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aliced25
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@ not_important
Thank you, that is pretty much what I was getting at. However I like the idea of the high multiplication/division rate on those PLL's, by using a
simple SAW Filter at a high wavelength one could reduce the signal width enough that when it was fractionated on the next PLL, the bandwidth was
smaller than is possible with DDS Chips. Shit, it may even be possible to utilize the pretty weak DAC's on many DSP's (using the onboard memory &
perhaps even the system clock). That removes two parts and leaves us with a monotonic signal on a narrow bandwidth, highly accurate wavelength. I have
seen numerous articles where the shape of the pulse produces different effects in MRI especially, but also with NMR. I do wonder how a simple sawtooth
pattern would run, it wouldn't be a perfect SINE wave, but it would approximate one. By the time it is put through a phase filter, multiplied to the
shithouse, filtered and then fractionated down to the target wavelength, I wonder what it would look like?
@ Twospoons
Yeah, but Lead Based solder & Bromine/Chlorine containing Dielectric "were" cheap because everyone did it too...
I strongly suspect the end is nigh for the etched PCB, bucketloads of waste polymer, more buckets of toxic waste from the etching process & a
board that cannot be recycled as anything other than fill.
IC manufacturer's use wire almost exclusively to join pressed metal components, which are then sandwiched into a small plastic box.
The size of the traces is going to be the start of the end for them anyway, 0.25mm traces are just way too small for decent signal quality, but they
are all that will fit with BGA's.
I strongly suspect Circuit Boards may go back to being wired, many components will have to be rejigged to enable this to be done in the most
expeditious manner, but the integrating circuit between the IC's is getting to be an IC in it's own right, wiring reduces the expense involved in
applying copper, then masking, exposing, cleaning, etching, cleaning then tinning, in terms of home users, they are going to be stuck between a rock
and a hard place. The Green's will control the Senate here (from the next changeover date, used to remember it, now I cannot be fucked). There is
little prospect that they will look favorably on any move to clean up the Electronics industry - changing to dielectric material that can be dissolved
in solvents, leaving behind the SMD's, IC's and Copper as solids, ready for reuse/recycling, which does not generate massive amounts of toxic and
unrecyclable waste during production, is a bigger step in that direction then Lead Free, Halogen Free Components.
As to the ease of mass production - mass producing circuit boards using the same technology used now to make smaller and smaller IC's, predominantly
as a means of connecting those IC's to one another - is no great stretch to imagine. Robots are used to locate, weld, string, cut & weld the wire
connectors between the pressed metal pads in IC's now. There is no solder (and little heat - they use Ultrasonic Welding/etc.). Why it should be any
more difficult for major manufacturer's to tool up to make bigger circuits, with higher clearances, etc. is beyond me (in fact, I suspect it may be
the next, necessary step in order to increase board density).
@ arsphenamine
There is a paper on the previous page from someone who tried to measure the field from 4 Neodymium Cubes (~0.6T IIRC) with a Hall Sensor. Their
results were indifferent, so they tested the field by reference to the resolution they could achieve with a syringe full of water. In terms of testing
techniques & sensors, that looks like it would be the most accurate by far (while it is rather easy to equate the testing procedure to the
end-use).
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watson.fawkes
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Quote: Originally posted by aliced25  | | There is a paper on the previous page from someone who tried to measure the field from 4 Neodymium Cubes (~0.6T IIRC) with a Hall Sensor. Their
results were indifferent |
Of course they were indifferent. If you're trying to measure a spatial derivative
of B, you don't go about measuring B in two different places and then subtracting. A Hall effect sensor measures B directly. If you had a measuring
device of infinite resolution, this might work, but since you don't, it doesn't work. If you're looking at inhomogeneities in the range 10E-4 or so,
it means you're knocking off 40 dB of dynamic range right off the bat, before any other noise sources. What you need is a differential sensor that
senses spatial derivatives of B directly.
I've thought of three ways of doing this. A pair of Hall sensors and a differential amplifier is conceptually feasible, but you've got to worry about
induced EMF in the other conductors, for example. The best way to deal with this is to make everything small and put everything on a single chip, but
I don't have a fab available.
You can mechanically vibrate a loop or coil in the field and rely on electromagnetic induction. This directly measures delta-B because the induced EMF
is proportional to the change in magnetic flux. Since we can assume the area of the coil doesn't change, the signal comes only from delta-B. This all
assumes that the axial direction of the coil is parallel to the direction of motion.
You can also change the induced EMF in a coil by switching a diode in and out of conduction. This requires some fancy electronics, because you have to
measure induced EMF on top of a changing forward bias current.
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arsphenamine
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| Quote: Originally posted by aliced25 |
There is a paper on the previous page from someone who tried to measure the field from 4 Neodymium Cubes (~0.6T IIRC) with a Hall Sensor. Their
results were indifferent, |
It means they had not the will to try a little harder.
Even the humble Allegro linear Hall sensors can resolve to .1 G (1e-5 T) if you take care of external noise adequately.
The 150µV noise floor over a 4.5 Volt range is ~15 bit resolution. Unfortunately, they only have a +/-1400 Gauss range at most.
| Quote: | | so they tested the field by reference to the resolution they could achieve with a syringe full of water. In terms of testing techniques & sensors,
that looks like it would be the most accurate by far (while it is rather easy to equate the testing procedure to the end-use). |
The reported value was necessarily taken from the median of a broad peak.
There was no mention of the peak's Q which means there were no hard figures on the field homogeneity.
They told us where the ballpark was but couldn't find the bases.
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aliced25
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That might be so, it is still the best route I can see to working out where the bases should be.
Anyone got any ideas on the shape & placement of Shim coils?
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Twospoons
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| Quote: Originally posted by aliced25 |
As to the ease of mass production - mass producing circuit boards using the same technology used now to make smaller and smaller IC's, predominantly
as a means of connecting those IC's to one another - is no great stretch to imagine. Robots are used to locate, weld, string, cut & weld the wire
connectors between the pressed metal pads in IC's now. There is no solder (and little heat - they use Ultrasonic Welding/etc.). Why it should be any
more difficult for major manufacturer's to tool up to make bigger circuits, with higher clearances, etc. is beyond me (in fact, I suspect it may be
the next, necessary step in order to increase board density).
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You've never been involved in electronics manufacturing, have you.
I mourn the loss of lead solder - its so nice to use, and the alternates are all more expensive. Its just a knee jerk reaction to the Lead Boogeyman.
Bet you car has more lead in its battery than all the electronics in your house.
PCB and IC manufacture may seem similar at first glance but your really can't compare EUV lithography at 32nm, class 10 cleanrooms, CVD, etc with
etching copper in a bucket of ferric chloride ! Wire bonding in IC's is done with gold - a flame formed gold ball is pressed onto the bondpad, the
other end is pressed and smeared onto the lead frame (which is usually etched, not punched, for fine pitch parts). The bonds are weak and gold is
soft, so the whole lot gets buried in epoxy. You cannot do this macro scale with copper! Ultrasonic welding is not trivial either - the welding head
has to be designed to suit the job.
You still need some way to hold the parts down, and something to stick them to. Copper clad PCB with solder is still the most cost effective way to do
this, and will remain so for a long time to come.
I think you are also missing the break between design and manufacture - manufacturing companies do only that, build stuff for other companies, hence
their processes are generic rather than specialised.
Helicopter: "helico" -> spiral, "pter" -> with wings
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12AX7
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| Quote: Originally posted by Twospoons | The bonds are weak and gold is soft, so the whole lot gets buried in epoxy. You cannot do this macro scale with copper! Ultrasonic welding is not
trivial either - the welding head has to be designed to suit the job.
You still need some way to hold the parts down, and something to stick them to. Copper clad PCB with solder is still the most cost effective way to do
this, and will remain so for a long time to come. |
Those black lumps you see on mass production stuff are direct mounted dies. I suppose they use wire bonds, which will be gold, but I doubt the traces
are even plated. The whole thing is covered in a goober of epoxy. What better way to not only reduce expense, but also add a layer of copy
protection, when you need a billion of something?
Tim
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Twospoons
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It can be done with wire bonds, or the chips can be attached upside down (flip-chip) using gold balls on the bond pads. Either way, a PCB is still
required.
It only makes economic sense, as you say, when making a billion of something. Assuming you can get the chip maker to sell you naked die.
Helicopter: "helico" -> spiral, "pter" -> with wings
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aliced25
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No, I wasn't thinking about working from the bare wafer. I was somewhere else entirely, I recall somebody saying that the density of Computers would
double every x years, then he came back and stated that based upon what was happening, it would double in half that time, the number of Integrated
Circuits on a single board is growing until it is veritably unsustainable. IIRC it was one of the old Microsoft people in an interview (or several
interviews).
Anyway, that is waaaayyyy off-topic...
I'm wondering about the design for the shim coils, what shape, where should they be placed and is there any prospect of modeling them with FEMM or
some other package to determine the interaction (good or bad) with the permanent magnetic field.
The other real issue to solve is how to process the digital signal, which would presumably be on the DSP (or in Flash Memory) waiting to be called.
What would we need a basic GUI, hooked up internally to one or more of the free Spectral Database(s), which can compare points that have been assigned
in order to find possible, probable and definate matches. As definate matches are found, their spectrum should be removed from the complex spectra, in
order to allow the spectrometer & the user thereof, to try and eliminate other possibilities.
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not_important
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| Quote: Originally posted by aliced25... |
The other real issue to solve is how to process the digital signal, which would presumably be on the DSP (or in Flash Memory) waiting to be called.
... |
If you're meaning that the FID data would be stored in Flash memory that might work. If you're intending to gather the FID data and filter+FFT using
Flash as working memory, that won't work.
Flash memory is closer to a fast disk drive than to RAM - either SRAM or DRAM. While individual memory words may be read at random as if in RAM or
ROM, writing is a different story. Common Flash memory requires updates to be done to blocks or memory, this usually means erasing the block so all
bits have the same value, then writing data which will flip the values of some bits - you can only write zeros or write ones depending on the type of
Flash, erasing sets all bits to the other value.
The erasure is moderately slow, so treating Flash like RAM makes for a very slow memory. If you just want to collect data for later analysis
elsewhere then you use a pair (or more) of buffers in true RAM filling first one and then writing it out to Flash as you use the next buffer to
capture the following sequence of data; this is similar to the way disks are used in data collection.
Another aspect of using Flash as RAM is wearout - writing/updating Flash causes it to slowly deteriorate past some point data errors start becoming
important. Most current Flash runs between 10<sup>5</sup>and 10<sup>6</sup> cycles exactly what this means depends on the type
and brand/style. The point is that while using Flash as you would a disk generally creates no data integrity issues, using as RAM may well do so; look
at how many accesses a 1K FFT processing would do.
As f or the automated analysis you'll need resolution to 1/10 ppm to make it worth while, spinning the sample controlling its temperature and so on.
You'll also need a good reference, be it TMS in the sample or locking to the <sup>2</sup>H of the deuterated solvent and adjusting the
proton frequency data from that.
[Edited on 30-10-2010 by not_important]
[Edited on 3-3-2015 by Polverone]
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aliced25
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Well, there ain't no deuterated solvent going to be used, that's for damn sure. I'm trying to look into the design of the coils and the placement
thereof. Are you certain that the original concept of spinning the sample was down to 10ppm? I do remember reading about it, but I do seem to recall
reading that when the concept was first suggested.
Anyway, we are slowly creeping down toward a couple of hundred ppm, without shims. I do remember reading that book/article you posted about shimming,
why do they spin the coil as well as the sample? It seems to me that if they did not, then they would not have repeating spikes as the coil passes
through different field strength zones.
Thanks for the heads up on the Flash, I'll have to look a little more closely. I may have to incorporate a memory chip on the board, but that would
put the FFT/etc. on the PC/Laptop, which would allow for a less complex MCU/DSP (well we really aren't utilizing a tenth of what the one in question
is capable of).
But memory can come later, I badly need some suggestions on shims. Anyone got any papers on the various approaches or ideas on what would be best
suited?
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not_important
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Spinning is used in just about all lab-analytical devices, just about needed to get the needed uniformity. A NMR that can only resolve to 10 PPM is
not much use save for measuring water content or the curing of glue/plastics that have active hydrogens that react with some other group. Most NMRs
only spin the sample, read that again. You spin the sample so it sees all the field values, slightly broadening the peak but preventing a given type
of proton from appearing as several peaks. remember the the FID is 100s of milliseconds long, each part ofg the sample sees much of the field a number
of times during that time; and their field is rather uniform to start with.
Without deuterated solvents you are pretty much limited to compounds that are liquids, and you'll need to get TMS as a reference. Without some real
trick fast sample swapping you can't depend on attempting to calibrate with a known then replacing it with an unknown, you need to calibrate while
measuring the sample. You could use fluorine as the reference, something like perfluor-oneopentane but then you need a solvent for that, and
fluorine's peaks aren't as clean as hydrogen's.
Shimming was what discouraged me from going ahead with building a FT-NMR ...
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aliced25
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OK, so we need as much information about shims and their design as possible, this is where we come when we cannot figure shit out on our own is it
not?
Alright, what do we know about shims?
They are small electromagnets, so PWM control should really allow for control of the same.
What do we know about the inhomogeneity of this design? The problem lies in the x,y coordinates approximately 1/4 of the x-axis from center and about
the same on the z-axis (presuming y is vertical & x,z = N-S and E-W respectively).* In terms of spatial homogeneity this is not as bad as the
small portable NMR-Mouse, etc. which do not use homogeneous fields at all.
But at least we are only shimming in the order of 50-100ppm (with some designs). It seems like we are on the verge of being able to come up with
something at least. arsphenamine seems to have plenty of information on shims and their design, I've seen some articles on the subject, but they
didn't spin as they were using MRI, in which the coils spin around the sample.
Can anyone work out how to model electromagnets in FEMM? That would have to be a start, even if we can only model them in 2D, it has to be better than
nothing.
* Always presuming (and I hate to do so) that the magnet is at least moderately homogeneous vertically, which is a big call.
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not_important
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PWM with really good filtering, any ripple there is going to show up in the FID signal as shifting/jittering of the peaks.
MRI spins the coil because spinning a patent would add to the discomfort, and the coil isn't for field homogeneity but rather creating a gradient that
provides a 'slice' through the patient that is the active imaging scan. Rreally quite different than you are attempting to do.
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aliced25
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Spinning the patient, damn that would sure take away from the boring side of
the discomfort though wouldn't it?
MRI came from NMR, it works on the same principles using electromagnetic coils to produce the magnetic field. There are several papers on using
permanent magnets to produce a field so the researchers could image small rodent brains. Magnetic Resonance Imaging used to called Nuclear Magnetic Resonance Imaging, which kind of shows the relationship (NMR Imaging).
If small DSP's can be used to recover the signal from that and plot the position (ie.2D NMR Image) and control the gradient coils, I think shimming
using smaller gradient coils should be possible. In fact looking at papers like this and this I suspect we can probably work it out (I don't expect it to be easy). What I find interesting with MRI is that they utilize the gradient
coils to re-intensify the FID, as a spin-echo, why can't we use the same concept?
[Edited on 4-11-2010 by aliced25]
[Edited on 4-11-2010 by aliced25]
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not_important
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You can use spin-echo, just good timing and control of the RF is needed.
Do realise that imaging is a _less_ demanding task than structure determination; MRI is mostly looking at H2O or H2O vs 'hydrocarbons' such as fat -
and how tightly it is hydrogen bonded to the tissues it is in while structure determination is looking at all the protons and their local-in-molecule
environments. Please don't try to apply too much from MRI to your NMR, the homogeneity needed for imaging is less than for structure determination to
start with and other factors are in play.
The 2nd ref - the PDF - gives most of what you need to design shims, now all you need is the tools.
[Edited on 5-11-2010 by not_important]
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aliced25
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Yeah, that is why I added it The plot with the Magnet Assembly shows that the
magnetic field is inhomogeneous in a curve from North to South around the diameter of the circle (and through the center of the sample area). So I was
thinking two coils or maybe even 10, (5 a side) to allow for the minute amount of additional magnetic assistance needed to get down to the ppm range.
Adjustment would be simplified by the fact that what you do to the first & fifth coil of each side is the same (so too the 2nd & 4th) - so 8
coils out of 10 would be adjusted as 2 coils (ie. 1,5,6 & 10 would be the same and 2,4,7 & 9 would be the same) the other two would be
equivalent (or close enough, ie. 3 & 8), allowing for only 3 adjustments. Alternatively, 12 coils would present only 3 adjustment ranges, coil
adjustment 1 would be 1, 6, 7 & 12; Coil 2 would be 2, 5, 8 & 11 & Coil 3 would be 3, 4, 9 & 10. If there were 16 coils, 4 PWM IO's
could run the lot. That would mean that there were 4 coil adjustments and the miniature coils would be better (in my view) than 2 large ones, insofar
as they allowed for finer adjustment.
Anyway, just for the hell of it, have a look at these (It'll take a little while for me to even get a slight grip on the maths):
Louis-S. Bouchard, 'RF Shimming for Ex-Situ NMR Spectroscopy and Imaging Using B<sub>1</sub> Inhomogeneities' (2007)
Shapira, et al, 'Spatially encoded pulse sequences for the acquisition of high resolution NMR spectra in inhomogeneous fields' J.Mag.Res. Vol.182 2006
pp.12-21
Sakellariou, Meriles & Moule 'Variable rotation composite pulses for high resolution nuclear magnetic resonance using inhomogeneous magnetic and
radiofrequency fields' Chem. Phys. Let. Vol.363(1) 2002 pp.25-33
Pryor & Khaneja, 'Fourier decompositions and pulse sequence design algorithms for nuclear magnetic resonance in inhomogeneous fields' J. Chem.
Phys.125 2006 pp.194111/1-94111/6
Heise, Sakellariou, Meriles, Moule & Pines 'Two-Dimensional High-Resolution NMR Spectra in Matched B<sub>0</sub> and
B<sub>1</sub> Field Gradients' J.Mag.Res. Vol.156 2002 pp.146–151
Chang, Chen & Hwang 'Single-sided mobile NMR with a Halbach magnet' Mag. Res. Img. Vol.24 2006 pp.1095–1102
Frydman, Scherf, & Lupulescu 'The acquisition of multidimensional NMR spectra within a single scan' Proc. Natl. Acad. Sci. Vol.99(25) 2002
pp.15858–15862
Chen, Cai, Chen & Zhong, 'Fast acquisition of high-resolution NMR spectra in inhomogeneous fields via intermolecular double-quantum coherences' J
Chem Phys. 2009 February 28; 130(8) 2009 pp.084504/1-08504/11
Blanton, 'BlochLib: a fast NMR C++ tool kit' J. Mag. Res. Vol.162 2003 pp.269–283
Many, many more... Seems if the maths can be sussed out, the need for greater homogeneity can be ruled out...
[Edited on 7-11-2010 by aliced25]
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not_important
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Do remember that often papers that you reference are dealing with specialised applications, or are bleeding edge. For example in Chen, Cai, Chen
& Zhong, 'Fast acquisition of high-resolution NMR spectra in inhomogeneous fields via intermolecular double-quantum coherences'
| Quote: | Our preliminary results on a 3 T whole-body MRI scanner indicate its feasibility.
...
Further work is needed to quantify the limit of the size and concentration of solute molecules to which the method can be applied. Further development
of the IDEAL methods is also necessary for better water suppression for practical applications.
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not to mention the increase in computing power, storage, and excitation signal control those methods require.
Keep in mind that old Project Triangle statement - Fast, Cheap, Good - pick any two. Cheap and good takes a lot of work to achieve.
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arsphenamine
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| Quote: Originally posted by not_important | | Keep in mind that old Project Triangle statement - Fast, Cheap, Good - pick any two. Cheap and good takes a lot of work to achieve.
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The Prototype Triangle is: fast enough, cheap enough, good enough.
I like to finish things before the Heat Death of the Universe.
Another point: the paper on rapid spherical field calcs for shim coils applies to superconducting solenoid bores and doesn't translate well to generic
rectilinear gaps.
The old patents from the 1950's and 60's are more applicable to the Halbach array previously discussed.
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not_important
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However they refuse to state what "good enough" is, I've ased several times. I assume that the intent is to develop a NMR that can be used to
determine molecular structure, which is one of the more demanding tass and requires a fairly high lovel of goodness.
I wored too long at a company that went by the prototype model, almost everything they shipped was little past the prototype stage. I'd say that at
least 3/4 of engineering manhours were spent on fixing problems on products in the field, which is usually more difficult and time consuming than
doing so on the worbench, and repeatedly fixing the same problem as it cropped up at other sites. As another engineer said on his way out the door,
"I want a job that's more than a ongoing 'quick hack'"
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arsphenamine
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| Quote: Originally posted by not_important | | However they refuse to state what "good enough" is, I've ased several times. I assume that the intent is to develop a NMR that can be used to
determine molecular structure, which is one of the more demanding tass and requires a fairly high lovel of goodness. |
Who are "they"?
I am not responsible for the short-sightedness of your previous employers
but sympathize with you since it sounds as if your managers would need
facial reconstruction were a clue stick a genuine physical entity.
A prototype is not a product, and ought to look like junk so no one will think of selling it.
"Good enough" means that the prototype proves the concept's validity and
informs you of production pitfalls that weren't obvious from ab initio reasoning.
It allows you to chart corrections before going to production.
It means you can fuck up a little without suffering a lot.
One more time:
It's easier to make a small 'X' and then a large 'X', than to make a large 'X'.
Regarding NMR magnets, I am deeply apprehensive about wrestling with
1"<sup>3</sup> magnets that exert a 75lb hold, out of fear for my fingers.
Watson.fawkes assures me that 75lb isn't so bad but I remain incompletely convinced.
When aliced25 gets to the magnet array construction stage, will his posts
decrease in number and length as a result of severe hand injury?
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aonomus
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In regards to spinning samples, it increases field homonogeneity, increasing the FID and decreasing peak widths.
At my workplace we run a 400MHz shielded bruker, and the spin function on our probe is broken (and has been for years), yet we can acquire 1H/13C NMR
spectra good enough for structure determination. It isn't an absolute requirement, we just shim it *very* well with X/Y coils (shimming with Z, Z^2,
Z^3 is the major requirement when spinning is required, but without, X and Y require extra work if the tube spinner isn't working).
If you guys increase the field strength to increase the FID, and shim it very well, you could probably get away with it....
Other tricks could be used to decrease SNR like cooling coils, using better preamps, and concentrated samples and give the instrument better
resolution.
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aliced25
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Hmmm, I'm not saying that I intend to try and use a completely inhomogeneous field, but with shimming we still would probably be fucking lucky to get
down past the ppm range. Spinning in such a small field is going to be hellishly difficult, but the use of a range of pulses and then a mathematical
model to derive the structure from the constants therein sounds like it might be part of the answer. The NMR-Mouse is actually in production and as
stated earlier, it does not use anything even vaguely recognisable as an homogeneous field, the field homogeneity in this model (particularly with
shimming) and the strength of the magnetic field (about twice that of the NMR-Mouse) suggest to me that while the answer is not the best answer
possible, it is one of the better solutions we'll come across (if it can be made to work). You'll note, I am not trying to utilize a field that is
completely inhomogeneous, I'm just trying to avoid the spinning once we are down to the sub-ppm range. Various solutions that were never before
plausible are becoming plausible due to the sheer size and speed of the processors in the humble PC (and people wonder why I roll my eyes when I see
it being used for idiotic games).
I have no intention of building something that is going to be a partially working prototype, followed by an equally partially completed prototype,
etc. I want to design something that will work. It doesn't need to be able to perform to the level of the 10-20T instruments, but it does need to be
able to perform. If raw processor power can be used to avoid having to come up with a motor design that will function in the presence of a fairly
solid magnetic field, so be it (even better if that means we don't have to open up the bore in order to accommodate magic-angle spinning. The fact
that the spectra can be mathematically derived from several broadened spectra developed by a variety of means, suggests that there is a degree of
feasibility to the concept, mathematics is too precise to admit a coincidence of that nature. Remember, people were burned at the stake for suggesting
less heretical ideas that turned out to be true, while I don't yet suggest it is going to be the answer to the problem, the articles on the subject
make interesting reading (and it isn't as outlandish as the fucking thermite in the WTC crap).
The previous suggestion that there be anything up to 8 or coils per side (4 pairs, or 4 sets of 4 around the outside of the bore) seems to have been
ignored. Of course, I have not been able to model the field in 3D yet, but the obvious inhomogeneity around the outside of the bore (which is matched
if one maps the field N-S) which is lowest near the poles and highest at the E-W centerline, suggests to me that if the mini-coils were set up so that
each set of 4 were adjusted equally, then we might be able to alter the field enough to get to the ppm level. We really aren't far from the PPM level
as it is.
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aonomus
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Re: motor that functions inside an intense magnetic field: air turbine.
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not_important
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(sigh)
Indeed, it has been said here several times before that the sample tube spinning is done with a small air turbine, commonly of plastic.
As for the NMR Mouse
| Quote: | | A hand-held NMR sensor, the MObile Universal Surface Explorer or the NMR-MOUSE® allows measurement of NMR relaxation and diffusion parameters in
surface-near volume elements of arbitrarily large objects. Considerable effort has been spent on development of NMR methods which are applicable in
inhomogeneous magnetic fields. Applications are in process and quality control of polymer and elastomer products as well as in Medicine
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Not structural determination, but things lie determining the crystallinity of plastics.
Also note that several of the papers you have referenced have stated that while the model of the field may show a few tens of ppm inhomogeneity, real
world physical magnet arrays are an order or two magnitude worse in their performance - 100 to 1000 ppm. Both permanent magnet and electromagnetic
shimming are needed to reduce that to acceptable values. Plus you still need a way to determine field strength on the fly - deuterium or TMS locking.
Shimming, an example - how many coils do they seem to be using:
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aliced25
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Air turbines require compressed air - that requires a compressor & a tank, which kind of excludes portability. The bore of the magnets discussed
by the researchers who've papers have been attached to here don't have the space for magic angle spinning & neither does the one I put up. Nor do
other researchers, who've authored papers that I have also linked/added to this this thread. There has to be a way around it, I'll keep looking, but
sending a broadish, inhomogeneous rf burst(s) at the sample, while alternating the current to the electromagnetic shims (so the sample "thinks" it is
spinning), while reducing the level of inhomogeneity in the magnetic field to ppm figures may not be as silly as it sounds. It has the potential to
avoid spinning the sample, by "spinning" the shim-current around the sample.
As there are, apparently, mathematical correlations between the responses, broad and narrow and the narrow, structurally determinant response can be
arrived at by algorithms, broadening the pulse so that it excites the whole sample, then narrowing it until it only excites part also makes sense,
kind of. There is a relationship between the width of the response and the width of the rf excitation, so obviously they must be extrapolating back
from that in order to determine the "theoretical" response of the sample if it were in a homogeneous field and excited with a narrow-pulse. I
visualize it as being like spraypainting a moving object (that moves within certain boundaries). The bigger the boundaries, the wider the spray, the
smaller the boundaries, the smaller the spray. Extrapolating back from that, you could work out what the pattern would be if the boundaries were such
that untoward movement was impossible and the spray was a marker pen instead. ie. it should be possible to determine based upon the multiple data
points (fan shaped) what the narrowest range would look like (provided there is at least a slight correlation between the broadest, incomprehensibly
wide response(s) and the narrower, semi-accurate responses. As that is what they appear to be saying in those papers (that such a relationship exists
and that there is a numerical construct that can be used to solve for x,y & z @ infinity), it kind of makes sense. That said, there seems to be
multiple formulae, which suggests the solution has yet to be found, as Einstein said, 'if you can't explain it, you don't understand it well enough'.
As to shims, I have mentioned a design several times, do you think current-output DAC's would be more useful than PWM controllers? That would give
precise control over the current sent to the coils (or sets thereof). I'm just trying to work out a decent way of getting the job done electronically,
such as would allow for someone using the control program to check the spectra when the shims are moved, then using the updated spectra, move one or
more shims then check again.
As to determination of field strength, water (so too Tequila/alcohol) has a known spectra and has been used by various Author's cited in this thread
to determine field strength (Or did I imagine that?). The known spectra would also allow for the correction of the shims, until the broad response(s)
narrowed to an acceptable level.
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