Shoulders, tangent skims, and defensible integration

Posting this under the heading it deserves: Shoulders, tangent skims, and defensible integration Everything below is what sits behind that.

Setting out an integration problem with the actual figures, because describing it in words has not worked.

Main peak with a trailing shoulder. Valley-to-valley gives 97.7%. A tangent skim gives 98.5%. Both are defensible and they are more than a point apart.

The question I actually have is which convention I should be asking suppliers to report, rather than which one is right.

Shoulders would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.

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.

A shoulder on the trailing edge is most often a closely related species rather than an artefact. The way to find out is to change the gradient slope, not to argue about the integration.

Take it as a starting point and not as a specification.

Taking post #4 at face value and following it one step further.

Where the shoulders reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.

That is consistent with mine, for whatever one more account is worth.

A chromatogram image at a resolution where you can see peak shape but not baseline detail is worth having and is not the same as the data. Ask for the integration table if the number matters.

Having read the whole shoulders thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.

Where the baseline is drawn is the least documented and most consequential choice in the whole determination. Valley-to-valley, tangent skim and forced-to-zero can span a couple of percentage points on the same chromatogram.

Not the whole picture, but the part of it I can speak to.

A definition problem is doing most of the work in this shoulders discussion. Once the term is pinned down I suspect the disagreement mostly goes away and what is left is small.

I would be cautious about generalising from the shoulders example above. It is a good example. It is one example.

Where I part company with post #9, and it is a narrow parting.

A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it.

Post #13 is the version of this I will quote in future. One addition.

If two laboratories disagree by more than two or three percentage points, work through method, integration, sample handling, whether it was the same lot and the same vial, and whether suitability passed. After all five, a gap needs an explanation.