What is a Magnet Quench?

Superconducting NMR magnets contain a large solenoid coil of superconducting wire in a closed loop. The wire is superconducting (i.e. passes current without resistance) only when cryogenically cooled by liquid helium. On installation, the coil is cooled below its critical point, a current is introduced by way of an external power supply until the specified magnetic field is reached. A superconducting switch is then closed forming a closed loop through which current perpetually flows without the need for an external power supply. Should any part of the wire increase in temperature beyond its critical point, the magnet will quench. During a quench, the wire becomes resistive and therefore generates heat. The magnetic field is lost. The heat boils off the liquid helium very quickly. Magnet quenches can be very dramatic.

To see a 900 MHz magnet quench, follow this link.

Measuring 2H NMR Spectra

Occasionally students will come to the NMR lab with the need to measure the ²H NMR spectrum of a synthetic product they are attempting to deuterate. Every so often, one will come with their product dissolved in a deuterium labelled solvent. This is not a good idea for the same reason it is not a good idea to run the ¹H NMR spectrum of a product dissolved in a protonated solvent. The solvent resonance will be orders of magnitude larger than the solute signal. When measuring the ²H NMR spectrum of a synthetic product, dissolve the product in a regular protonated solvent. Since there is no deuterated solvent, DO NOT ATTEMPT TO LOCK. You will have to shim the magnet as described here or use proton gradient shimming if possible. It is usually possible to observe the natural abundance ²H resonance of the solvent. This signal can be conveniently used as the chemical shift reference. The chemical shift scales are the same for ²H and ¹H, therefore the natural abundance ²H chemical shift of the solvent is (to a very good approximation) equal to the ¹H chemical shift of the same solvent. The figure below shows the ²H NMR spectra of two partially deuterated synthetic products run in chloroform and acetone. The natural abundance ²H signals of the solvents are highlighted in yellow.

Hartmann-Hahn Match as a Function of MAS Spinning Speed

Cross polarization with magic angle spinning (CPMAS) is a means of obtaining high resolution, high sensitivity NMR spectra of dilute isotopes in solids. The dipolar coupling between the protons and the dilute isotope to be observed ( typically ¹³C, ¹⁵N, ²⁹Si etc....) is exploited as a means for magnetization transfer from the abundant protons to the dilute isotope. The transfer is only possible when the product of the gyromagnetic ratio and applied power for the dilute isotope equals the product of the gyromagnetic ratio and applied power for the protons. For static or slow spinning samples, one can vary either the X or ¹H power level during the contact time and one will observe a maximum signal when the Hartmann-Hahn condition is met. The situation is more complicated when a sample is spinning at a rate comparable to or faster than the magnitude of the heteronuclear dipolar coupling used for the magnetization transfer. In such a case the MAS interferes with the dipolar coupling. The effect is that the Harmann Hahn matching curve (intensity vs X or ¹H power) is split into a series of maxima and minima separated by the spinning speed. This is illustrated in the figure below. When fast MAS CP experiments are to be used, it is important to set up the Hartmann-Hahn condition on a maximum for a standard sample at the same spinning speed to be used for the sample of interest.

NOESY vs ROESY for Large Molecules.

NOESY experiments work well for molecules of very low and very high molecular weight. They do not work well for molecules with molecular weights of approximately 1000 - 2000 g/mol at typical field strengths, where the NOE's are very close to zero. A ROESY experiment can be used to get NOE information for molecule in this intermediate molecular mass regime. For high molecular weight molecules a NOESY and a ROESY experiment will give very similar results with the exception that the cross peaks will be in phase with respect to the diagonals for a NOESY and 180 degrees out of phase with the diagonals in the case of a ROESY. The figure below shows both a NOESY and a ROESY for gramacidin at 300 MHz The red contours are negative and the black contours are positive.

Setting the Magic Angle

High resolution solid state NMR employs a technique called "Magic Angle Spinning" where the sample is spun at an angle of 54.736 degrees with respect to the magnetic field at a rate fast with respect to the interactions being averaged. In order to achieve high resolution, the angle must be set very precisely. This is commonly done by looking at the ⁷⁹Br signal of KBr. The intensity of the spinning sidebands for the satellite transition of the I = 3/2 ⁷⁹Br is strongly related to the precision of the magic angle. The sidebands appear as rotational echos in the ⁷⁹Br FID. The more rotational echoes - the stronger the sidebands and therefore the more precise the angle setting. Although any quadrupolar isotope with strong satellite transition sidebands can be used for this purpose, the ⁷⁹Br of KBr is particularly convenient as the resonance is very close to ¹³C, has a short T₁, and can be seen easily in one scan. The spectrometer is set up to scan without adding the signals and the angle adjustment is made while observing the FID until a maximum number or rotational echos is observed.

How to Assess a Used Superconducting Magnet

The superconducting magnet is the single most expensive component of an NMR spectrometer. There is no reason why a properly maintained superconducting magnet should not have a lifetime spanning decades. The console of an NMR spectrometer, on the other hand, begins to fail or becomes obsolete long before the end of the useful life of a superconducting magnet. As a result many laboratories find themselves buying new consoles for existing magnets. Also, when spectrometers are decommissioned, their perfectly good magnets may be available at a very much reduced price as "used". Acquiring a used magnet can represent a huge financial saving when obtaining an NMR instrument. It is useful therefore to address the points to consider when assessing a used magnet. These are outlined here.

1. Cryogen consumption - All NMR laboratories maintain detailed logbooks for the maintenance of their magnets. If you are considering a used magnet it is essential that you have access to these logs. They will detail the date of liquid helium and liquid nitrogen fills as well as quantities of cryogens used and regularly measured cryogen levels. From this information, you will know the boil off rates for both helium and nitrogen. A comparison of these boil off rates to the published boil off rates for new magnets will allow you to do a financial assessment of cryogen costs for the used magnet vs. the capital cost of purchasing a new magnet. It is very common for cryogen boil off rates to slowly increase over time. This is often the result of helium leaking into the vacuum through the magnet seals. The seals of magnets should be replaced periodically (every 10 to 20 years). This should also be considered when evaluating the cryogen boil off for a used magnet. The seals should always be replaced when recommissioning a used magnet.

2. Drift rate - Superconducting magnets are remarkably stable, however they do have measurable drift rates. It is important to know whether or not the drift rate is acceptable for you applications. Many laboratories will periodically measure the drift rate and keep a record of it.

3. Homogeneity - Superconducing magnets have cryoshims which are used to make the magnet homogeneous. This is done by the service engineer during installation. He/she will have noted the linewidth and lineshape (typically of water) the last time the magnet was charged up. The currents in the cryoshim coils will have also been noted. These currents should not be near their maximum values. Magnets are further shimmed by the user with a set of room temperature shim coils which are inserted into the magnet. NMR laboratories will routinely evaluate the lineshape and resolution using the globally accepted lineshape sample. If the same room temperature shim set will be used, this lineshape data should be compared to that for a new magnet. If the magnet is to be used for solids NMR, the homogeneity requirements are not as critical as for liquids NMR.

4. How was the magnet decommissioned? - The magnet should be decommissioned by a qualified service engineer. Magnets that are decommissioned by quenching may have suffered damage as a result of the quench.

5. Shipping - Superconducting magnets are very sensitive pieces of equipment and must be shipped only with the proper shipping restraints. Any magnet moved without the proper restraints should be avoided.

6. Stray magnetic fields - Most new NMR magnets are actively shielded and have very small stray fields. Many older magnets are unshielded and have much larger stray fields. It is essential that you know whether these stray fields are acceptable for your workspace. Unshielded magnets require significantly more space than shielded magnets.

Ultra-high Resolution NMR in the Earth's Magnetic Field

Many NMR spectroscopists take great pride in their ability to shim their very expensive magnets well and acquire NMR spectra with beautiful sharp lines. Despite the skill of the spectroscopist, the line widths are still limited by the homogeneity of the magnetic field. There are NMR instruments designed to measure NMR spectra in the earth's magnetic field. If these instruments are set up outside of any external magnetic interference, then the magnetic field at the sample (the earth's field) is extremely homogeneous and one can obtain very sharp NMR lines - much sharper than those easily obtained in an expensive superconducting magnet. In the figure below, the ¹H NMR spectrum of TMS is shown at 9.4T and in the earths magnetic field. The vertical scale of the spectrum has been increased to show the ²⁹Si satellites. Note that the spectrum obtained in the earth's magnetic field (Stephan Appelt, Holger Kühn, F. Wolfgang Häsing and Bernhard Blümich, Nature Physics 2, 105-109 (2006)) has far superior resolution compared to the one acquired in a well shimmed 400 MHz magnet. Of course, this resolution comes at a price - there is essentially no chemical shift dispersion at such low fields and the sample must be polarized by an external magnet before data are collected.

Field Dependence of 13C Line Shapes for Nitrogen Bearing Carbons in the Solid State

The frequency separation between the two peaks in the 1:2 doublets typically observed for ¹³C nuclei bonded to ¹⁴N in ¹³C CPMAS spectra is inversely dependant on the strength of the magnetic field. As a result narrower resonances are observed for nitrogen bearing carbons at higher field. The figure below shows the ¹³C CPMAS spectrum of D,L-alanine acquired on both a 200 MHz and a 500 MHz instrument.

High Resolution CPMAS NMR of 13C Bonded to 14N

In ¹³C CPMAS spectra, the resonances of ¹³C bonded to ¹⁴N can give unusual line shapes. They often show up as 1:2 doublets which should not be interpreted as seperate resonances. This effect is observed because the dipolar coupling between the spin I=1/2 ¹³C and the quadrupolar, spin I=1 ¹⁴N is not fully averaged by the magic angle spinning as is the case for a pair of spin I=1/2 nuclei. The appearance of the spectrum is a function of the quadrupolar coupling constant for the ¹⁴N, the orientation of the electric field gradient tensor with respect to the internuclear vector, the distance between the ¹³C and the ¹⁴N, the strength of the magnetic field and the ¹⁴N T₁ relaxation time. This effect is not specific to ¹³C and ¹⁴N, but a general effect between and spin I=1/2 and quadrupolar neulei. In the figure below, the 50 MHz ¹³C CPMAS spectrum of D,L-alanine is presented as an example. The resonance shaded and expanded in yellow is the carbon directly bonded to nitrogen.