Saturday, February 20, 2016

What's that Crud in My NMR Sample?

Scene:

The reaction finished in 20 minutes by TLC. You grabbed a quick aliquot for LCMS; one peak! Quickly, you quenched, extracted, perhaps pushed through a silica plug for good measure. After concentration, a gorgeous white powder formed, so you pulled high vac for 20 minutes and rushed down to "get your proton on." But, darn it! Still wet with traces of, well, something...

Friends, has this ever happened to you? Trace impurities in otherwise perfect spectra lead to much head-scratching and SI docs labeled "final product_spectrum 5." 

The three papers linked to this post should help.

The new chart offers recommendations (colored arrows) based on Chem21 assessments of environmental impact, safety, and toxicity. Shown above are chemical shift tables (1H) in deuterated chloroform, acetone, and dimethyl sulfoxide.

If I were joining a synthetic lab this year, or starting an internship / work-study, I'd download 'em all and thumbtack liberally to the back of my bench. Guaranteed utility.

3 comments:

  1. Solvent impurities can come from the weirdest places! The most unlikely I've run across is pipette bulbs - if you have a wet pipette w/bulb that's been sitting around, the bulb gets saturated with solvent vapors. Take the bulb off, put it on a new pipette, make up your NMR sample - hey weird, this reaction never saw ether! And it was on the pump anyway!

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  2. Hey See Arr Oh,

    I am the Editor of The Analytical Scientist. I believe some of your posts would fill our readers with glee... If you're interested in writing the odd post for us, please get in touch. You should be able to find my contact details easily on the website...

    Cheers!

    Rich Whitworth

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  3. Something that should have been mentioned is that the article is FREE to download under the "ACS AuthorChoice license". That was a nice surprise.

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