University of Ottawa NMR Facility Web Site

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Thursday, March 7, 2013

Thermal Noise in NMR Data

The University of Ottawa has recently been funded for a 600 MHz NMR spectrometer with a cryogenetically cooled probe.  Cryoprobes differ from conventional NMR probes in that the rf circuits and preamplifiers are cooled with cold helium gas while the sample is maintained at ambient temperature.  The benefit of cryogenically cooled electronics compared to room temperature electronics is that the thermal noise in the system is reduced at cryogenic temperatures while the NMR signal remains constant for the sample at ambient temperature.  The signal-to-noise ratio in an NMR spectrum acquired in a cryoprobe is therefore increased dramatically compared to a conventional probe, typically by a factor of 4.  This allows for data collection times on the order of 16 times shorter than those using conventional probes as well as lower detection limits.  This principle can be crudely demonstrated by replacing the NMR probe with a 50 Ω  load and collecting "NMR" data on the load at both high and low temperatures.  The "NMR spectra" in the figure below were collected (without using an rf pulse) on a 50 Ω load outside of the magnet at room temperature (left panel) and in a dewar of liquid nitrogen at 77 K (right panel).  The noise collected in the 77 K spectrum is 35% lower than that in the room temperature spectrum demonstrating the lower thermal noise at lower temperatures.


This effect is dramatically increased in a crypoprobe which cools the electronics of both the rf probe circuits and preamplifiers to temperatures much lower than 77 K.

4 comments:

Anonymous said...

Is this considered Johnson noise that is being eliminated by cooling the electronics?

Glenn Facey said...

Anonymous,

Thank you for the question. Yes, Johnson noise and thermal noise are synonynmous terms.

Glenn

Unknown said...

Glenn,

Nice little demonstration of thermal noise. One little caveat in your description of the cryoprobe system, however, is that the preamplifier electronics are only cooled to 77K in the Bruker systems (I can't comment on the Varian or Jeol systems). The NMR coil is cooled down to about 15K and then the cool helium gas is warmed up to 77K in the cryoplatform and pumped back out to the preamp module before returning to the coldhead.

Great blog by the way, I look forward to the new posts.

Cheers,

Ian.

Glenn Facey said...

Thanks for the information, Ian.

Glenn