University of Ottawa NMR Facility Web Site

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Friday, May 24, 2019

Fast 2D Data Collection - NOAH and NUS

One always strives to collect high quality 2D NMR data in a short period of time.  This is particularly important for samples of limited stability or perhaps for monitoring chemical reactions.  High magnetic fields and cryogenically cooled NMR probes have allowed for a higher signal-to-noise-ratio for a given quantity of sample, thereby reducing data collection time as a fewer number of scans are required.  Gradient enhanced 2D NMR data collection gained widespread use in the 1990s.  This represented a tremendous time saving as multi-step phase cycles required for coherence selection could be reduced or eliminated as they were replaced by pulsed field gradients.  Some pulse sequences which required 16 scans per increment to accommodate the necessary phase cycle could be run with a single scan for every increment with the use of pulsed field gradients, thus reducing the data collection time by a factor of 16.  Now, 2D data collection with coherence selection via pulsed field gradients is considered "conventional".  More recently, Non-Uniform Sampling (NUS) was introduced.  Data collection with this technique samples only a limited number of increments in the t1 domain.  The unsampled increments are calculated based on the sampled increments prior to Fourier transformation. The data collection time is reduced in accordance with the number of increments not sampled.  Recently, Kupce and Claridge1,2 have developed a technique where multiple 2D methods are concatenated in a single super pulse sequence employing a single relaxation delay. They have called the technique NOAH (NMR by Ordered Acquisition using 1H detection) The time saving of the NOAH technique compared to individually collected 2D spectra results from waiting a single relaxation delay for all experiments rather than a single relaxation delay for each separately acquired spectrum. The data for each spectrum is acquired in separate memory blocks which are separated after data collection allowing the data for each 2D method to be processed individually.  Very recently, both NUS and NOAH have been used together to further reduce data collection times3.  A comparison of the time saving is shown in the figure below for a sample of sucrose in DMSO-d6 collected on a Bruker AVANCE III HD 600 NMR spectrometer equipped with a cryoprobe.
All spectra were collected with 2 scans and a 1 second recycle time.  Individually, both NOAH and NUS offer a significant time saving but when used together they permit very fast, high quality data collection.  A COSY, edited HSQC and HMBC can be collected in a total time of only 4 minutes and 8 seconds.  Other ultra-fast techniques have been developed by others where an entire 2D spectrum is collected in less than one second.

1. Eriks Kupce and Tim D. W. Claridge.  Chem. Commun. 54, 7139 (2018).
2. Eriks Kupce and Tim D. W. Claridge. Angew. Chem. Int. Ed., 56, 11779 (2017).
3. Maksim Mayzel, Tim D. W. Claridge and Ēriks Kupce. Bruker User Library (2018).

Tuesday, April 2, 2019

Dying iPhones and Liquid Helium Fills

As a habit I do not expose my iPhone to the large stray magnetic fields of high-field or unshielded NMR magnets.  I do however feel safe carrying it near low-field shielded magnets with 5 Gauss stray fields within the croyostat of the magnet. That is - until lately.  Last year, after topping up the liquid helium on a 300 MHz shielded magnet in a fairly small room, I noticed that my iPhone 8 had become completely unresponsive.  The only stimulus it appeared to respond to was gravity.  As it was under warranty, I sent it back to Apple.  After a week or so, they sent it back to me with a note saying that there was nothing wrong with it.  I found this very strange and did not make a connection between the helium fill and the problem with the phone.  I had done many helium fills in the past while carrying an older iPhone 5.  Approximately 9 months later, my iPhone 8 suffered a similar problem again after filling the same shielded 300 MHz magnet with liquid helium.  This time, I took it to a local Apple Store while it was dead.  The technician examined it, ran it through a software protocol, confirmed it was dead and issued me a new phone as it was in its last weeks of its warranty.  Shortly after this, I read about problems others have had with iPhones and Apple watches around helium gas and finally made the connection between the problem I was having and my helium fills. Some Apple iPhones (apparently, iPhone 6 and higher) will completely die when exposed to helium gas. As if in the spirit of Easter, however, they will resurrect themselves after the helium has dissipated from the phone and the battery has been allowed to discharge.  The problem is that in newer iPhones, Apple has swapped out a quartz oscillator, used in older versions of the phone, with a microelectromechanical systems chip (MEMS) which is sensitive to the presence of helium gas. This sensitivity is indeed mentioned in the User Guide of the iPhone.

Exposing iPhone to environments having high concentrations of industrial chemicals, including near evaporating liquified gasses such as helium, may damage or impair iPhone functionality. Obey all signs and instructions. 

Android phones apparently do not use MEMS and therefore are not vulnerable to the problem. Several weeks ago, I absent-mindedly entered a room with my iPhone 8 while a magnet was being filled with liquid helium.  Again, the same thing happened.  This time, I allowed the phone to sit for a week after which I was able to charge it.  After charging, it worked well with no loss of data.

Warning: If you see that a magnet is being filled with liquid helium, Do not enter the room with an iPhone 6 or higher.

Thursday, January 17, 2019

Comparison of Broadband Decoupling Schemes

Many NMR measurements such as HSQC or HMQC rely on broadband X nucleus decoupling (X = 13C, 15N, 31P .... etc.)..  Broadband decoupling schemes, using conventional rectangular pulses (e.g GARP) require fairly high power levels leading to undesired sample heating.  They are also limited in their effective decoupling bandwidth.  Adiabatic decoupling schemes use shaped adiabatic pulses and have become more and more common over the last couple of decades due, in large part, to the flexibility of modern NMR instruments to generate shaped pulses.  Adiabatic decoupling schemes (e.g. WURST) use much less power than those using conventional rectangular pulses. thereby reducing or eliminating problems associated with sample heating.  Due to the lower power requirements and increased effectiveness over wider frequency ranges, adiabatic decoupling schemes are ideally suited for X nucleus decoupling at higher field strengths. The figure below shows 500 MHz 1H [31P] NMR spectra measured with inverse gated decoupling for the P-CH3 methyl resonance of dimethyl methylphosphonate  The single scan spectra were collected in a pseudo-2D fashion, as a function of the decoupler offset frequency from -256 ppm to +256 ppm from the 31P resonance frequency in 1 ppm steps.  The acquisition time and recycle time for each FID were 2 sec and 4 sec, respectively.   In the right-hand panel, broadband GARP decoupling was employed at a power of 1.22 W  (60 µsec 90° pulses).  In the left-hand panel, WURST decoupling was used at a peak power of 0.755 W (2 ms WURST pulses, bandwidth = 250 ppm).
Clearly, the data collected with WURST decoupling, at lower power, have a much larger decoupling range (250 ppm) compared to the data collected with GARP decoupling (98 ppm).  Furthermore, while the GARP data were collected, the sample temperature increased and had to be compensated for by the variable temperature unit.  No such temperature increase was observed while collecting  the WURST data.  It is also interesting to note that, in the case of GARP decoupling, distorted line shapes are observed just outside of the decoupling range, while for WURST decoupling, the spectra are fully coupled just outside of the decoupling range with a very sharp transition between being fully coupled and fully decoupled.  For broadband decoupling, WURST is best!