Column chemistry choices for a 40-residue peptide posts 31–44
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Answering the question post #28 raises rather than the one it answers.
The confident answers on column chemistry choices and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
A shallow gradient resolves close-eluting species that a steep one merges. Two honest laboratories running different gradients can report genuinely different purities on the same vial.
That has been true for the cases I have seen and I have not seen many.
For anyone finding this later: the short answer on column chemistry choices is that it depends on one thing, and the rest of the thread is people identifying which thing.
Noted, and thank you for writing it out rather than summarising it.
Post #34 is the version of this I will quote in future. One addition.
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.
A modest claim, modestly supported.
Adding the boring version of column chemistry choices, because the interesting version keeps getting posted and the boring one is usually right.
Check the ordinary explanations, in order, and stop when one of them accounts for what you are seeing. Most of the time the second one does.
Answering the column chemistry choices question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken.
Collapsed as off-topic by two members at trust level 3 or above
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.
Anyone who has looked at this more carefully, please correct the record.
Post #37 and I disagree about the size of the effect, not about the direction.
On column chemistry choices I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
Taking post #41 at face value and following it one step further.
Counterpoint on column chemistry choices, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.
Column temperature affects retention and selectivity and is omitted from most certificates. Two runs at twenty-five and forty degrees are not the same method.
The general answer and the answer for your case may diverge here.
This topic was referenced in
- Comparing two chromatograms from different laboratories, properlyAnalytics › HPLC & UHPLC · 40 replies
- Coming back to: Column chemistry choices for a 40-residue peptideAnalytics › HPLC & UHPLC · 2 replies
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