University of Ottawa NMR Facility Web Site

Please feel free to make suggestions for future posts by emailing Glenn Facey.

Tuesday, March 26, 2013

Removing t1 Noise from Heteronuclear 2D NMR Data - Video Tutorial

The often troublesome stripes of vertical noise in 2D NMR spectra are called t1 noise (i.e. noise originating in the t1 domain). When t1 noise occurs in hereronuclear 2D correlation experiments such as HMBC, HSQC, HMQC or HOESY, there is a simple trick to remove a great deal of the noise and make the data more presentable. The technique was described in a previous post and is demonstrated in this video tutorial using a 19F - 1H HOESY spectrum as an example.

Monday, March 18, 2013

Exponential Line Broadening - Video Tutorial

Exponential line broadening is an important NMR data processing tool.  It involves multiplying the time domain signal by a decaying exponential function prior to Fourier transforming the data into the frequency domain.  It is used to improve the signal-to-noise ratio and is more fully described in a previous post.  The following short tutorial video demonstrates its use.

Wednesday, March 13, 2013

Phasing a 2D NMR Spectrum - Video Tutorial

The following video demonstrates how to phase a 2D NMR spectrum in TOPSPIN 3.

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 cryogenically 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.