SS-31 identity is confirmed by mass spectrometry read together with chromatography, ideally in one LC-MS run so the integrated peak is demonstrably the peak that produced the mass. Parent mass plus a fragment series fixes composition and sequence order. Chirality needs a separate chiral method or an authenticated reference standard.
- Purity by HPLC area percent describes homogeneity under one condition and never establishes what the molecule is.
- SS-31 is D-Arg-Dmt-Lys-Phe-NH2, free base composition C32H49N9O5, monoisotopic mass near 639.39 with [M+H]+ near 640.39.
- Mass spectrometry cannot see chirality, so an all-L analogue passes every MS test on a certificate.
- Sequence isomers within the Szeto and Schiller series share a parent mass and are separated only by MS/MS fragments and retention time.
- Net peptide content, water and counterion figures determine how much peptide a labelled milligram actually contains.
- Identity evidence is strongest when the mass and the integrated UV peak come from the same LC-MS injection.
Identity and purity are different measurements
A certificate reporting 99.1% purity has said something about homogeneity and nothing about what the material is. Area percent on a UV trace means one peak accounted for 99.1% of the absorbance the detector saw, under one gradient, at one wavelength. It does not name the peak. A cleanly synthesised, entirely unrelated tetrapeptide can produce that same number and that same tidy chromatogram.
Identity answers a different question: does the molecule in the vial match the structure on the label? ICH Q6B states the principle for biotechnological and biological products in terms that transfer well to synthetic peptides, that the identity test should be highly specific and based on unique aspects of the molecular structure. For a tetrapeptide the working answer is mass spectrometry interpreted alongside chromatography, and the two are far stronger when they come from the same injection.
That last detail is the one most often lost. A certificate carrying an HPLC purity figure from Monday and a mass spectrum from a separate direct-infusion run on Thursday has established that a peptide of the right mass exists somewhere in the sample and that some peak is 99% of the UV signal. It has not tied those two facts to each other. An LC-MS run does tie them, because the mass trace and the UV trace share a time axis.
- Accurate parent mass - elemental composition, nothing about order
- MS/MS b and y ions - residue order and the amidated C-terminus
- Retention time versus standard - stereochemistry and isomer exclusion
- Chiral amino acid analysis - the D-arginine configuration
- Net peptide content and water - how much peptide the vial holds
What SS-31 is as a molecular object
SS-31 is the research designation for the aromatic-cationic tetrapeptide also known as elamipretide, and as MTP-131 and Bendavia in earlier development literature. It comes out of the Szeto and Schiller series of small cationic peptides, and its sequence is D-Arg-Dmt-Lys-Phe-NH2, where Dmt is 2',6'-dimethyltyrosine, a non-coded residue. The C-terminus is a primary amide, so the molecule carries no free carboxyl group.
Three basic sites — the N-terminal amine, the arginine guanidinium and the lysine epsilon-amine — give a net charge near +3 at neutral pH. That is why the material is supplied as a salt rather than a free base, and why the counterion has to be named on the paperwork. Hydrochloride, acetate and trifluoroacetate all appear in practice, and each carries a different weight penalty.
The free base composition is C32H49N9O5. From standard atomic weights that gives an average mass near 639.8 and a monoisotopic mass near 639.39. The observables below are calculated from residue masses rather than copied from a vendor sheet; check them against your own instrument and your own arithmetic before you rely on them.
| Observable | Calculated value | What it tells you |
|---|---|---|
| Monoisotopic neutral mass | 639.39 | Elemental composition of the free base |
| [M+H]+ | 640.39 | Primary parent ion; the number most certificates quote |
| [M+2H]2+ | 320.70 | Common in ESI given three basic sites; half-integer spacing confirms charge state |
| b2 (D-Arg-Dmt) | 348.20 | Places Dmt at position 2 rather than position 1 |
| y2 (Lys-Phe-NH2) | 293.20 | Places Lys before Phe at the C-terminal end |
| y1 (Phe-NH2) | 165.10 | Confirms the amidated C-terminus |
What the mass spectrum settles and what it does not
A parent ion at nominal 640 on a single-quadrupole instrument is weak evidence. Nominal mass is shared by a large number of plausible synthesis products and by a larger number of implausible ones. High-resolution accurate mass narrows the field enormously: an error inside about 5 ppm on a time-of-flight or Orbitrap measurement, together with an isotope envelope that matches the predicted pattern, is consistent with one elemental composition and excludes most alternatives.
Composition is still not sequence. Reorder the four residues and nothing about the parent mass changes. This is where MS/MS earns its place. A b/y series that brackets each amide bond tells you the order, and for SS-31 the informative ions are the ones that fix Dmt at position 2 and the amidated phenylalanine at the C-terminus. The Dmt residue itself is a useful marker because it is 28.03 Da heavier than tyrosine, so a batch in which ordinary tyrosine was coupled by mistake shows a parent 28 Da light — trivially visible in accurate mass, easy to overlook in a nominal-mass printout.
What mass spectrometry cannot do is see chirality. D-arginine and L-arginine are the same mass, produce the same fragments and give the same accurate-mass answer. An all-L impostor of SS-31 passes every MS test in the certificate. That limitation is structural, not a matter of instrument quality, and it is the reason chromatography and reference standards remain part of the argument.
Chromatography carries the information mass cannot
Reversed-phase HPLC on a C18 column with a water/acetonitrile gradient and 0.1% trifluoroacetic acid remains the ordinary condition for a peptide of this size. Detection at 214 nm reads the amide backbone and is the trace purity is normally integrated from. A second channel around 275 to 280 nm reads the phenolic and aromatic absorbance, and the ratio between the two channels across a peak is a cheap consistency check: a peak whose 280/214 ratio drifts across its width is more than one thing.
Retention time is the property that responds to stereochemistry and to sequence order. Diastereomers separate on ordinary achiral columns often enough to be worth checking, and sequence isomers of the same composition generally separate cleanly. What makes retention time evidence rather than anecdote is a reference standard. A single retention time on its own means nothing without one; a match against an authenticated standard under two orthogonal conditions, plus a co-injection that gives a single symmetric peak rather than a shoulder, is a real identity argument.
USP general chapter 621 sets out the system suitability expectations that make any of this defensible: resolution, tailing, repeatability, and the requirement that the system be shown to perform before sample results are read. A chromatogram with no system suitability data attached is a picture, not a measurement. Peak purity by diode array adds a further layer, though it detects only co-elution of spectrally distinct species and will miss a co-eluting stereoisomer entirely.
Near neighbours worth explicitly excluding
The Szeto and Schiller series contains close relatives, and the synthesis of SS-31 itself generates a predictable set of related substances. Some of these differ in mass and fall out immediately. Others do not.
| Species | Mass relationship | Distinguishing method |
|---|---|---|
| SS-02, reported as Dmt-D-Arg-Phe-Lys-NH2 | Same composition, same mass | MS/MS fragment series; retention time against standards |
| SS-20, reported as Phe-D-Arg-Phe-Lys-NH2 | 44.03 Da lighter (Phe in place of Dmt) | Accurate parent mass |
| Tyr-for-Dmt analogue | 28.03 Da lighter | Accurate parent mass; isotope envelope |
| All-L stereoisomer at Arg | Identical in every respect to MS | Chiral amino acid analysis; retention time versus standard |
| Free-acid form (incomplete amidation or hydrolysed amide) | 0.98 Da heavier | High-resolution MS; earlier retention on RP |
| N-terminal acetylated by-product | 42.01 Da heavier | Accurate mass; distinct retention |
| Deletion sequences from a failed coupling | Lighter by one residue mass | Accurate mass and MS/MS |
The row that matters most is the stereoisomer, because it is the only one that no amount of mass spectrometry will find. Where a laboratory needs that assurance, the route is acid hydrolysis followed by chiral derivatisation and separation — Marfey's reagent, or an equivalent chiral tag, with LC or GC detection. USP general chapter 1052 covers the amino acid analysis side of this for biotechnology-derived articles. One practical wrinkle: hydrolysis liberates the non-coded Dmt as well, and quantifying it requires a Dmt standard that most contract laboratories will not have on the shelf. Arginine configuration is usually the question actually being asked, and that one is answerable.
Net peptide content, counterion and water
Purity and content are also different numbers, and the gap between them is where vial arithmetic goes wrong. A lot can be 99% pure by HPLC area and still be well under 99% peptide by weight, because the weighed powder includes counterion, residual water and residual solvent. For a molecule with three basic sites the counterion contribution is not a rounding error. Trifluoroacetate is the usual offender, since it comes along from both the synthesis and the mobile phase.
A certificate that reports net peptide content, water by Karl Fischer or loss on drying, and either a TFA figure or evidence of a salt exchange, is telling you how much peptide is in the vial. One that reports HPLC purity alone is not. The two documents look similar at a glance and are worth different amounts.
Because concentration calculations run off stated mass, this feeds straight into anything downstream. A vial labelled 10 mg that is 82% net peptide delivers roughly 8.2 mg of peptide, and a protocol calculating a molar concentration from the label rather than from the content figure is out by nearly a fifth before any pipetting error. The vial concentration calculator handles the arithmetic for laboratory documentation; feed it the content-corrected mass rather than the number printed on the cap. The same correction changes any price comparison, which is the point the cost per mg tool exists to make.
Reading the certificate
Certificates vary in quality more than in format. The fields below are the ones that either carry information or reveal its absence.
| Field | What competent documentation shows | Failure mode |
|---|---|---|
| Lot identifier | Unique, and repeated on the vial label | Generic certificate not tied to the container in hand |
| Named compound and sequence | Full sequence written out, D-configuration marked, amide indicated | Trade name only; no sequence; silence on stereochemistry |
| CAS number | Stated, with clarity on whether it refers to free base or a salt | Salt-form registry number quoted against free-base mass, or none at all |
| Mass spectrometry | Instrument type, observed and theoretical mass, ppm error, spectrum attached | A single number in a table with no spectrum and no resolution stated |
| MS/MS or fragment data | Assigned b/y ions supporting sequence order | Absent; composition presented as if it were sequence |
| HPLC | Column, gradient, wavelength, run time, integrated chromatogram, system suitability | A purity percentage with no conditions and no trace |
| Net peptide content | Reported with method | Omitted, leaving purity to be misread as content |
| Water and counterion | Karl Fischer or LOD, plus TFA or acetate figure | Both absent |
| Analysis date and analyst | Present, and after the manufacturing date | Undated, or dated before the lot existed |
Two habits catch most of the bad paperwork. Read the chromatogram, not the summary table: the axis labels, the gradient, whether the peak the integration claims is actually the largest feature. And check that the mass spectrum's stated theoretical value matches the structure named at the top of the page. Mismatches between the claimed sequence and the quoted theoretical mass are more common than they should be, and they are the fastest signal that a certificate was assembled rather than measured. Our own documentation expectations are set out in the quality standard.
Regulatory position
SS-31 is the research designation for elamipretide, a compound that has been through clinical development for mitochondrial disease indications including primary mitochondrial myopathy and Barth syndrome, with orphan designation in the United States. Regulatory activity around it has continued to move, and I am not going to assert an approval status or date I cannot verify here. Check Drugs@FDA and the FDA orphan designation database on the day the answer matters.
What does not depend on that check: research-grade powder supplied against a certificate of analysis covering identity and purity is not a pharmaceutical product. It is made outside the quality systems that govern sterility, endotoxin, formulation and lot release for a licensed medicine, and it has never been a lawful route to human use at any point in this compound's history.
Status statement written 26 August 2026; verify before relying on it.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. NOT FOR PERSONAL, MEDICAL, DIAGNOSTIC, THERAPEUTIC, OR RECREATIONAL USE.
Documentation for a lot has a shelf life of its own, in the sense that identity data describe the material as it was on the analysis date. How the powder was held after that is a separate record, and the reasoning there is covered in the storage and stability guide, which applies to lyophilized peptides generally rather than to one sequence.
What is SS-31 studied for?
Published research on SS-31 investigates the areas below, which is a different question from what SS-31 will do for anyone, a claim about a living system that nothing on this site is sold for.
What it is. A man-made four-building-block peptide that gathers in mitochondria, also published as elamipretide.
What the research looks at. Animal and clinical-stage research on mitochondria and on cardiolipin.
How it is thought to work. Described in the research as sticking to cardiolipin, a fat molecule in the inner membrane of mitochondria. Its alternating pattern of bulky and positively charged building blocks is the feature associated with it ending up there.
What is not established. No approved product. SS-31 and elamipretide are two names for the same peptide, and certificates may use either; that is a naming difference, not a chemical one.
The full record, including the certificate for the lot in stock, is on the SS-31 product page.
Common questions
Can mass spectrometry alone confirm SS-31 identity?
Why does a 99% purity figure not establish identity?
What distinguishes SS-31 from SS-02 analytically?
What is net peptide content and why does the certificate need it?
Which single certificate field is the most common red flag?
More documentation guides
Sources
- PubChem compound record for elamipretide. Establishes the synonym set (SS-31, MTP-131, Bendavia), the sequence D-Arg-Dmt-Lys-Phe-NH2 and the free-base molecular formula and mass used for the calculated observables.
- ICH Q6B, Specifications for Biotechnological and Biological Products. Sets the principle that an identity test should be highly specific and based on unique aspects of the molecular structure, which is the basis for treating identity and purity as separate tests.
- United States Pharmacopeia general chapters 621 (Chromatography) and 1052 (Biotechnology-derived Articles: Amino Acid Analysis). Supports the system suitability requirements that make a chromatographic identity claim defensible, and the amino acid analysis route used for configuration checks after hydrolysis.
- The mitochondria-targeted peptide SS-31 binds lipid bilayers and modulates surface electrostatics as a key component of its mechanism of action. Journal of Biological Chemistry . Basis for the description of the aromatic-cationic tetrapeptide series and its D-Arg-Dmt-Lys-Phe-NH2 architecture, which is what identity confirmation for this compound has to match.
