plausibly be present. That is why a named method beside each result is not decoration.
There is no FDA-approved product containing Selank and no United States pharmacopeial monograph defining an acceptable batch, so the specification a lot is released against is the supplier's own. That raises rather than lowers the value of knowing which methods produced each number.
What a research certificate can honestly establish is what a named laboratory measured, on a named lot, by named methods.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. NOT FOR PERSONAL, MEDICAL, DIAGNOSTIC, THERAPEUTIC, OR RECREATIONAL USE.
What is Selank studied for?
Published research on Selank investigates the areas below, which is a different question from what Selank will do for anyone, a claim about a living system that nothing on this site is sold for.
What it is. A man-made peptide seven building blocks long, based on an immune peptide called tuftsin.
What the research looks at. It appears in animal work coming largely from a narrow set of research groups, which is worth carrying in mind while reading it.
How it is thought to work. Built from tuftsin with a short tail added for durability. How it works is not settled in the published research.
What is not established. No approved product in the United States and no official standard. The evidence base is smaller and more geographically concentrated than commercial writing implies.
The full record, including the certificate for the lot in stock, is on the Selank product page.
Common questions
What mass should Selank show on a certificate?
What related molecule should be ruled out?
Why does the Selank peak split?
How do you prove a split peak is not an impurity?
Why does the counterion matter more on a small peptide?
Published certificates for Selank
Every figure below is read from a report the laboratory issued for that lot; each page carries the PDF and the lab's own verification link.
More documentation guides
Sources
- PubChem Compound Summary for CID 11765600, Selank. The openable record giving the molecular formula C33H57N11O9 and average mass near 751.9 that an identity result is read against, and showing the absence of sulfur and aromatic residues.
- ICH Q2(R2), Validation of Analytical Procedures . Defines specificity for identity procedures, including that a method must distinguish the target from closely related substances, which is what the tuftsin fragment check demonstrates.
- FDA guidance, Q6B Specifications: Test Procedures and Acceptance Criteria. Sets out how identity, purity and content are treated as separate attributes and why orthogonal evidence is expected rather than a single confirmatory test.
- Finnrick independent verification portal. Third-party lookup used to confirm a certificate resolves to the lot it claims to describe, rather than confirming a document with the seller who supplied it.
