A big part of my research thesis that uses NMR has been getting the sample prep right. I showed the filters that I was using in an earlier post, but these were creating problems of their own by absorbing too much of my precious material and skewing concentrations. Since particles or clumps left in solution mess up the needed homogenous nature of an NMR sample, I couldn’t just forego this filtering step. And so I decided to try centrifuging the samples! This forces the suspended, undissolved particles to the bottom of the container and leaves the lovely, uniform liquid – the supernatant – available to be removed.
So, to back up a bit, what is a centrifuge and what does it do? A centrifuge, pictured above, contains a fixed motor that spins objects around the center at high speeds. I set this one at 25000 rotations per minute, for five minutes. The more dense, solid material in the liquid solution will settle to the bottom due to centripetal acceleration. Since the centrifuge spins the samples so quickly, it’s very very important to have opposite sides balanced. For example, in the centrifuge pictured above, there are four slots in which samples may be inserted. If a test tube or container with sample is inserted on one side, it must be balanced out with a test tube or container with sample (or solvent) that collectively has the same mass. If not balanced, the centrifuge will shake frantically and likely fall off the counter.
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