Construction of a permanent magnet NMR spectroscope - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 05:31:20 pm
content: Hi!I've been taking some courses on NMR spectroscopy, and I wonder- Is it possible to build an NMR machine yourself?Obviously not wanting to deal with liquid helium and superconducting magnets, that'd be super dangerous, but I'm thinking a 1960's style permanent magnet machine.I'm just unsure what's involved in terms of electronic interfacing machinery - Is there any references on older NMR generators/aquisition gear? Magnet aside, as I see it, you'd only need a probe with the coils for RF pulse and FID signal pickup - then you'd need an RF pulse generator, and some manner of gear for collecting/processing the data, maybe even just a spectrum analyzer.It's more of a "can this be done" than a thing I'm actually starting, and it'd never rival anything scientific that has been used in the last 46 years, but it would be neat to see working.What are your thoughts? There has been a lot of development in tiny, tabletop NMR apparatus these last few years.--Christoffer

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2016, 05:51:07 pm
content: Shimming is hard, no matter how you do it, so there's that...Tempco is the hardest thing you'll have to deal with. Permanent magnets have a pretty good negative tempco, and may not be reproducible (i.e., exhibit hysteresis). Which means your center frequency is always changing, and if it's changing significantly (<1ppm!) during an acquisition alone, you're kind of screwed, as far as getting good data.Temperature regulation helps, obviously! But any residual (convection, drafts?) manifests as 1/f noise, which is hard to deal with.The first ones, I think, were electromagnet because it's easier to control, and more stable(!), thanks to a nice large lead-acid battery. The original articles by Purcell and Pound spoke of controlling the magnetic field by varying the position of the window (no doubt actuated by a grad student).FYI, it might be tempting to regulate magnetic field with feedback -- that's a good idea too, but mind its limitations. If your field sensor is quite noisy (as is the case of a cheap Hall effect sensor!), your results will only be worse. Perhaps considerably so!It would be nice to have a sample of a pure substance, usually H2O (? ppm) or TMS (0 ppm, by convention), and do a PLL servo using that as the field sensor and frequency standard. But that's tricky because the sensor has to be in the same place as your sample, physically speaking, and the extra reference signal probably means a poor SNR against whatever your test sample is. Producing, and shimming, a larger volume of magnetic field, is quite energy intensive, too.Mind that your solution doesn't need to be exclusively one or the other. You could use permanent magnets for the bulk of the field (a Halbach array at, say, 1-2T), and put electromagnets around it to tweak the XYZ fields (using perpendicular Helmholtz coils), and their gradients (by applying differential current to the Helmholtz coil pairs). This definitely saves DC power!Tim

author: max_torque
author_rank: Super Contributor
timestamp: August 22, 2016, 05:56:49 pm
content: This sounds like on of those interesting "Projects" where, by the time you get it working, you've either forgotten why you needed it, or it's morphed into an entirely different thing

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 06:19:36 pm
content: Quote from: T3sl4co1l on August 22, 2016, 05:51:07 pmThanks for that rundown! Yeah, a electromagnet-tweakable permanent magnet field might be the best course of action.The original oldies had the magnetic field varying to scan across the bands, while the newer just bursts an RF pulse that covers the entire band - that might be more efficient (and easier to implement in practice), the issue there is generating VERY short RF pulses...But if I read you correctly, you're saying getting a stable magnetic field is by far the biggest challenge.The good thing about having a short aquisition period is that temp. drift in magnets (and anywhere else) is minimized.Quote from: max_torque on August 22, 2016, 05:56:49 pmBut those are the best ones!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 07:28:28 pm
content: Obviously there'd be a lot of issues involved, but if goal #1 is to just SEE a free induction decay in a test reference, it doesn't sound completely unachievable.For an initial test magnet array, could ordinary commercial rare earth metal magnets be used, perchance? I think it must be either that, or see if an old NMR magnet is obtainable from a university or maybe Ebay.. Getting a custom made permanent magnet might make it more pricey than fun. Probes are relatively easy to come by too.

author: xygor
author_rank: Regular Contributor
timestamp: August 22, 2016, 07:39:11 pm
content: There was an article in Scientific American from the '50s or '60s on building one using a magnetron magnet. It was later republished in a book. If you use the correct RF frequency, it should be possible using the earth's magnetic field. The signal would be noisy though.Edit:Book:The Scientific American book of projects for the amateur scientist.Author: Clair L StongPublisher: New York, Simon and Schuster, 1960.Article:http://www.scientificamerican.com/article/the-amateur-scientist-1959-04/

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 07:56:05 pm
content: Quote from: xygor on August 22, 2016, 07:39:11 pmMagnetron magnet! not a bad idea! Thanks a bunch, that looks interesting.

author: langwadt
author_rank: Super Contributor
timestamp: August 22, 2016, 08:36:47 pm
content: Quote from: T3sl4co1l on August 22, 2016, 05:51:07 pmwhether you need 1ppm depends on what you want to do.electro magnets are weak, and SNR is proportional to field^(3/2) so you want the highest field you can getit might just be possible to make a 2T halbach array but even with high temperature magnetic material itstarts to permanently change at ~40'C

author: CD4007UB
author_rank: Contributor
timestamp: August 22, 2016, 08:38:58 pm
content: We have a 1960s' vintage Newport pulsed NMR spectrometer that we use in our UG lab for teaching NMR.It uses a permanent 'Watson' magnet and the NMR frequency is approx 2.3 MHz. The pulse generator is based on an astable at 4.6MHz (so as not to saturate the RF receiver), divided down by 2 with TTL logic during the excitation pulses. (Nowadays, of course, one could use an MCU for the pulse generation.) The RF receiver is a FET cascode followed by a video amp and a Ge diode detector to show FID and spin echoes, which can be recorded on a DSO.To make a Watson magnet, you need two approximately identical rectangular permanent magnets (magnetized parallel to a short edge). The magnets are sandwiched between two soft-iron sheets (a few mm or more thick), which act as pole pieces (N & S). The NMR sample and coils go inside the narrow cavity formed by the magnets and pole pieces. The field is fairly uniform over ~1cc because the soft iron acts as a magnetic equipotential - so, the pole pieces form the analogue of a parallel-plate capacitor.We operate with a vertical gap, so that the B field is horizontal. The RF pickup coil is then just a solenoid wound on a vertical cylindrical former. The excitation coil is slightly trickier, as it has to produce a uniform horizontal field (to rotate all the spins by the same amount), perpendicular to both the static B and the pickup coil. The solution to this problem is to use vertical windings on the cylindrical former to produce a current density that varies as aproximately cos(theta), where theta is the azimuthal angle. This produces the uniform horizontal RF field inside the coil former.With this setup, you can just about see the FID by inserting your finger into the excitation coil (but that's not really recommended). Glycerol is a better sample (fairly short lifetime, so no problems with saturation). It can be diluted with water to explore the amazing phenomenon of motional narrowing. Pure water does not work well (long lifetime, so tends to saturate). But a little bit of copper sulphate reduces the lifetime. All described in a classic paper by Bloembergen, Purcell & Pound.Our local hospital built a clone of our apparatus some years ago for training staff in the early years of MRI. We haven't replicated the setup with home DIY, but it would be possible if you can find the material to make a Watson magnet - a fairly uniform static B field is essential to see the FID (which decays as T2*, determined by field inhomogeneity). The electronics side should be fairly straightforward.Hope that helps.Neil.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 09:03:44 pm
content: Quote from: CD4007UB on August 22, 2016, 08:38:58 pmThat is certainly interesting! And having RF in the 1MHz-10MHz range solves a lot of trivial issues concerning my 80's instrumentation.Is it still the case with this type of magnet, that you have two magnets with a spacing of some centimeters?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 09:28:35 pm
content: I also found this:http://www.conspiracyoflight.com/NMR/NMR.htmlwhich has a lot of details on the electronic side of things.It might be good to develop an NMR receiving coil simulator (basically just a function gen.) to calibrate the 'tronics.

author: CD4007UB
author_rank: Contributor
timestamp: August 22, 2016, 09:43:54 pm
content: Quote from: ChristofferB on August 22, 2016, 09:03:44 pmYes, the permanent magnets are about 2cm from pole to pole, so there's an approx 2cm gap between the soft-iron pole pieces, which are about 10cm square. The coild former is mounted on a valve socket that fits inside the gap, and we insert a test-tube containing ~1cc of liquid as the sample, placed near the centre of the B field, where it's presumably most uniform.With this setup, you need field uniformity of ~1 part in a thousand (not ppm), so no shims are required with the Watson magnet.NeilP.S. An alternative DIY NMR approach is the proton-precession magnetometer described athttp://www.gellerlabs.com/PMAG%20Articles.htm. This uses a strong electromagnet to polarize the sample (H2O). When that is removed, the protons precess in the earth's field - but this has to be done outside, away from anything magnetic.

author: CD4007UB
author_rank: Contributor
timestamp: August 22, 2016, 10:09:27 pm
content: Quote from: ChristofferB on August 22, 2016, 09:28:35 pmWorking at 2.3MHz or so, you don't need to use double-balanced mixers or anything fancy. You could generate the pulses from an MCU (e.g., Arduino) and drive the excitation coil with a single-transisitor common-emitter amp (with the coil in a tuned LC circuit). Our RF receiver is a tuned FET cascode followed by a wideband op-amp (which only needs a few MHz bandwidth at ~40dB gain - you can't use a 741, but wideband op-amps are not hard to get hold of).We inject a test signal using a small coil in series with a 1k resistor soldered at the end of a piece of co-ax and driven by a signal generator. (An MCU could be used instead.) This is useful for initial tuning of the RF receiver. One can also beat a low-level injected signal against the FID. Observing the beats on a scope allows very precise measurement of the FID frequency, and hence precise determination of the B field.Neil.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 10:39:06 pm
content: That does sound good. The largest issue with the article I linked is the 100W RF power amplifier - Seems like that'd be less desirable.Keeping it at a couple of MHz might be a wise plan as of first. I assume you'd always be able to ramp up the freq. and thus resolution with more advanced parts, reusing magnet and probe.Now finding a proper magnet / materials to make one..

author: CD4007UB
author_rank: Contributor
timestamp: August 22, 2016, 10:49:27 pm
content: Quote from: ChristofferB on August 22, 2016, 10:39:06 pmSince the magnetic field determines the NMR frequency, with a permanent magnet you have to stick with more or less the same frequency - the hospital system that we worked on had a small coil wound on one magnet so that the field could be tuned, but only by a tiny amount.As NMR frequencies for research have increased over the years, our university has scrapped some large high-quality permanent magnets (unfortunately, much too large and heavy for home use). So, when you up the frequency, you need to buy a lot of new kit. For teaching purposes, there's never been much point in us going above the original 2.3MHz.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 10:56:42 pm
content: Hm. That's true, thinking about it. Is there a relatively painless way of determining the appropriate frequency for a given magnet?

author: CD4007UB
author_rank: Contributor
timestamp: August 22, 2016, 10:58:14 pm
content: Quote from: CD4007UB on August 22, 2016, 10:49:27 pmOf course, this assumes that you stick with proton NMR. In principle, you can change to a different nucleus and hence work at a different frequency with the same magnet. I don't know if this is actually practical with our simple equipment, as it relies in part on the high concentration of protons in H2O etc to get strong enough signals.

author: langwadt
author_rank: Super Contributor
timestamp: August 22, 2016, 11:10:50 pm
content: Quote from: CD4007UB on August 22, 2016, 10:58:14 pmthere are lots of other nucleus but they all have lower sensitivity and you have a probe tuned for thefrequency

author: CD4007UB
author_rank: Contributor
timestamp: August 22, 2016, 11:19:50 pm
content: Quote from: ChristofferB on August 22, 2016, 10:56:42 pmThe B-field, determines the Larmor frequency for the nuclear precession. For protons, this comes out as about 42.58 MHz/tesla.There's a table herehttp://kodu.ut.ee/~laurit/AK2/NMR_tables_Bruker2012.pdffrom Bruker, who make NMR spectrometers (starting at a field of about 7 teslas!). You can see how modest our 2.3MHz setup is compared to modern equipment - but it's still great for learning about NMR.Neil.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 11:25:08 pm
content: Quote from: CD4007UB on August 22, 2016, 10:58:14 pmBut you can get combination probes, with outputs for multiple nuclei. I've seen a 1H, 2H, and 13C combi probe once, i think.I actually assumed 13C would be more efficient, since they're usually taken at lower frequencies.With the proton concentration in H2O, you could use another solvent to get a nice 13C peak. CDCl3 is sometimes used, TMS would be good too.

author: zappedy
author_rank: Regular Contributor
timestamp: August 22, 2016, 11:36:26 pm
content: This might be stupid, but can't you just capture a s**tload of data and then simply chose to look at a sub sample where the temperature and other parameters happened to be stable? They couldn't do that in the 60s, perhaps that could be a way to "cheat" using modern consumer computer tech.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2016, 11:38:36 pm
content: That's not a bad idea, and I did plan on using computer-aided data aquisition. The issue with NMR is that you sometimes need to average the spectra over a long period of time (hence why modern NMR experiments can take up to 48Hrs!).Definately an option over building an analog system to do FT and then 2d pen-plot the spectrum.

author: CD4007UB
author_rank: Contributor
timestamp: August 23, 2016, 12:00:50 am
content: Quote from: ChristofferB on August 22, 2016, 11:38:36 pmThis is possible with research equipment, but way beyond what's possible with the setup that we use. For FT-NMR spectroscopy, you're looking at the chemical shift between protons (or other nuclei) in different chemical environments. That's ~ppm, so I think you need corresponding field homogeneity of ~ppm. With our setup, the field homogenity is ~parts in 10^3, so you can't distinguish between -CH2, -CH3 groups etc. A research chemist would therefore not regard our pulsed NMR apparatus as a useful 'spectrometer'. But it is, nontheless, useful for looking at the FID, spin echoes, measuring T1, T2, T2* and exploring things like motional narrowing and paramagnetic relaxation - the basic physics of NMR, rather than its use in chemistry.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 23, 2016, 12:30:44 am
content: True. You probably wouldn't get anything sensible out of FT'ing a 2,something mhz spectrum.Even if you had the fieldwise precision, it'd likely be all muttled.

author: zappedy
author_rank: Regular Contributor
timestamp: August 23, 2016, 12:34:40 am
content: Sample a lot of parameters and put an FPGA genetic algorithm / neural network AI on the stabilization job?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 23, 2016, 12:50:35 am
content: Can't really be done. It's the issue of signals physically overlapping, and thus becoming indecipherable. That's why professional instruments go UP in operating frequency. it gives more room between signals.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2016, 01:13:45 am
content: Note that excitation frequency isn't very critical.Consider the electrical circuit. The nuclear spins manifest as resonances coupled to the coil.It behaves much like a quartz crystal very weakly coupled to a coil. You can cancel out the coil's parameters (reactance and loss) using an impedance bridge (first, a capacitor to cancel reactance, then a complementary arm of the bridge with the same L and C, but a variable resistor to adjust loss to match). What's leftover is higher order effects, like skin effect, cable and chamber resonances, etc. Most of which vary slowly with frequency, so are easily corrected for, over the narrow frequency range needed here.The tiny peaks in that narrow ~Hz wide window are your signal: a measurement of atomic resonances.To resolve the peaks, as a practical instrument, you need quite good resolution, at least 0.1ppm I would guess.If you just want to see a peak, a somewhat more crude magnet and receiver can be used. (This is definitely a development milestone!)Conversely, the stimulus doesn't need to be very special. The burst can be quite long in real time, as long as it's a suitable fraction of the FID time constant. The Fourier transform of a rectangular windowed tone burst is a sinc(F) spectrum, centered at the tone's frequency. More pulse duration means narrower bandwidth means more power at the frequency it's needed; shorter duration means more bandwidth and less precision needed in lining up stimulus and actual resonance.Tim

author: zappedy
author_rank: Regular Contributor
timestamp: August 23, 2016, 02:27:58 am
content: I was more thinking that you would use a couple of calibration / control samples and let an AI learn how to drive the various parameters to get a better signal than would have been available in the 60s, from a 60s type instrument. Replacing the poor grad student with a computer positioning the window 100000 times per second in just the right pattern. Or possibly letting the AI learn how to interpret data based on the temperature fluctuations, etc. Again based on known control samples.This would probably be difficult to accomplish and I'm sure somebody who is experienced in AI stuff is laughing at me right now, it's just a half baked idea of again using modern computer technology to "boost" old tech while keeping it within the reach of DIY.Anyway, I don't know anything about spectrometers except what I learned in high school and the odd article in science magazines so I'll quietly leave this thread...

author: knotlogic
author_rank: Regular Contributor
timestamp: August 23, 2016, 06:59:18 am
content: Quote from: ChristofferB on August 22, 2016, 05:31:20 pmPossible, but a lot of work I would have thought. I think a few years back there was a news article making the round about a guy who build a low-field imager at home and was just about able to image his torso. I didn't see the images, and I don't know if you could actually make anything out, but still...TheAberdeen Mark-Iwas the clinically used scanner (First clinical scanner?) That was a water cooled electromagnetic system. I also knew some guys doing low field work at university. IIRC, one of them build a permanent magnet imager for his PhD. You do want better field homogeneity for spectroscopy than for imaging though.I was at a place that built a specialised 0.5T imaging unit. Two permanent magnets and an iron frame to channel the field. Samples had to be under an inch in size though.

author: knotlogic
author_rank: Regular Contributor
timestamp: August 23, 2016, 07:07:12 am
content: Quote from: ChristofferB on August 22, 2016, 11:25:08 pmIf you're imaging tissue (human or animal) there's a lot more hydrogen than carbon available, so a lot more signal there. I suppose the same applies to organic chemistry and spectroscopy. Also, the relative abundance of carbon-13 is 1.1%, which makes things a whole lot worse. A chemist told me that it wasn't uncommon to do a ton of signal averaging with some samples just to be able to make out the spectroscopic peaks.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 23, 2016, 10:20:53 am
content: I thought the RF burst had to be short, in order to uncertainty out over the entire spectrum? In earlier models, the magnetic field was varied. I was thinking maybe simplifying things with a tracking generator.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 23, 2016, 04:15:01 pm
content: For a normal NMR the exciting pulses don't need to be that short. The length of the pulse determines the bandwidth that is excited. Usually the needed bandwidth is still relatively small, e.g. way less than 1% - so pulses can be 100 periods at least. It does not make sense to make the pulses longer than the relaxation time though, but up to that limit the signal output gets higher, though it might mean one would need different excitation signals to cover the whole range, e.g. to find the peaks at first.A more difficult part is that the excitation might need quite some power. So I would guess the main difficulty for excitation is the power amplifier and matching to the probe. Signal generation might be as simple as a digital pattern. Or at lower frequencies a DDS chip controlled by an µC. Today even the Pulse pattern for pulse echo measurements is not that difficult to produce.For a permanent magnet version I would keep the field constant and adjust the frequency as needed. Adding an electromagnet generates heat and thus caused thermal drift. So if at all have an large external coil to compensate for an changing external (earth) field - no need to cancel out, just keep it constant. It's still a question if the sensor causes more noise than the natural changes in field. Changing temperatures can also influence flied homogeneity.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 23, 2016, 04:44:28 pm
content: Keeping the field constant is a good idea. I think my first experimental setup would be a permanent magnet, maybe with a small tuning coil, and then 3D Helmholtz coils (hula hoop ring based?) around it for homogeneity. Does Helmholtz coils need anything special to drive them? Or do you just throw some DC at them all? I've never actually handled them in practice.

author: Wolfram
author_rank: Frequent Contributor
timestamp: August 24, 2016, 08:42:02 am
content: Quote from: xygor on August 22, 2016, 07:39:11 pmThe whole book is available for free on archive.org:https://archive.org/details/TheAmateurScientist

author: KE5FX
author_rank: Super Contributor
timestamp: August 24, 2016, 11:01:29 am
content: Quote from: Wolfram on August 24, 2016, 08:42:02 amHere's an actual decentscan. (45 MB) The NMR article begins on page 355 of the .PDF.

author: knotlogic
author_rank: Regular Contributor
timestamp: August 24, 2016, 03:13:47 pm
content: I'm not sure about spectroscopy, but in imaging the length of the pulse affects the width of the slice (in the presence of a slice select gradient).Quote from: ChristofferB on August 23, 2016, 04:44:28 pmSay a pair of permanent magnets? Sample placed in between them. Some sort of thermal stabilisation for the magnets, otherwise the field drifts. Field inhomogeneities are handled with shim coils, they're driven with DC I think. It's not an area I'm familiar with either, but there are a few - XY, XZ, YZ or something like that, and more for higher orders.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 24, 2016, 08:38:31 pm
content: KE5FX: thanks for the scan! That's not bad reading, and not only for the NMR stuff!Quote from: knotlogic on August 24, 2016, 03:13:47 pmThat was something similar to what I was thinking. The largest problem remains in finding a good magnet. Some older homebrew's uses magnetron magnets, but I think microwave oven ones are too puny..

author: KE5FX
author_rank: Super Contributor
timestamp: August 24, 2016, 10:56:24 pm
content: Quote from: ChristofferB on August 24, 2016, 08:38:31 pmYeah, it's an awesome book. Lots of politically-incorrect experiments for the doughty amateur.QuoteI wouldn't mess with those old alnico jobs unless there's some other reason to (like field homogeneity?)Modern Nd magnetsare 10x stronger than the WWII-era magnetron magnet in the Amateur Scientist article.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 25, 2016, 06:18:06 pm
content: Nah, you're very right, you can get a lot juicier magnets today, but if one were to use a stack of common modern neodymium's, i'd just think they would be more poorly made, and be harder to get to behave homogeneously. Maybe if you mounted them on a large block of soft iron...

author: Akra
author_rank: Contributor
timestamp: August 25, 2016, 08:16:00 pm
content: If you use the magnets with an iron yoke (or other ferromagnetic material), you probably can get a much better and more homogeneous field.Don't know if this would get homogeneous and stable enough...But even a lab NMR needs to lock the frequency to an known standard (solvent signal) for every new sample.If i remember correctly, than there are handheld NMR devices. They should be low resolution, so you don't see coupling,but you can determine what elements are in your sample.The high resolution NMR used in the lab gets really fast really complicated...PS: Would really love to see someone do something this complicated (but really awesome) at home.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 26, 2016, 04:31:49 pm
content: Neat project. I've built ribbon speakers using magnets which could be used for this project.To take this to a realistic level you will need to model the magnetic field. I suggest;http://www.femm.info/wiki/HomePageIts, good, free software.If you can come up with a design using some number of Neodymium N42 rectangular magnets of size 0.5 inch x 0.5 x 2 inches, I will donate them to you(*). The scientific american article calls for 1450 gauss which should be practical. The hard part is getting the field uniform using obtainable materials. Usually I use 1018 steel or similar. Of course, constructing something using individual magnets is pretty difficult and dangerous. (Two magnets of this size in free air can amputate small digits when they come together). Be aware that is very difficult to construct a design where a N-N field is in close proximity along the long edge of a magnet. The 0.5 edge is do-able.Magnets i have are like these:http://www.kjmagnetics.com/proddetail.asp?prod=BY088As you can see, not cheap. Obviously, bigger magnets are better but also very much more expensive.Randy(*) Reasonable restrictions apply. i have about 25 available and will not pay shipping costs. Donation obligates recipient to do good documentation!

author: LaserSteve
author_rank: Super Contributor
timestamp: August 26, 2016, 07:37:08 pm
content: Early References:1. N. Bloembergen, E. M. Purcell, and R. V. Pound, Phys. Rev. 73, 679 (1948).2. G. E. Pake, J. Chem. Phys. 16, 327 (1948).3. P. Grivet, M. Soutif, and R. Gabillard, Physica (Utr.) 17, 420 (1951).4. F. Bloch, W. W. Hansen, and M. Packard, Phys. Rev. 70, 474 (1946).5. N. J. Hopkins, Rev. Sci. Instrum. 20, 401 (1949). [ CAS ?]6. R. V. Pound and W. D. Knight, Rev. Sci. Instrum. 21, 219 (1950).7. E. Lustig and W. B. Moniz, Anal. Chem. 38, 331R (1966).8. B. V. Rollin, Nature (Lond.) 158, 669 (1946).9. Gabillard, C. R. Acad. Sci. (Paris) 232, 324 (1951).Steve

author: ale500
author_rank: Frequent Contributor
timestamp: August 27, 2016, 04:48:45 am
content: TMS, tetramethylsilane is only used as a reference for 0 ppm shifts, not as a solvent. As a solvent you need something without H, if you want to measure protons, to avoid having a strong H signal. Anyways modern machines use the residual H in deuterated solvents to calibrate the scale, TMS is not required anymore.I used to work with a 400 MHz Bruker machine (like 15 years ago), it needed manual shimming, lower frequency machines had automatic shimming.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 27, 2016, 07:23:59 am
content: The old AlNiCo Material has 3 good properties: It has lower temperature dependence than other modern magnets. But I think by keeping the temperature reasonable constant this is not a big problem. Also the mechanical strength is quite good, but usually the magnets are under mechanical compression, so not a problem anyway. The higher (differential) permeability of Alnico (about 3 compared to about 1) can be an advantage when, combining it with an electromagnet - however this might not be a good idea anyway, if one needs to keep the temperature constant. I would consider extra coils only for shimming and maybe compensation of changes in the external field - so more a low power part.Using the FEM simulation is a good idea, as the geometry can be different from old designs using AlNiCo or ferrite magnets. This is because the Nd magnets are usually magnetized along there short axis.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 29, 2016, 04:50:42 pm
content: Thanks for all the feedback! there's more than enough to go on!- RandallMcRee: That's very generous, but this is more of a "collect parts and ideas over 10 years" project for me. There'll probably be a loong time before I start on anything actually resembling a project.