The method section is present, which is unusual enough that I want to make use of it.
The claim is plausible, which is exactly why I want to check it.
I am able to read a paper if someone points me at one.
Is there data behind this, or is it received wisdom?
The response factor computed from a standard — area per unit mass — assumes the standard and sample ionise identically, which is only true if they are treated identically.
Purity of the standard is not the same as traceability of the standard — purity tells you the composition, traceability tells you the standard came from a credible source.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
Specifically, comparing your quantitation result to a standard certificate requires that the sample purity be accounted for — if you measure a 95 per cent pure sample against a 99 per cent pure standard, the content result is biased.
The pharmacopoeial standards specify system suitability requirements for quantitative methods that include replicate injection agreement and that agreement should be tighter than two per cent.
Ask for the standard's purity when you buy it.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemWorth being precise here: a reference standard is a material of known identity and established purity against which you compare everything else, and it is the foundation of quantitation.
Blank injections using only the mobile phase should give a clean baseline, and any peak in the blank is a contaminant that will affect your sample result.
On the detail: a reference standard that has been opened and used many times is more exposed to contamination and moisture than a fresh standard is.
Certified reference materials are available from national metrology institutes and carry full documentation of their purity and traceability.
The practical summary: use a standard of declared purity and verified traceability, and check that the certificate is current.
edited 26 Sept 2024 by dead_volume — added the method parameters
The standard is not the sample, and the standard is not a stability indicator — it is a measurement baseline against which all inference depends.
A standard purchased from a supplier carries a certificate stating its purity, and that purity is your starting point for understanding your result.
Worth being precise here: using a standard that is older than its certificate validity window is using a material of unknown purity, which defeats the purpose of having a standard.
Worth noting that the cost of a certified standard is a fraction of the cost of a wrong conclusion based on an unverified one.
Do not use an in-house standard without independent verification of its purity.
Concretely, the cost of a certified standard is cheaper than the cost of one wrong conclusion based on an unverified standard, which is the argument for using them.
The single highest-value check is verifying that the standard's certificate specifies its purity and gives a traceable source.
The limitation is that a standard only establishes a baseline; it does not establish that your method is correct.
Ask for the standard's purity when you buy it.
More usefully, certified reference materials from national metrology institutes carry a purity statement and a statement of traceability, and both matter.
The response factor changes if the method changes — a different gradient time, a different temperature, a different column — so the standard must be run under the exact method conditions as the sample.
The pharmacopoeial standards specify system suitability requirements for quantitative methods that include replicate injection agreement and that agreement should be tighter than two per cent.
One qualification: using a standard from a reputable source and checking its purity on receipt is worth the effort.
The practical summary: use a standard of declared purity and verified traceability, and check that the certificate is current.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.