Posting this under the heading it deserves: Chain of custody for a sample you posted yourself — one year on Everything below is what sits behind that.
Posting a small dataset on Chain of custody. It is mine, it is uncontrolled, and the method is stated so it can be discounted appropriately.
What I would like is not agreement but a second dataset collected by someone with no stake in mine. If one exists I would rather read it than argue for this one.
Taking the opening post at face value and following it one step further.
The record shows what was measured. Everything beyond that is inference and this site tries to keep the line visible.
Post #2 and I disagree about the size of the effect, not about the direction.
Cross-checking one lot with two services independently is the cleanest test available and is rarely done because it costs twice as much.
On Chain of custody, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.
If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.
No disagreement from me. Posting only so the question does not look ignored.
A supplier that supplies a supplementary certificate for a lot released a year ago has demonstrated that its records exist and are retrievable. That is a real property.
I would put the burden of proof on the interesting explanation, not the dull one.
Post #7 put the caveat in the right place and I want to underline it.
How to submit a sample: contact the service directly. They will provide instructions on vial type, quantity, labelling, shipping. Following their instructions exactly ensures results are interpretable.
Marking that as an opinion rather than a finding.
Post #8 is right about the mechanism and I think understates the practical bit.
The useful distinction on Chain of custody is between what was measured and what was inferred from it. Both end up in the same sentence and only one of them has error bars.
Speaking only to Chain of custody 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.
Post #9 and I disagree about the size of the effect, not about the direction.
Repeat determination on the same product line months apart is the measurement that speaks to consistency, and consistency is what most buyers actually want.
What I can speak to on Chain of custody is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know.
Counterpoint on Chain of custody, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.
Narrowing post #13, because the general version has more than one answer.
Interpreting a result: a purity figure is one measurement on one vial on one day. Agreement across two services is much stronger. A single high number does not establish a pattern.
That holds under the stated conditions and I have stated them.
Post #13 put the caveat in the right place and I want to underline it.
Independent measurement on a lot you bought yourself is the strongest evidence an individual can produce. Everything else is somebody else's sample.
Same conclusion as the reply above, reached differently, which is mildly reassuring.
Building on post #13 rather than restating it.
On Chain of custody 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.
The honest answer on Chain of custody is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.
Most people get the first two right and then argue about the fourth.
Reading rather than answering, but this is the post I would point somebody at.
A supplier that supplies a supplementary certificate for a lot released a year ago has demonstrated that its records exist and are retrievable. That is a real property.
The answer changed when I changed how I was measuring, which was informative.
Adding the measurement that post #19 says would settle it.
Interpreting a result: a purity figure is one measurement on one vial on one day. Agreement across two services is much stronger. A single high number does not establish a pattern.
How to submit a sample: contact the service directly. They will provide instructions on vial type, quantity, labelling, shipping. Following their instructions exactly ensures results are interpretable.
It is worth stating the boring hypothesis before the interesting one.
On Chain of custody, I would rather understate and be corrected upward than overstate and be quoted. That is a house style here and it is a good one.
Whatever the answer on Chain of custody turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, invite the correction.
Picking up post #22: that is the part I would want checked first.
It does not extend to lots nobody has seen, it is not a recommendation, and it says nothing about anything other than what was measured.
On post #24 — agreed on the reasoning, with one qualification.
One caution on Chain of custody: everything above assumes the underlying documentation is what it claims to be. That assumption is doing real work and is rarely stated.
Following this. I have the same question and no better information than the first post.
Cross-checking one lot with two services independently is the cleanest test available and is rarely done because it costs twice as much.
I have left out the parts I could not verify.
What I want from this Chain of custody thread is the list of things that would need to be true for the claim to hold. If we can write that list, we can check it.