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Topic: GC-MS versus NMR  (Read 16867 times)

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Offline DrCMS

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Re: GC-MS versus NMR
« Reply #15 on: October 21, 2013, 03:06:37 PM »
@ Archer if you'd read his messages clearly and did not overlay your own expectation you might have realised curiouscat is not trying to ID a main product with some impurities.  He has already said the compounds he is trying to ID are in the 0.1%-5% range.  It is those impurities your are trying to tell him how to ignore that he is actually trying to ID so NMR of the crude reaction mix will not be very useful.

Offline Archer

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Re: GC-MS versus NMR
« Reply #16 on: October 21, 2013, 04:15:49 PM »
@ Archer if you'd read his messages clearly and did not overlay your own expectation you might have realised curiouscat is not trying to ID a main product with some impurities.  He has already said the compounds he is trying to ID are in the 0.1%-5% range.  It is those impurities your are trying to tell him how to ignore that he is actually trying to ID so NMR of the crude reaction mix will not be very useful.

My apologies, I misunderstood.

DrCMS is absolutely right, the minor impurities need to each be purified to be the major components if NMR is going to provide any useful data.
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Offline DrCMS

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Re: GC-MS versus NMR
« Reply #17 on: October 21, 2013, 05:31:26 PM »
@Archer The thing to remember is that curiouscat is a chemical engineer involved with process improvement & control so identifying small impurities are important to work out how & why the yields are less than he wants.  Then he can work out if improving the yield is cost effective for that particular process.   

Your background is I think in analysis for regulatory control and proving a particular component is a controlled substance is your focus and so other minor impurities are not that important.

Those two roles are not really complementary.

My background is as a practical lab/plant chemist involved with process development & scale up so I can see where curiouscat is coming from which is why I posted the advice I did.  Then you posted the exact opposite which wound me up so I apologise if my reply to you seemed a bit sharp.  I can see from your reply to me that your posts were due to a minor comprehension error rather than a case of Maslow's hammer.

Offline curiouscat

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Re: GC-MS versus NMR
« Reply #18 on: October 21, 2013, 11:52:55 PM »
@Archer @DrCMS:

Perhaps I added to the confusion because of notation: These are impurities with respect to my process, but OTOH for this analysis since they are the molecules of interest it may be a misnomer to call them an "impurity" except in the sense that they mess up each other's analysis by interference.

Offline Archer

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Re: GC-MS versus NMR
« Reply #19 on: October 22, 2013, 02:18:56 AM »
@Archer The thing to remember is that curiouscat is a chemical engineer involved with process improvement & control so identifying small impurities are important to work out how & why the yields are less than he wants.  Then he can work out if improving the yield is cost effective for that particular process.   

Your background is I think in analysis for regulatory control and proving a particular component is a controlled substance is your focus and so other minor impurities are not that important.

Those two roles are not really complementary.

My background is as a practical lab/plant chemist involved with process development & scale up so I can see where curiouscat is coming from which is why I posted the advice I did.  Then you posted the exact opposite which wound me up so I apologise if my reply to you seemed a bit sharp.  I can see from your reply to me that your posts were due to a minor comprehension error rather than a case of Maslow's hammer.

No harm done, I was contradicting sound advice so your response was considered and polite relative to what you were probably thinking.

You and I have similar backgrounds in processing, my current role is the analysis you describe.

If we made something then minor impurities were things which were removed after the initial identification of the target molecule, then they would be identified at a later date after purification often used as standards for future process monitoring. One time an impurity turned out to be a valuable commodity.




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Offline curiouscat

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Re: GC-MS versus NMR
« Reply #20 on: October 22, 2013, 02:29:01 AM »
Quote
curiouscat is a chemical engineer involved with process improvement & control so identifying small impurities are important to work out how & why the yields are less than he wants.  Then he can work out if improving the yield is cost effective for that particular process.   

Couldn't have put it better. :)

In particular here's what happens. A process gets commercialized in a hurry and no one cares about what exactly is the 10%  that they form as by products.

Over time margins get squeezed and people start digging deeper. Unfortunately most often these side peaks have no characterization which makes it hard to try and reduce them in any systematic way.

Of course, often even after we know what they are there's no way to reduce them but that's another story. :)

Offline discodermolide

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Re: GC-MS versus NMR
« Reply #21 on: October 22, 2013, 04:17:28 AM »
@Archer,
I was in the Pharma business and the impurities were and by the time I left still a valuable commodity. That was the state of affairs not very long ago.
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Offline DrCMS

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Re: GC-MS versus NMR
« Reply #22 on: October 22, 2013, 04:35:39 AM »
I know I can be rather harsh to people so I'll try to tone it down.  Thankfully as grown ups you and I hope me can take a bit of criticism without having a hissy fit and flaming everyone before running off to our mums. 

For what curiouscat is trying to do GC-MS is probably the best starting technique. 

If you could separate the components and get enough of a pure component NMR might give more structural info but the time and effort to do that purification and get an NMR run If you do not have in onsite might not be justified.  The GC-MS may well give you enough of an idea of the identity of the components of interest to figure out what side reactions are occurring and how you might reduce their formation.  You may well find that the cost of increasing the yield is more than the value of the extra yield.  A process we are fine tuning here just got the yield deliberately reduced because the cost in plant time and waste disposal of pushing the yield from 90% to 95% was costing more than the extra 5% of product sold for.  The people looking at process improvements had focused too much on the % yield rather than looking at the profit.  Once I did that is became quite clear that accepting the lower yield was a valid way to maximise the profits. 

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