September 27, 2022, 12:25:35 AM
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Topic: Purity Estimation - Instrumental Method  (Read 654 times)

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Offline rjb

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Purity Estimation - Instrumental Method
« on: February 12, 2022, 05:08:33 AM »
I’ve always been lucky enough to work with analytes where a reference standard is readily available, so determining purity of compounds purchased from a supplier has been a fairly trivial process in the past. Unfortunately, I am working with a compound where no reference standard is available and I would very much like to determine the purity of my compound as purchased. I suspect that the compound contains somewhere between 0-3% of impurity, but I have no idea what this impurity (if it’s there at all) could be, at least until my GC-TQ is fixed.   

I understand that purity of a compound can be fairly readily estimated (with various caveats) by using HPLC or GC to determine the total peak area (i.e. analyte + impurities) and peak area of analyte. Cleary however this is not ideal given that:

   If using GC-FID – Any impurities might not be volatile (although I suspect that this is unlikely) and that the response factors of the analyte and impurity might be different.

   If using HPLC-UV – The response factors are again unlikely to be the same or we might have a situation where my chosen detector wavelength is inappropriate for the impurity or it’s UV inactive.

Given these issues, can the method touched on above give reasonable accuracy (I recognise that this is a judgement call of course!) and if not, can anyone suggest any method by which the accuracy of the estimation of purity (using GC-FID or HPLC) can be improved at all?

I should know the answer to this, but I don't, so any advice would be most welcome!

R

Offline Orcio_87

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Re: Purity Estimation - Instrumental Method
« Reply #1 on: February 12, 2022, 09:29:16 AM »
I heard that purity of the compound can be estimated by its melting point temperature (impurities lowers the melting point).

"Compound contains somewhere between 0-3 % impurity" - maybe its just a moisture, not that awful.

Offline rjb

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Re: Purity Estimation - Instrumental Method
« Reply #2 on: February 13, 2022, 04:29:28 AM »
"Compound contains somewhere between 0-3 % impurity" - maybe its just a moisture, not that awful.

This so reminds me of "3.6 roentgen, not great, not terrible" of Chernobyl meme fame!

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