Purity & Certificates of Analysis
Purity is the single most important specification of a research peptide, and the one most often taken on trust rather than verified. This page sets out how purity is actually measured, what each method does and does not tell you, what a certificate of analysis must contain to be worth anything, and how to request the certificate for the specific batch you would receive.
What HPLC measures, and what it does not
High-performance liquid chromatography (HPLC) separates the compounds present in a sample as they travel through a column, and detects them, most commonly by ultraviolet absorbance, as they elute. The peptide's own peak is compared against the total signal detected, and the result is reported as a purity percentage — every peptide listed on SemaxBuy is supplied at ≥99% purity by this measure. Reported alongside the number should be the chromatography conditions used, since a purity result only means something in the context of the method that produced it.
HPLC purity is best read as a well-supported estimate, not an absolute count of every impurity present. Two limitations follow directly from how the method works. First, an impurity that elutes at almost the same time as the main peptide peak can be partly or fully hidden underneath it, rather than showing up as a separate, countable peak. Second, UV detection does not see every compound equally well: a related fragment or truncated sequence left over from synthesis that absorbs UV light weakly can be present in the sample without being fully reflected in the purity number. HPLC is the standard method for a reason — it is sensitive, reproducible and well understood — but a purity result from HPLC alone does not, on its own, prove that every trace impurity has been separated and accounted for, and it says nothing about whether the main peak is even the intended molecule.
What mass spectrometry adds
That last gap is exactly what mass spectrometry closes. Mass spectrometry measures the mass of the molecule or molecules present and compares it against the mass expected for the target sequence. Where HPLC answers "how much of this sample is one thing," mass spectrometry answers a different question: "is that one thing actually the peptide it is meant to be." A sample can return a high HPLC purity score while still being the wrong compound, or a truncated or modified version of the intended sequence, if identity is never separately confirmed. A complete quality package reports both figures together — purity by HPLC and identity by mass spectrometry — because neither test on its own answers the full question of what is in the vial.
Independent laboratory versus the manufacturer's own control
Who performs the test matters as much as which test is performed. A specification sheet produced in-house by the manufacturer describes what that manufacturer's own quality-control process found, using equipment and reference standards the manufacturer controls, with no external check on the result. A certificate issued by an independent, third-party laboratory — one with no commercial stake in whether the batch passes — is a materially stronger form of evidence, because the party running the test has no incentive to round a borderline result upward. Every certificate we reference is issued by an independent laboratory, not produced in-house, and is tied to a specific manufacturing batch rather than reused as a generic specification sheet across every shipment we send.
What a valid certificate of analysis must contain
| Field | What it should state | Warning sign if missing |
|---|---|---|
| Batch or lot number | The exact batch identifier, matching the number printed on the vial label you receive. | No batch number, or a number that does not match your vial, means the certificate cannot be tied to the product in your hands. |
| Test date | When the analysis was carried out, relative to manufacture and to your order date. | An undated certificate, or one dated long before the batch was produced, cannot confirm the condition of what you actually received. |
| HPLC purity result | The measured purity percentage and the chromatography method or conditions used to obtain it. | A bare percentage with no method stated cannot be checked or reproduced by anyone else. |
| Mass-spectrometry result | Confirmation that the measured molecular mass matches the expected mass of the target sequence. | A certificate with a purity figure but no identity confirmation has not actually shown the vial contains the right molecule. |
| Issuing laboratory | The name of the independent laboratory responsible for the result. | An unnamed laboratory, or one that turns out to be an internal department of the seller, is not third-party verification. |
| Compound and sequence identifier | The specific peptide name and, where relevant, its CAS number. | A generic certificate with no compound identifier could apply to any product and proves nothing about the one you ordered. |
Requesting the certificate for your specific batch
The certificate that matters is the one for the batch you are actually going to receive, not a representative example from an earlier production run. Request it by Telegram or at orders@semaxbuy.com before you order, and again once your vial arrives; include the batch number from the vial label in the second request. Checking that the batch number on the certificate matches the number on the label in front of you is the single step that turns a certificate from a general reassurance into evidence about the specific unit you hold — a certificate for the right compound but the wrong batch tells you nothing about the vial in your hand.
What "≥99%" actually means, and why it does not compare cleanly across suppliers
A stated purity of ≥99% means that, by the specific HPLC method used and reported on the certificate, at least ninety-nine percent of the detected signal corresponds to the target peptide. It is not an absolute, method-independent measurement. Two suppliers can each publish "≥99% purity" using different column chemistries, different detection wavelengths, or different integration settings for how a peak's edges are drawn, and arrive at numbers that are not directly comparable, even when both are reported honestly. This is exactly why the method and conditions belong on the certificate alongside the number, and why a bare percentage with no method stated, from an unnamed laboratory, is worth very little on its own — the figure only becomes meaningful once you can see how it was produced and who produced it.
Chain of custody and storage
Batches are stored and shipped under the conditions each compound's stability data calls for, with packaging designed to preserve the cold chain in transit where the specification requires it. On receipt, laboratory best practice is to store the lyophilised peptide frozen and protected from light until use, and to reconstitute only the quantity a given protocol requires; our reconstitution and storage guide covers this in detail. All products described on this page are supplied for laboratory research use only.