Wednesday, June 1, 2011

Process Wednesday: yeah, make up that sample any way you want.

I don't know about other chemists, but I know that I'm tempted to take samples from reactions and take a squirt of this and a squirt of that and chuck it into the ol' HPLC to monitor the reaction. Our mentor-by-literature, Neal Anderson, suggests that's not such a great idea with his guidelines for sample preparation for in-process controls:
  • Dilute sample properly. Dilution effectively slows or quenches reactions, allowing for reliable assays. 
  • Consistently measure volume of diluent. Use volumetric flasks to prepare dilutions. 
  • Accurately measure sample to be diluted. Deliver aliquot for dilution by a microliter syringe or volumetric pipette. Quickly weigh solid samples. If it is necessary to deliver quantities larger than those a pipette can accommodate, weigh the aliquot and dilute. 
  • Be consistent about order of dilution
  • Be sure that the sample is completely dissolved. This is particularly important if the sample purity is to be assessed. 
  • Note if dilution of the sample significantly changes the microenvironment of the analyte sample relative to the conditions of the surrounding solvent. For instance, dilution of a reaction aliquot into a HPLC mobile phase may change the pH of the mobile phase as the analyzed sample travels through the column, resulting in changes in retention times or peak shapes. 
This, of course, from Practical Process Research and Development, pg. 156. 

Doing things the right way every time is difficult and annoying sometimes. That being said, it's the only way of generating a consistent body of data. It's kind of funny how (even at this level) you I need to be reminded of that. 

1 comment:

  1. It all depends on what you're doing. In some cases, yields don't even matter, they're almost entirely irrelevant. With process chemistry, everything matters. It's a matter of what your priorities are.

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