This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.
Colorimetric tests: colour intensity correlates with concentration, but the relationship is non-linear at extremes and the correlation depends on the exact conditions (pH, temperature, timing). Reading the colour visually introduces subjectivity.
Adding the measurement that post #29 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.
Worth separating Lateral-flow and colorimetric approaches as a question about the compound from Lateral-flow and colorimetric approaches as a question about the documentation. They get answered by different people and only one of them is answerable here.
Adding the boring version of Lateral-flow and colorimetric approaches, 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.
On post #33 — agreed on the reasoning, with one qualification.
Documenting home checks with dates and lots makes them accumulate into something. Undocumented ones evaporate and get remembered selectively.
Picking up post #33: that is the part I would want checked first.
Small correction to my own earlier position on Lateral-flow and colorimetric approaches. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
The question underneath Lateral-flow and colorimetric approaches is usually "how would I tell?" rather than "what is true?", and that one has a method attached to it.
Write down what you would expect to see under each hypothesis before you collect anything. If they predict the same observation, collecting it will not help.
Worth separating two things that post #37 runs together.
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.
I would hold that lightly until someone with a larger sample weighs in.
Speaking only to Lateral-flow and colorimetric approaches as I have actually seen it, rather than as it is usually described: the effect is real, it is smaller than the thread suggests, and the variance between people is larger than the effect.
Lateral-flow and colorimetric approaches looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
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.
What physical quantity does the device measure? A colorimetric or lateral-flow method typically responds to a class of chemistry rather than to a specific molecule. It can distinguish "peptide present" from "nothing present" but generally cannot distinguish the intended peptide from a closely related one.
Post #42 is right about the mechanism and I think understates the practical bit.
Careful with the language on Lateral-flow and colorimetric approaches. "Not detected" and "not present" are different findings and the first is a statement about the method.
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.
That is a description of practice, not a recommendation of it.
On Lateral-flow and colorimetric approaches the community has more anecdote than the confidence in this thread implies, and I include my own contribution in that.
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.
Answering the question post #48 raises rather than the one it answers.
Lateral-flow devices: like a rapid COVID test. They have a reagent strip and produce a colour result. They are quick but not precise and not intended for quantitative work.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
I have three months of notes on Lateral-flow and colorimetric approaches and the honest summary is that the trend is real and the week-to-week numbers are noise. I nearly drew the opposite conclusion from the first fortnight.
Worth separating two things that post #51 runs together.
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.
If this contradicts something upthread, the upthread version may well be the better one.
Adding thanks rather than a view. I do not have a view worth the space.
If you are new and reading this thread for the answer to Lateral-flow and colorimetric approaches: the answer is conditional, the conditions are in the third reply, and the rest of the thread is worth skipping.
Post #55 is right about the mechanism and I think understates the practical bit.
False positives and false negatives: no test has 100% sensitivity and 100% specificity. A negative result does not guarantee absence; a positive result does not guarantee presence. The predictive value depends on prevalence.
The literature is thinner on this than the confidence in the thread implies.
Worth separating two things that post #55 runs together.
Second-hand on Lateral-flow and colorimetric approaches, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.
Answering the question post #55 raises rather than the one it answers.
Reading a colorimetric result at the wrong time is the commonest procedural error. Development is time-dependent and the window is usually narrow.
I would treat that as a working assumption and revisit it.