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Research

Methodology

How material is identified, what the laboratory actually measures, and what the resulting document does and does not prove. Written for a reader who intends to check it, because that is the only reader worth writing this page for.

Section one

A report belongs to a lot, not to a compound

A lot is one production run of one compound, made at one time, under one set of conditions, and given an identifier that follows it from the manufacturer to the vial in your hand. It is the smallest unit anything can honestly be said about. Two lots of the same compound are two different physical objects, and a measurement taken on one is not evidence about the other.

We do not manufacture. Compounds are produced by manufacturing partners and arrive here as identified lots, which is the ordinary arrangement in this category. What we can attest to is narrower and more useful than a claim about a reactor we do not own: which lot is in the vial, and what an independent laboratory reported about that lot. Where the two are connected, the certificate is published in full. Where a laboratory has not reported, the entry says pending and stays pending.

This is also why a certificate is tied to a strength. A report is issued against the lot that was sampled, and a 10mg lot and a 20mg lot of the same compound are different lots. A supplier who prints one tick beside a compound name is describing a vial nobody tested. The comparison table lists every vial separately for exactly this reason.

Compounds in the catalogue
39
With a published certificate
3
Reports read and recorded
4
Of those, carrying an identity row
3

Section two

What a chromatographic purity assay measures

The method on our reports is high-performance liquid chromatography with ultraviolet and visible-light detection, written on the certificate as HPLC-UV/VIS. A small, precisely measured amount of the lot is dissolved and pushed under pressure through a packed column. Components travel through it at different speeds depending on how strongly each one interacts with the packing material, so a mixture that went in as one sample comes out separated in time.

A detector at the far end records how much light the eluting stream absorbs, moment by moment. The output is a chromatogram: a trace of detector response against time, in which each separated component appears as a peak. The software integrates the area under every peak, and chromatographic purity is the area of the main peak expressed as a share of the total area integrated.

That definition is worth sitting with, because almost every misunderstanding of a purity figure comes from skipping it. The number is a proportion of a signal, not a count of molecules and not a measure of how much material is in the vial. It is an excellent and hard-won piece of evidence about what else came off the column alongside the thing you ordered. It is not a general verdict on the vial, and the list below is what it leaves out.

A result also carries a margin of error, and the margin is part of the result. A figure quoted without it has been tidied up by somebody between the instrument and the page. Ours are published with theirs attached, in the laboratory’s own wording, on the scanned report.

Section three

Seven things a purity figure does not tell you

None of these are faults in the method. They are the boundaries of what the method is for, and a supplier who lets a purity figure stand in for all of them is either not reading their own certificates or is counting on you not to.

  1. 01

    It does not establish identity on its own

    A retention time says a substance left the column at a particular moment under particular conditions. It becomes evidence of identity only when the same conditions are used to run a reference standard for the compound, and the two line up. That comparison is a separate row on the report, and it is either there or it is not.

  2. 02

    It cannot see what the detector cannot see

    A UV/VIS detector responds to the part of a molecule that absorbs light at the monitored wavelength. A contaminant with no such absorbance produces no peak, so it contributes nothing to the total area and does not appear in the denominator of a purity figure. Salts and solvent residues are the ordinary examples.

  3. 03

    It cannot separate what co-elutes

    Two substances that leave the column at the same moment are integrated as one peak. A purity figure is a statement about the chromatogram, and anything hiding inside the main peak is counted as part of it. This is why the column, the gradient and the wavelength are part of the result rather than housekeeping detail.

  4. 04

    It does not weigh the vial

    Purity is a proportion, not an amount. A high proportion in a vial holding less material than the label states is still a high proportion. How much is actually there is the quantity row, measured separately, and it is the number to check against what you paid for.

  5. 05

    It says nothing about sterility or endotoxin

    Chromatography detects chemical species eluting from a column. Microbial content, endotoxin and particulates are different tests with different instruments, and a purity certificate is not evidence about any of them.

  6. 06

    It describes one day

    A result is a measurement with a date on it, not a property the material carries around. Everything after that date — transit, temperature, light, how many times a vial was opened — is outside what the report covers and inside what you control.

  7. 07

    It is not comparable across laboratories

    Change the column chemistry, the gradient or the detection wavelength and the same material returns a different number. Figures from two laboratories are two measurements, not a ranking. This is the single most common way purity numbers are misread in this category.

Section four

What an ISO/IEC 17025 accreditation covers

ISO/IEC 17025 is the international standard titled General requirements for the competence of testing and calibration laboratories. A laboratory does not award itself one. An accreditation body assesses it against the standard, on site, and re-assesses it periodically to keep it.

Crucially, an accreditation is granted for a defined scope — a published list of the specific tests, methods and materials the assessment covered. Accreditation bodies publish those scope documents, and they can be read.

The part that gets misusedAn accreditation tells you that a laboratory runs an audited quality system and submits to outside assessment. It does not, by itself, tell you that the specific assay on your certificate was performed under that accreditation. An accredited laboratory may quite legitimately run a test outside its accredited scope; the result is a real result, it is simply not an accredited one. Unless the report says the test was performed under the accreditation, the accreditation is a fact about the laboratory and not about your number.

Which is why a logo is not evidence and a scope document is. If a supplier shows you the mark next to a purity figure and cannot show you the scope behind it, the mark is doing the work of a claim nobody has made in writing.

Our posture follows from that and is deliberately unglamorous. Where a laboratory holds an accreditation we say so and we say what it covers. We do not print an accreditation mark beside a measured result to imply the two are connected, and we do not describe this site’s testing as accredited in general terms. Each report names the laboratory, the method and the result; that is the record worth checking.

What the assessment looks at

  • The laboratory's management system, and whether it is followed rather than merely written down
  • The competence, training and authorisation of the people who run the tests
  • Calibration of equipment and traceability of measurements to recognised standards
  • Validation of methods, and how measurement uncertainty is established and reported
  • Handling, identification and storage of samples once they arrive
  • Participation in proficiency testing, where the laboratory is measured against its peers

What it does not look at

  • Your particular sample, which the assessors have never seen
  • Any test outside the published scope, even when the same laboratory runs it
  • Whether the figure on your certificate was produced under the accreditation, unless the report says so
  • The material itself, which is what the certificate is for

Section five

Reading one of our certificates

Seven checks, in the order worth doing them. The first two decide whether the document is about your material at all, and they are the two people skip.

  1. Match the laboratory ID to the vial

    The identifier on the report must be the identifier printed on the vial in front of you. If they differ, the report is a true document about somebody else’s material. Every published report is searchable by that identifier in the certificate library, without an account and without having bought anything.

  2. Check the compound line

    The report states what was submitted for testing. It has to be the product you ordered, spelled as the catalogue spells it. A mismatch here is the failure mode that survives every other check, because every path resolves and every number is internally consistent — the document is simply about a different substance.

  3. Check the date the result was reported

    A result describes the material as it was on the day of the test. An old date on a recently shipped vial is not automatically wrong, but it is worth asking about, and it tells you how much of the vial’s history the report does not cover.

  4. Read the method line

    Ours say HPLC-UV/VIS. The method is what makes the number mean something, and it is what makes two numbers comparable or not. A figure from a different method, column or wavelength is a different measurement rather than a better or worse one.

  5. Read the purity with its margin of error

    The margin is part of the result, not a footnote to it. Where a laboratory states its result as a bound rather than a point value, that is the laboratory’s wording and we reproduce it rather than rounding it into something tidier.

  6. Check the quantity row against the label

    The assay also reports how much material it found, so a labelled mass can be checked rather than trusted. Expect the two to differ: across the reports we have read, measured quantities land both above and below the labelled mass — the widest gap on our own published reports is GHK-Cu, labelled 100 mg and measured 57.87 mg. Gross vial mass includes counter-ions, residual water bound into the cake and, for a metal complex, the complexing ion itself; net peptide content is a different and smaller number. Our reports state the measurement, not the reason for it, and we are not going to supply a reason on the laboratory’s behalf. Use the net figure for stoichiometry — the guide to net content versus gross vial mass walks through the corrections.

  7. Check whether there is an identity row at all

    Not every report carries one. Of the 4 reports we have read and recorded, 3 confirm the compound against a reference standard and the remainder report purity and quantity only. A site that badges all of them as verified is asserting something one of those documents does not say, so ours says which is which — on the certificate card, on the product page and in the comparison table.

Section six

What a certificate is not

Not a safety assessment

A purity assay is an analytical chemistry result. It is not a toxicological evaluation, and it has not been reviewed by any regulator. These materials are supplied for in-vitro laboratory research only.

Not a statement about effect

Nothing on this site describes what a compound does in or to a person, because that is not a claim a supplier of research material is in a position to make. Sequence, mass and measured purity are the whole of what we publish.

Not a guarantee that travels

The report ends at the test date. Temperature in transit, light exposure, how a vial is stored and how often it is opened are all downstream of it, and all yours.

Certificate library

Every published report in full, searchable by laboratory ID.

Open

Compare the catalogue

Every vial, its price per milligram and its certificate status.

Open

Handling guides

Reconstitution, storage, stability and measurement at the bench.

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For research use only. Figures quoted on this page are read from the reports published in our certificate library and apply only to the lots those reports name. Nothing supplied by PepXtide is for human or veterinary consumption, and none of it has been evaluated by the FDA.