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Author: Subject: Microscale Techniques
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[*] posted on 13-4-2012 at 09:18
Microscale Techniques

So, I thought I was pretty good working on the millimole/submillimole scale with some of my synthesis and getting okay yields, but someone was showing me some of their natural products work and I sharing some difficulty characterizing the product of a certain step.

So I sez "carbon NMR should clear that right up though, right?" And he grinned and politely informed me that he was working with .7 milligrams of material.

This... Blows me away. He is getting 80+ percent yields on some of these steps working with MAYBE 2 milligrams of material.

I want this skill. Are there any good references that can help me make the transition from microscale to... Nanoscale and beyond? Or is this just something one picks up after 10 years of dedicated labwork?
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[*] posted on 13-4-2012 at 11:42

Ask him how much solvent he uses.

NMR also doesn't destroy the compound :P

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[*] posted on 14-4-2012 at 12:37

For a carbon NMR he's going to need at least 3-4mg even with very NMR facilities. The recommended quantity for a standard 5mm NMR tube with 0.75ml solvent is 2mg for 1HNMR, 10mg for 13CNMR, on a 400MHz machine. He might be lucky to get a reasonable resolution if he has access to a 900MHz, or even maybe a 700MHz machine.

Regarding microscale work, I frequently partake on reactions using somewhere between 100mg and 1g of material. In some instances, I even work down to 10-25mg, but I try to avoid it if I can; this is often impossible when I'm working with a small stock of material (300mg-ish atm) that has taken 7-8 steps to make and need to do lots of different screening reactions with. Recovering material from such failed reactions is a pain, but its far better than when the material gets chewed up :( I can imagine theres probably a decent amount of error in your colleagues weighings, even with a "standard" organic lab balance (0.1mg resolution).
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