Monday, March 17, 2008
Probe Coil Geometry
The NMR sample sits inside a coil within the NMR probe. The coil is part of a tuned circuit which delivers radio frequency (RF) pulses to the sample and detects the RF signals from the sample. The RF fields delivered by and detected from the coil must be perpendicular to the static magnetic field, Bo. The most efficient way to do this is with a horizontal solenoid coil. Such coils are commonly used in NMR probes for solids. The probe must be removed from the magnet when samples are changed. A less efficient, but more convenient, Helmholtz coil is commonly used for liquids samples. Like horizontal solenoid coils, vertical Helmholtz coils deliver and detect RF fields perpendicular to Bo, but due to their geometry they allow samples to be inserted and ejected form the probe with a pneumatic system. The probe need not be removed from the magnet to change samples.
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4 comments:
Glenn, reading this post made me think about MAS probes where you have a solenoid coil but the coil is at the magic angle rather than horizontal. Perhaps I'm being naive but this must mean the rf is not perpendicular to Bo and therefore these probes are less efficient than, say, a wideline probe. Would you agree?
nmrfreak,
You are anticipating a future BLOG post! You are indeed correct. The efficiency of an MAS soenoid coil is only sin(54.7) = 81.6% that of a horzontal solenoid.
Glenn
Dear Glenn,
I am trying to get quantitative data from solution NMR with samples of different lengths in standard and Shigemi tubes.
I wonder if the scaling of the peak integrals should be linear with the length of the sample (compared to the coil length) when using a Helmholtz coil, or if calibration should be performed...
In other words, imagine a reference sample of known concentration in either a full length standard tube (2-3 x the coil length), a Shigemi tube covering the length of the coil (e.g. 18 mm), and a Shigemi tube of only a quarter of the coil length (18/4 = 4.5 mm). Will the first two samples give the same integral ? And will the third sample give an integral 4 times smaller ?
Thanks a lot for your insights !
Séb :)
Sebastian,
Would it not be easier to put an internal concentration standard in your sample to use as an integration standard? In this case you would be measuring relative integrals within the sample and the whole problem of sample size would be eliminated.
I do not think that the scaling of the integrals would be linear with sample size. Not only is the excitation not perfectly linear inside the entire coil but you would also have to worry about differences in probe tuning and matching from one sample to another affecting the signal intensity. NMR is not easily adapted to "absolute" quantitation but is very easily adapted to "relative" quantitation.
In your last paragraph you asked, "Will the first two samples give the same integral ? And will the third sample give an integral 4 times smaller ?" I think the answer is no.
Glenn
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