Friday, February 12, 2016

Twelve reasons why it could not be that your sample is not pure enough

So you get your results back from QC or maybe from your friendly local NMR or mass spec person and they don't look like what you want them to look like. OF COURSE, it couldn't be your fault, so here are some handy questions to ask while you're denying reality being thorough :
  1. Uh, did you switch the sample with someone else's? 
  2. When was the last time your instrument was maintained? 
  3. We did have an outage last week.......
  4. Run it again, please. 
  5. Look, I recrystallized this stuff twice. 
  6. I know this is a difficult method.....
  7. Say, do you have a calibration curve for that measurement? 
  8. I knew I shouldn't have taken that left turn at Albuquerque...
  9. Have you power cycled your instrument? 
  10. How does it compare to a standard? 
  11. Is there a different instrument we can run it on? 
  12. Why won't your magic silver god give me the right answers?!?!? 
NB actually saying these things out loud is not a good idea 


  1. Also could ask if you have a liquid crystalline compound if you have two "melting points" in the DSC trace. Happened to me once.

  2. Just last night I saw some weird olefin looking signals in an energetic precursor. Turns out (after much detective work) someone had stashed a flask of 1-octadecene in the same 100 C oven in which I store my NMR tubes.

  3. When I was in grad school we had a CHN analyzer in our group. A few of the graduate students were tasked with running the samples for the group, and we'd have to give a copy of the results to our advisor. Inevitably he'd ask why certain compounds didn't pass. Certain types of molecules didn't burn well, but sometimes I'd have to tell him that someone gave me material that didn't look very crystalline or had bits of glass/kimwipe/etc in it.

  4. Wow. Unexpected new compound in standard reaction = high-impact publication!

  5. The one where you reflux something under nitrogen, and forget to cool the water in your condenser or are just too lazy to refill the ice, then end up with a new complex, compound in your NMR and GC/MS that promises a new glamour mag pub, but later turns out to be a plasticizer from the tygon connected to the top of your setup... That one is a classic.

  6. I have miserable grad school memories of running crude peptides through columns, sending the fractions off to a 'core facility' for mass spec, and weeks later getting results indicating that there was 'nothing there'. Apparently 'nothing' was an off-white solid which, in solution, absorbed at 220 and 280. Once I got in-house walk-up access to ESI, samples of 'nothing' ceased to plague me. I'll never know what really was going on.

  7. It only gets more fun in industry. Sudden spike in calcium levels in your low-metal product? Turns out the neighboring tenant was doing some drywall work. Molecular weight of your product goes haywire? ppm-level catalyst quenchers, chain terminators or crosslinkable species will do that in a heartbeat. Trying to figure out what those minute impurities are, then synthesis authentic mostly-pure samples of them so that you can spike your process with them in order to determine specification limits? fun fun!

  8. My favorite is when people shoot impure crap on GC, contaminating the liner and column.