I have the report as a PDF with the chromatogram on page two, so I can quote specifics.
I would like the arithmetic checked rather than the conclusion asserted.
I have deliberately not used an online calculator because I want to be able to check the result.
Can someone show the working rather than just the answer?
Worth being precise here: comparing results from different laboratories requires knowing that both used standards of declared purity and traceability, and often they did not.
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.
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 caveat is that a standard certificate is only as good as the issuing laboratory is reliable, and not all certificates are equally credible.
The practical summary: use a standard of declared purity and verified traceability, and check that the certificate is current.
edited 9 Aug 2024 by kwn_analytical — added a caveat about sampling
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsStart from the certificate on the standard itself, because that tells you the uncertainty in every result that uses that standard.
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.
Most research quantitation work assumes the sample and standard are treated identically through every step, which is almost never true if they come from different sources.
I would not accept a standard that does not come with a certificate stating its purity.
Ask for the standard's purity when you buy it.
Concretely, certified reference materials from national metrology institutes carry a purity statement and a statement of traceability, and both matter.
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.
Stated carefully, storing a standard in a freezer is better than storing it at room temperature, but how much better depends on the chemical stability of the standard.
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.
Do not use an in-house standard without independent verification of its purity.
Using different standards for different samples is a recipe for systematic disagreement, which is why validation requires that the same standard be used for the entire series.
System suitability for a standard includes that replicate measurements of the same standard give consistent results, and if they do not, something in the method or the instrument is wrong.
The ICH Q3A and Q3B thresholds for reporting, identification and qualification of impurities are the framework the pharmaceutical industry works to, and they are worth reading even though nothing in the research-grade supply chain is obliged to meet them, because they tell you which numbers a competent analyst would consider worth reporting at all.
Worth noting that the cost of a certified standard is a fraction of the cost of a wrong conclusion based on an unverified one.
If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.
Most research work uses in-house standards of unknown provenance, which is exactly the situation where a mistake in the standard produces systematic error in every sample measured against it.
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.
The relevant pharmacopoeial reference points here are the general chapters on peptide purity and on bacterial endotoxins; both specify method validation requirements that research-grade certificates almost never claim to meet, and the absence of that claim is itself information.
The limitation is that a standard only establishes a baseline; it does not establish that your method is correct.
In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.
edited 9 Aug 2024 by Dr_Bram_Verhoeven — corrected a unit error in the worked example
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.