System suitability injections: if they failed, the run did not happen — the long version

System suitability injections: if they failed, the run did not happen — the long version — setting out what I have, and where I think it stops being reliable.

A narrow question about System suitability injections, deliberately narrow, because the broad version has been asked here four times and produced four long threads and no answer.

One question, stated units, stated method, and what I have already ruled out.

Gradient slope is the single biggest driver of apparent purity differences. A shallower gradient over a longer run resolves more impurities and gives a higher purity figure. A steep gradient produces a tidier-looking chromatogram with fewer visible peaks and gives a lower purity figure. Both are legitimate methods and they will not produce the same number.

I checked the source rather than the summary, and they differ.

Typical suitability criteria are a replicate area relative standard deviation below about two per cent, a tailing factor inside a defined window, a resolution minimum against a specified peak, and a plate-count floor.

The arithmetic in post #3 is right; the assumption feeding it is the part to check.

Gradient delay volume differs between instruments and shifts the whole chromatogram. It is why a transferred method rarely reproduces retention times exactly on a different system.

Right — I had this wrong and I am glad to have read it before it mattered.

System suitability testing: injections run before and during the sample run to establish whether the instrument, column and method were performing when the sample was analysed. If suitability did not pass, the sample results from that run are uninterpretable.

Everything in post #3 holds. The case it does not cover is the one I have.

System suitability is not paperwork. If the replicate injections failed, the run did not happen — the numbers from it are uninterpretable rather than approximate.