What does %RSD on a system suitability run tell me?

I have the report as a PDF with the chromatogram on page two, so I can quote specifics.

This is presented as though it settles something, and I am not convinced it does.

I have two documents that appear to disagree, which is what prompted this.

What is the correct interpretation, and what is the common misreading?

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.

A standard purchased from a supplier carries a certificate stating its purity, and that purity is your starting point for understanding your result.

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.

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.

The underlying point is that 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 of uncertain purity used to measure an unknown sample produces a result whose uncertainty is larger than you will have estimated.

More usefully, 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.

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.

The practical summary: use a standard of declared purity and verified traceability, and check that the certificate is current.

A standard of low purity is worse than no standard at all, because low purity introduces a systematic error that is invisible without independent verification.

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 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.

The limitation is that a standard only establishes a baseline; it does not establish that your method is correct.

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.

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.

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 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 caveat is that a standard certificate is only as good as the issuing laboratory is reliable, and not all certificates are equally credible.

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.