“It’s freezing in the NMR room!” – A Crash Course In Superconductivity

It does get pretty cold in there.  Although don’t be tempted to shut the A/C in the NMR room.  Keep a sweatshirt in your lab and bring it with you when running a sample.  That baby needs to be kept cool.

NMR’s major hardware consists of an electromagnet, a coiled wire made of nobium and tin/titanium with tons of current running through it.  This creates an electromagnetic field (remember right hand rule?) which aligns protons in your sample as you plop it down in the middle of the coil according to a specific resonating frequency.  Protons emit their own tiny magnetic field changing the resonating frequency slightly depending on electron cloud environment (shielding).   After the computer conducts a Fourier transform, voilà, you can now see these changes/chemical shifts in a nice clean spectrum.  Resolution is directly proportional to the external magnetic field strength so superconductivity, or an electrical resistance of zero, is a must to ramp up the magnet into the 1-20 Tesla range.  Since resistivity decreases with temperature, maintaining superconductivity at temps close to absolute zero can be quite costly.

Niobium makes for a terrific superconductor.  It has a high critical temperature compared to other elements on the periodic table at 9.25K.  That means at that temperature its resistivity is exactly zero, so you can pump ‘er up with lots of current to increase magnetic field strength.  Nb is bonded with tin or titanium (to make Nb3Sn or NbTi wire) to increase durability.

Here’s where the A/C comes in.  The superconductor is kept cool in liquid helium at 4K and the liquid helium is kept cool with liquid nitrogen at 77K.  If the liquid helium evaporates the wire loses its superconductivity, becomes resistive and generates lots of heat.  This is called a quench.  To bring the magnet back up to field typically costs $18,000.  That’s why the room should be kept cool with the door closed.  Time to make an Old Navy run for that sweatshirt, eh?

400 MHz NMR spectrometer in St. Albert Hall (illustration credit to Connections digital education ecosystem)

400 MHz NMR spectrometer in St. Albert Hall (illustration credit to Connections digital education ecosystem)

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