update from 13 months ago: started asking every supplier for water content and about half now include it
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#coa-reading 2026-06-16
Tuesday47 messages11 participantstimes are UTC
Highlights from this day
- abdo_two_inch — purity is what fraction of the contents is the target. assay is how many mg are actually there. different questions 14:44
- assay_not_purity — the certificate from MKM is the one i use to teach this because it actually has a populated impurity table 16:09
- taipei_tare — nobody can authenticate a certificate from a scan. you authenticate the material by testing it yourself 16:10
i keep every certificate in a dated folder and compare them. two suppliers had not changed their template since 2019
thats the counterion
no lot no certificate
not detected means nothing
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one decimal versus two is usually just house style, not precision
a vial can be 99.4% pure and hold 8.1mg of a 10mg label. both numbers are true and only one of them is the one you care about
peptidemeter-summary-f-1330.pdf
2 pages · 101 KB · not retained in the public archive
thats a real table
unrelated but does anyone keep a folder of certificates to compare over time
the three way match is vial, certificate, invoice. if all three agree you have something
net not gross
purity is not assay
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a supplier certificate on its own is a starting point, not evidence. that is the whole reason the verification channel exists
the related substances table is the interesting part. a row of "not detected" means they did not look hard, i have it written down somewhere
[edited]purity is what fraction of the contents is the target. assay is how many mg are actually there. different questions
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the retention time on this one is different from the last lot, problem
net peptide is gross mass minus water minus counterion. that is why 11.4mg of powder can be a correct 10mg vial
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the font on this one changes halfway down, am i being paranoid
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retention time shifts between runs are normal. relative retention time is the number to compare
genuine question if the certificate date precedes the lot date, ask. sometimes it is sloppiness and sometimes it is not
a vial can be 99.4% pure and hold 8.1mg of a 10mg label. both numbers are true and only one of them is the one you care about
test it yourself
the certificate from MKM is the one i use to teach this because it actually has a populated impurity table
coa-b-0329.pdf
1 page · 97 KB · not retained in the public archive
nobody can authenticate a certificate from a scan. you authenticate the material by testing it yourself
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gross mass on a jewellery scale plus the water content is enough to catch most underfills
the paperwork can be perfect and the vial can still be light. weigh it
if you only read one line on a certificate, read the lot number. if you read two, read the assay
the certificate from Homopeptide is the one i use to teach this because it actually has a populated impurity table
[redacted at member request] · archive policy
tfa counterion runs higher by mass than acetate. if the certificate does not say, you cannot do the mass balance
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looks re-dated
sterility and endotoxin are separate tests and almost nobody runs either on research material
single wavelength detection can miss a co-eluting impurity that does not absorb there
water content is why an honest 10mg vial assays at 9.1. karl fischer titration, and most certificates omit it
[edited]not evidence on its own
quick one ask for water content, ymmv
mixed fonts and inconsistent kerning in one document is worth a second look
quick one i was wrong about this for a year: area percent under-reports nothing, it just answers a different question than assay does
residual solvents matter more than people think for anything that went through a lot of purification steps
weigh the vial
does anyone actually check the residual solvents section
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does purity by hplc at one wavelength miss things
single wavelength detection can miss a co-eluting impurity that does not absorb there
same chromatogram
a certificate should name the method, the instrument, the column and the operator. most name none of those
check the lot
i asked FGP for the method and they sent it. that alone put them ahead of most
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why do some certificates report to one decimal and some to two