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.

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.

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.

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.

On integration: where the baseline is drawn matters more than most people realise. On a clean chromatogram with well-resolved peaks the choice is inconsequential. On a chromatogram with a trailing shoulder or a rising baseline it matters. Differences of one to two percentage points between defensible integrations are ordinary.