Identity is confirmed by mass evidence read together with chromatography. Intact mass by LC-MS fixes the molecular formula, MS/MS or a peptide map fixes the sequence and the N-terminal acetyl, and retention against a reference standard places the peak. A purity percentage describes a chromatogram, never which molecule produced it.
- A purity percentage describes chromatographic homogeneity in one method and says nothing about which molecule produced the peak.
- Thymosin Alpha-1 is a linear 28-residue peptide, Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN, with formula C129H215N33O55 and average mass near 3108.3.
- The peptide has no aromatic residues, so detection and quantitation at 280 nm fail and analytical work runs at 210 to 220 nm.
- Loss of the N-terminal acetyl shifts mass by 42.011 Da; deamidation of Asn28 shifts it by 0.984 Da and needs high-resolution MS or a charge-based separation.
- ICH Q6B requires an identity test that is highly specific and based on unique aspects of molecular structure, which points to sequence-level evidence.
- Peptide content by amino acid analysis, not vial fill weight, is the correct basis for quantitative work and for supplier comparison.
Purity and identity are separate claims
A certificate that reports 99.1% by HPLC has told you one thing: that in a particular chromatographic system, one peak accounted for 99.1% of the absorbance summed across the run. It has not told you what that peak is. Swap the contents of the vial for a similarly sized acidic peptide with comparable hydrophobicity and the number will look much the same, produced by an entirely different molecule.
This distinction is written into the regulatory guidance for biotechnological products. ICH Q6B states that the identity test should be highly specific and based on unique aspects of the molecular structure. Area percent from a single gradient is not a unique aspect of anything. It is a measure of chromatographic homogeneity, which is a useful and necessary thing to measure, and a different measurement from identity.
So the question for Thymosin Alpha-1 becomes: what evidence actually distinguishes this 28-residue sequence from everything it could plausibly be confused with, including the near-misses that share its formula to within a dalton? The answer is mass spectrometry, read alongside chromatography rather than instead of it, plus at least one orthogonal check on composition.
- Intact mass — fixes the formula, with accuracy stated in ppm
- MS/MS or peptide map — fixes the sequence and the N-terminal acetyl
- Retention against a reference standard — places the peak, supporting only
- Amino acid analysis — composition and true peptide content
- Chiral analysis — the question mass spectrometry cannot answer
The structure the test has to match
Thymosin Alpha-1 (INN thymalfasin) is a linear 28-residue peptide with an acetylated N-terminal serine and a free C-terminal asparagine. The sequence, in single-letter code, is Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN. PubChem lists the molecular formula as C129H215N33O55 with an average molecular weight near 3108.3.
| Feature | Detail | Consequence for testing |
|---|---|---|
| Length | 28 residues, linear, no disulfides | Full sequence coverage by MS/MS is realistic on one injection |
| N-terminus | Acetylated serine | A 42.011 Da marker that the des-acetyl impurity lacks |
| Aromatic residues | None; no Trp, Tyr or Phe | Absorbance at 280 nm is effectively zero; detect at 214 nm |
| Sulfur residues | No Cys, no Met | Oxidation is not the leading degradation route here |
| Acid/base balance | Three Asp, six Glu, four Lys, blocked N-terminus | Strongly acidic, calculated pI near 4; retains weakly on C18 |
| Amide side chain | Asn at position 28 | Deamidation gives a +0.984 Da species needing high resolution |
The absence of aromatic residues catches people out more than anything else on that list. Any protocol carried over from a tryptophan-containing peptide and left at 280 nm will show a flat trace, and more than one laboratory has concluded a vial was empty on that basis. Peptide-bond absorbance in the 210 to 220 nm region is the working detection window, with the practical cost that mobile-phase and solvent purity now matter to the baseline.
What chromatography establishes, and where it stops
Reversed-phase HPLC does two jobs. It separates the target from process-related impurities so that the mass spectrometer sees a resolved peak, and it provides a retention time that can be compared against a reference standard run in the same system on the same day.
Co-elution with a qualified reference standard is legitimate supporting evidence for identity. It is not sufficient on its own, and it is worth being precise about why. Retention time is a function of the column lot, the gradient, the temperature, the ion-pairing agent and the instrument's dwell volume. Two laboratories reporting different retention times for the same material is unremarkable. Two different molecules sharing a retention time in one system is also unremarkable. What carries weight is a spiked co-injection showing a single symmetric peak rather than a shoulder, and even that is a statement about chromatographic behaviour.
For this peptide the method needs some thought. With nine acidic residues and a blocked N-terminus, Thymosin Alpha-1 is poorly retained on C18 under conditions tuned for hydrophobic sequences. Trifluoroacetic acid as ion-pairing agent and a shallow acetonitrile gradient are the usual starting point; a 100 Å pore stationary phase is adequate at 28 residues. If the peak elutes near the injection front, the method is not resolving anything and the purity figure derived from it means correspondingly little. Note also that TFA suppresses signal in positive-ion electrospray, which is why the analytical HPLC method and the LC-MS method are often not the same method.
Mass evidence: the intact molecule first
Electrospray on a basic-site-poor, acid-rich peptide gives a modest charge-state envelope in positive mode, driven by the four lysines. The doubly and triply protonated species dominate in most sources. Negative-ion mode is a reasonable alternative for a peptide this acidic and sometimes gives cleaner spectra.
| Species | Calculated m/z | Note |
|---|---|---|
| Neutral monoisotopic mass | 3106.50 | Average mass 3108.3; do not mix the two conventions |
| [M+2H]2+ | 1554.26 | Usually the base peak |
| [M+3H]3+ | 1036.51 | Isotope spacing 0.33 confirms the charge state |
| [M+4H]4+ | 777.63 | Present with a harsher source |
Two things should be checked on any intact-mass report before it counts as evidence. First, whether the number quoted is monoisotopic or average, because the 1.8 Da gap between them is larger than the accuracy claim on a decent instrument and reports that silently swap conventions are common. Second, whether a mass accuracy figure in ppm accompanies the measurement. A deconvoluted mass with no accuracy statement and no calibration reference is a number, not a measurement.
Intact mass on a low-resolution instrument narrows the field but does not close it. Deamidation of Asn28 shifts the mass by 0.984 Da, which a unit-resolution quadrupole will not separate from the parent within a charge state. Loss of the acetyl group shifts it by 42.011 Da, which any instrument sees. Sequence isomers, and the substitution of a D-amino acid for an L, shift it by nothing at all.
Sequence-level confirmation: MS/MS and peptide mapping
Fragmentation is where the sequence claim is actually made. Collision-induced dissociation of the 2+ or 3+ precursor gives a b and y ion series; the useful target is coverage across the whole chain, with particular attention to the first few residues, because that region carries the acetyl and is where a des-acetyl or truncated variant declares itself. A report showing coverage only through the middle of the sequence has left the informative ends unexamined.
Peptide mapping after enzymatic digestion is the orthogonal route, and USP General Chapter <1055> sets out how such a method is developed and controlled for biotechnology-derived articles. Trypsin cleaves after the four lysines here, giving a large N-terminal fragment spanning residues 1 to 14 that carries the acetyl, three very short internal fragments around the Lys17-Lys19-Lys20 cluster, and a C-terminal fragment covering residues 21 to 28. The adjacent lysines make missed cleavages likely, so the expected map includes those partial products rather than treating them as impurities. Endoproteinase Glu-C gives a complementary cut pattern for a peptide this rich in glutamate.
Amino acid analysis after acid hydrolysis is a third kind of evidence and answers a question the mass methods cannot. It reports composition, and because Thymosin Alpha-1 has no chromophore, it is also the defensible way to establish peptide content as distinct from gross vial weight. Chiral analysis of the hydrolysate, by Marfey's derivatisation or chiral GC, addresses the D-amino acid question that no mass measurement can touch.
Reading the certificate
Certificates for research peptides vary from a full analytical package to a single line of text under a letterhead. The useful ones share a shape.
| Field | What to look for | Why it matters |
|---|---|---|
| Lot identifier | Unique, and matching the vial label | A certificate that cannot be tied to the vial documents nothing |
| Sequence as written | Full one-letter or three-letter sequence, acetyl shown | The claim being tested has to be stated |
| Intact mass | Found vs theoretical, convention named, accuracy given | Distinguishes measurement from transcription |
| Sequence confirmation | MS/MS coverage or a peptide map, not just the intact mass | Isobaric and near-isobaric variants survive intact mass alone |
| Purity method | Column, gradient, wavelength, run time | A percentage without a method is uninterpretable |
| Peptide content | By amino acid analysis or nitrogen determination | Net peptide differs from vial fill weight |
| Counterion and water | Acetate or TFA content; Karl Fischer moisture | Explains much of the gap between weight and content |
| Dates and signature | Analysis date, release date, named analyst or lab | Attribution is the part that is hardest to fake and easiest to check |
Three specific red flags recur. A chromatogram image with no axis labels or with the time axis cropped. An intact-mass figure quoted to two decimal places on an instrument class that cannot support it. And a certificate that names the compound only as "Thymosin" without the alpha-1 qualifier, which matters because thymosin beta-4 is a different, longer, also N-acetylated peptide, and the supply chain confuses the two often enough that the ambiguity should never be tolerated on a document. Our own release requirements are described in the quality standard.
The near-misses a purity figure hides
Synthesis and storage produce a small set of related species that a single-gradient purity number tends to bury inside the main peak or dismiss as noise.
- Des-acetyl material, from incomplete N-terminal capping. Mass shift 42.011 Da, obvious by MS, sometimes resolved chromatographically and sometimes not.
- Deamidated Asn28, from storage in water at neutral or higher pH. Mass shift 0.984 Da. High-resolution MS or a charge-based separation such as anion exchange or capillary electrophoresis is what finds it.
- Deletion variants missing a single residue, which shift the mass by that residue and are the most common failure of a long stepwise synthesis.
- Aspartimide and related side reactions at the Asp-containing stretches, which this sequence has three opportunities for.
- Residual scavengers, protecting groups and solvents, which fall under the general residual-solvent limits in ICH Q3C and are invisible at 214 nm if they do not absorb.
None of these is exotic. They are the ordinary consequences of making a 28-mer, and a supplier who reports them is demonstrating that the analytical method can see them. A certificate showing 99.9% with no impurity profile at all is usually reporting a method that cannot resolve anything, not a material without impurities.
Where an aliquot is prepared in solution for analysis, the concentration arithmetic and its documentation are covered by the vial concentration calculator, and diluent selection for analytical work is discussed in the note on bacteriostatic water. Peptide content rather than fill weight is the correct basis when comparing suppliers on a cost per mg basis, since counterion and residual water can account for a tenth of a vial's mass.
Regulatory position
Thymalfasin is the international nonproprietary name for synthetic Thymosin Alpha-1. It is registered and marketed as a prescription medicine in a number of countries outside the United States. It has received orphan drug designations from FDA for several indications, which is a development-stage designation and not a marketing approval.
FDA has placed thymosin alpha-1 in Category 2 of its list of bulk drug substances nominated for use in compounding under section 503A of the Federal Food, Drug, and Cosmetic Act, the category for substances raising significant safety risks. Material supplied for laboratory use carries a certificate covering identity and purity, which is a narrower claim about a different class of article than a licensed medicine.
Status verified 26 August 2026.
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 Thymosin Alpha-1 studied for?
Published research on Thymosin Alpha-1 investigates the areas below, which is a different question from what Thymosin Alpha-1 will do for anyone, a claim about a living system that nothing on this site is sold for.
What it is. A 28-residue acetylated peptide originally isolated from thymic tissue; the pharmaceutical form is called thymalfasin.
What the research looks at. Immune suppression associated with infection, cancer and ageing, and as a vaccine adjuvant. Registered and used in a number of countries; not approved in the United States.
How it is thought to work. Acts through Toll-like receptors on myeloid and plasmacytoid dendritic cells, initiating signalling that leads to production of immune-related cytokines. The mechanism is described as pleiotropic — it affects multiple immune cell subsets rather than acting on a single target.
What is not established. No United States approval. Effects on immune cell subsets are better established than effects on clinical outcomes.
The full record, including the certificate for the lot in stock, is on the Thymosin Alpha-1 product page.
Common questions
Can intact mass alone confirm identity?
Why does the certificate show no absorbance at 280 nm?
How do you detect a des-acetyl impurity?
Is co-elution with a reference standard enough?
What does a 99% purity claim actually cover?
Should the certificate report counterion content?
More documentation guides
Sources
- ICH Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. Establishes that the identity test should be highly specific and based on unique aspects of molecular structure; supports the separation of identity from purity claims.
- USP General Chapter <1055>, Biotechnology-Derived Articles — Peptide Mapping. Describes development, control and interpretation of enzymatic peptide mapping methods; supports the digestion and coverage discussion.
- PubChem compound record for thymalfasin, National Library of Medicine. Source for the molecular formula C129H215N33O55 and average molecular weight near 3108.3 used in the mass calculations.
- ICH Q2(R2), Validation of Analytical Procedures. Defines specificity and the supporting data expected of an analytical procedure; supports the requirement that a purity method state its conditions and demonstrate resolution.
