Shoulders, tangent skims, and defensible integration — the long version

Shoulders, tangent skims, and defensible integration — the long version Writing it up because I had to work it out twice and would rather nobody else did.

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 98.2%. A tangent skim gives 99.3%. 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.

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.

The most useful thing anyone has posted about Shoulders in this category was a table of what had been measured and by whom. That is what I would want again.

Picking up the opening post: that is the part I would want checked first.

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.

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

What I would tell a new member reading about Shoulders for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.

Worth separating two things that post #5 runs together.

Change the wavelength and the proportions change even though the sample has not. That is the reason the wavelength has to be on the document for the number to mean anything.

This follows post #5 rather than contradicting it.

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.

An update on my earlier Shoulders post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.

That is a fair summary of where the discussion has got to.

Confirming post #9 from a second method, which matters more than confirming it from a second person.

Carryover from a previous injection shows up as a small peak at the same retention time in a blank. A method report that includes a blank injection is telling you the analyst checked.

The confident version of this sentence would be wrong, so here is the hedged one.

I had written a reply contradicting post #8 and deleted it. Here is what survived.

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.

Whatever the answer on Shoulders turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, invite the correction.

The version of Shoulders that I was taught turned out to be a teaching simplification. Useful, and not true in the way I had assumed it was.