Why quantitation is the hard part, not detection — that is the question, and I have not found it answered plainly anywhere I have looked.
Collecting what is known about quantitation in one place, because it is currently spread across a category, two tag pages and a thread that is hard to find.
This is a summary rather than new work, and I have attributed each part to where I found it.
Answering the question the opening post raises rather than the one it answers.
A home refractometer is not measuring what people hope it is measuring in these preparations, and the readings are dominated by everything other than the peptide.
One more caveat and then I will stop qualifying: the sample selected itself.
I came in to disagree and I am leaving without a disagreement.
Post #2 describes the usual case. This is about the unusual one.
Research-use-only material is not certified for anything by anybody, and no home procedure changes that. What home checks buy you is early detection of the obvious problems.
The right question for any home test is: what physical quantity does it measure, and what would have to be true for that quantity to answer your question? Most disappointment comes from skipping it.
Speaking for myself and not for anyone else who has posted here.
Picking up post #8: that is the part I would want checked first.
Summarising the quantitation thread so far, since it is long and the answer is buried: the first reply has the method, the fourth has the correction to it, and the rest is people agreeing at length.
Molecular tests: DNA or RNA tests are not applicable to peptides, which do not contain nucleic acids. Peptide-specific tests rely on protein recognition, not nucleic-acid detection.
I am describing what is, rather than arguing for what should be.
A note on how quantitation gets discussed rather than on quantitation itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
Answering the question post #9 raises rather than the one it answers.
My position on quantitation is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.
Documenting home checks with dates and lots makes them accumulate into something. Undocumented ones evaporate and get remembered selectively.
The short answer was in the first line; everything after is the working.
The arithmetic on quantitation is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.
Post #13 describes the usual case. This is about the unusual one.
Offering a way to settle quantitation rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
Adding the measurement that post #16 says would settle it.
Ultraviolet absorbance at 280 nanometres estimates concentration for peptides containing aromatic residues and gives nothing for peptides that do not. Knowing which yours is comes first.
Second-hand on quantitation, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.
Building on post #18 rather than restating it.
A home refractometer is not measuring what people hope it is measuring in these preparations, and the readings are dominated by everything other than the peptide.
The claim is narrower than it sounds, and deliberately so.
Something worth flagging about quantitation: the strongest-sounding claims in this thread are the ones with no source attached, which is the usual pattern and not a coincidence.
Post #22 is the version of this I will quote in future. One addition.
Quantitation has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet.
That reframing is the whole thing. The facts I already had.
The right question for any home test is: what physical quantity does it measure, and what would have to be true for that quantity to answer your question? Most disappointment comes from skipping it.
Quantitation at home is the part that does not survive scrutiny. Precision adequate to distinguish 96 from 98 per cent requires instrumentation and calibration that a kit does not have.
I would hold that lightly until someone with a larger sample weighs in.
I had written a reply contradicting post #28 and deleted it. Here is what survived.
Where the quantitation discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.