Taking quantitation seriously for a moment rather than deflecting: the honest position is that the community has observations and no controlled comparison, and those two things support very different sentences.
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Analytics · Home & field testing · continued
Coming back to: Why quantitation is the hard part, not detection posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Post #31 describes the usual case. This is about the unusual one.
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.
Adding the caveat now so it does not have to be extracted later.
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t.waldenstrm, post #6: 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. Go to post
Reading back through the quantitation threads from last year, the same three questions come up every time and only one of them has ever been answered properly. That seems like a documentation gap rather than a knowledge gap.
Coming back to post #31, because the follow-up matters more than the original answer.
A methods point on quantitation rather than a substantive one: if the comparison is not like for like, the difference you are measuring is the difference in method.
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Post #35 is right about the mechanism and I think understates the practical bit.
Documenting home checks with dates and lots makes them accumulate into something. Undocumented ones evaporate and get remembered selectively.
On reflection I would soften that slightly.
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If someone has run quantitation properly I would rather read that than my own reconstruction of it. Posting mine only because the thread has gone quiet.
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endo_fellow_rk, post #11: 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. Go to post
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.
Reading back through, this was answered upthread and I missed it. My fault.
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f.espinoza, post #5: I came in to disagree and I am leaving without a disagreement. Go to post
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.
m.ibarra, post #14: Right, and stated more narrowly than I would have dared to state it. Go to post
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.
Lundqvist, post #10: What I can speak to on quantitation is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know. Go to post
Documenting home checks with dates and lots makes them accumulate into something. Undocumented ones evaporate and get remembered selectively.
Quantitation: I would want to see the raw numbers rather than the summary before agreeing. Summaries lose exactly the information that would settle this.
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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.
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c.amankwah, post #16: 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. Go to post
Taking post #48 at face value and following it one step further.
I have three months of notes on quantitation 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.
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.
I looked this up rather than remembered it, which is the right order.
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j.habermann, post #39: Reading back through, this was answered upthread and I missed it. My fault. Go to post
That is a cleaner way of putting what I was circling around.
Everything in post #49 holds. The case it does not cover is the one I have.
Quantitation sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.
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Narrowing post #53, because the general version has more than one answer.
Trying to state the quantitation position in a way that someone who disagrees would recognise as fair, because I do not think the version in this thread passes that test.
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f.espinoza, post #5: I came in to disagree and I am leaving without a disagreement. Go to post
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.
Quantitation looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
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I read post #53 twice before replying, because I had assumed the opposite.
Having read the whole quantitation thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.
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an.zamora, post #24: The documentation on quantitation is better than this thread and I say that as someone who has posted in the thread. Go to post
Documenting home checks with dates and lots makes them accumulate into something. Undocumented ones evaporate and get remembered selectively.
That is what I would do. It may not be what is correct.
Post #57 is the version of this I will quote in future. One addition.
Two questions I would want answered before drawing anything from the quantitation data above: how were the cases selected, and what happened to the ones that dropped out.
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