Does an in-house standard invalidate a content result?

I have both a purity figure and a content figure, which is why the discrepancy is visible.

I suspect the usual explanation for this is wrong, or at least incomplete.

I am aware this may have a boring answer. I would still like the boring answer stated clearly.

So what is the mechanism, and how well established is it?

Concretely, a standard should be stored as carefully as the samples you test it against, because a degraded standard skews all of your results in a consistent direction.

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

In practice, 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.

Certified reference materials are available from national metrology institutes and carry full documentation of their purity and traceability.

I would not accept a standard that does not come with a certificate stating its purity.

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

The standard is not the sample, and the standard is not a stability indicator — it is a measurement baseline against which all inference depends.

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 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.

Do not use an in-house standard without independent verification of its purity.

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.

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.

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

Certified reference materials are available from national metrology institutes and carry full documentation of their purity and traceability.

Ask for the standard's purity when you buy it.

The part that matters: 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 pharmacopoeial standards specify system suitability requirements for quantitative methods that include replicate injection agreement and that agreement should be tighter than two per cent.

Worth noting that the cost of a certified standard is a fraction of the cost of a wrong conclusion based on an unverified one.

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

edited 15 Jan 2025 by vialroom — fixed an arithmetic slip in the third paragraph

Put another way, 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.

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.

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

Do not use an in-house standard without independent verification of its purity.