Author Topic: Nuclear Magnetic Resonance  (Read 4320 times)

Offline uselessmass

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Nuclear Magnetic Resonance
« on: May 14, 2013, 08:53:39 PM »
I was wondering if you could do a show on the topic of Nuclear Magnetic Resonance instrumentation, or rather the theory behind it. Why do atoms with an uneven number of protons and neutrons have the spin property, etc...

This might not be an interesting topic for a show, I don't know.

*Walks away looking at his feet*

Offline bn

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Re: Nuclear Magnetic Resonance
« Reply #1 on: May 15, 2013, 04:13:31 AM »
that's a GREAT idea!
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline CthulhuKid

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Re: Nuclear Magnetic Resonance
« Reply #2 on: May 15, 2013, 07:17:27 AM »
Would be interesting, especially since even the question makes me go "huh?"

Offline scikopas

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Re: Nuclear Magnetic Resonance
« Reply #3 on: May 15, 2013, 05:31:20 PM »
Oh man, NMR is crazy interesting!  I just wrote a couple paragraphs to start explaining it for you, but then I realized that this is TiPhi topic suggestions, and I don't want to undermine ben's show, so I will only post it here in very tiny white text.

[Potential Spoilers: copy-paste without formatting to read]
To answer a few of the questions with much too little background information;
 the reason atoms with uneven number of proton/neutrons have spin is because of the total sum spin of their constituent particles.  Each one of these protons and neutrons have a spin of +/- 1/2hbar (dangit ben where is your forum LaTeX plugin!?) so they are all added up to have either integer spin (+/- 0,1,2,3,4...) or spins of (+/-) 1/2, 3/4, 5/2,...    If you put enough energy into these nuclei though, you can "flip" the positively oriented protons/neutrons to a negative value, and the energy required is different for every species of atom (because of how many protons/neutrons each atom or molecule has) 

The way very basic NMR works is that you put a bunch of the stuff that you want to analyze in a strong magnetic field, so that the spins will orient themselves in the same direction as the magnet.  Then you have a radio frequency transmitter that fires a radio frequency at the sample, sweeping from low energy to high energy (low frequency to high frequency), and a receiver around the sample that records what frequencies are absorbed and then re-emitted by the sample. (this is almost the same as how a light emission spectrum or X-Ray emission spectrum works, except with magnets! and the Delta E changes with different background magnetic field by the equation mu*B/I where mu is the magnetic moment of the nucleus [every one is different!] B is the magnetic field, and I is the value of the spin state [1/2] ) You will see a little emission from the sample when the negative spins that came from positive spins suddenly flip back down to their original lower-energy (positive)h position.  Each nuclei has a "characteristic" energy that it will require, so you can use that to determine what you have!

[/spoilers]
PhD student in Materials Science at Arizona State University currently working on high-temperature superconductors and quantum computers or something.
my (materials) science podcast: LASER (Let's Agree Science and Engineering are Rad!) twitter @scikopas

Offline bn

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Re: Nuclear Magnetic Resonance
« Reply #4 on: May 15, 2013, 06:03:36 PM »
that's awesome and i'm going to retroactively blow up your text and make it visible.

Quote

To answer a few of the questions with much too little background information;
 the reason atoms with uneven number of proton/neutrons have spin is because of the total sum spin of their constituent particles.  Each one of these protons and neutrons have a spin of +/- 1/2hbar (dangit ben where is your forum LaTeX plugin!?) so they are all added up to have either integer spin (+/- 0,1,2,3,4...) or spins of (+/-) 1/2, 3/4, 5/2,...    If you put enough energy into these nuclei though, you can "flip" the positively oriented protons/neutrons to a negative value, and the energy required is different for every species of atom (because of how many protons/neutrons each atom or molecule has) 

The way very basic NMR works is that you put a bunch of the stuff that you want to analyze in a strong magnetic field, so that the spins will orient themselves in the same direction as the magnet.  Then you have a radio frequency transmitter that fires a radio frequency at the sample, sweeping from low energy to high energy (low frequency to high frequency), and a receiver around the sample that records what frequencies are absorbed and then re-emitted by the sample. (this is almost the same as how a light emission spectrum or X-Ray emission spectrum works, except with magnets! and the Delta E changes with different background magnetic field by the equation mu*B/I where mu is the magnetic moment of the nucleus [every one is different!] B is the magnetic field, and I is the value of the spin state [1/2] ) You will see a little emission from the sample when the negative spins that came from positive spins suddenly flip back down to their original lower-energy (positive)h position.  Each nuclei has a "characteristic" energy that it will require, so you can use that to determine what you have!
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline bn

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Re: Nuclear Magnetic Resonance
« Reply #5 on: May 15, 2013, 06:04:52 PM »
after all, it might be a while before we get around to it as a topic.

Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline CthulhuKid

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Re: Nuclear Magnetic Resonance
« Reply #6 on: May 16, 2013, 09:54:09 AM »
That's...well...still a bit difficult for me to grasp.

What if the molecules were bees?

Is this in any way related to how quarks are all multiples of 1/3 to make up their own particles?  Can you mess with quark's energy the same way?


Offline stephako

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Re: Nuclear Magnetic Resonance
« Reply #7 on: May 22, 2013, 02:19:41 PM »
Is this in any way related to how quarks are all multiples of 1/3 to make up their own particles?  Can you mess with quark's energy the same way?

The spin of nuclei are not related to the 1/3 charge of quarks. This 1/3, 2/3 is the electrical charge, while the spin is somewhat differnt (it is more like a vector than a number).
In principle you can do the same with quarks but it is much more difficult, because the nuclear forces conspire to give you many other effects that make the interpretation of the data more difficult. Plus EM waves that can resolve individual quarks can not be controlled in the lab so you need to use particles instead of the radio waves in NMR. So suddenly you are building a particle collider ;)

What if the molecules were bees?

OK I'm going to try this but I'm not sure my analogy will help you:
Imagine you have a bunch of bears (the bee will be introduced in a minute) sitting on a log floating on a river. This will be our nucleus and the tail-head direction of the bear will be the spin direction. If you want to have neutrons and protons you could mix grizzly bears and polar bears. The bears can either look to the left or the right shore of the river. If they try to look in a different direction they will fall into the water because they can't balance.
Now imagine you have a whole lot of such logs floating along. There can be different numbers of bears on the logs (i.e. different nuclei for different elements). To polarize the system you put honey pots along one side, lets say the left, of the river. Because the bears want to eat the honey they will all look to the left (the nuclei are now spin polarized). Now you steal a genetically engineered super bee from the biology lab next door (maybe you are already working in a biology lab or a zoo if you have access to a lot of bears?) and strap a pot of honey onto it. You send this bee along the right hand side of the river. Because the bee is carrying much more honey than is placed on the left side of the river the bears try to turn on the log to face the bee. Of course this will cause quite some uproar as the bears turn around. They will see the bee pass and then turn back to the left shore. You take a recording of the commotion going on on the river and because you now the grunts that come from a single log (maybe you tried the experiment before with just one bear on one log and worked your way up) you know on how many logs the bears have changed direction
Of course not every bear will want to turn for the bee. Maybe some aren't as hungry, or they prefer a different type of honey. So you repeat the experiment with different bees and record your findings. In the end you should know how what bears are on what logs.
The bee in this analogy is the radio wave and changing the honey it is carrying amounts to changing the frequency.
« Last Edit: May 22, 2013, 02:23:47 PM by stephako »
Null results, open questions and a bit of my writing: JUnQ

Offline CthulhuKid

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Re: Nuclear Magnetic Resonance
« Reply #8 on: May 23, 2013, 08:33:35 AM »

OK I'm going to try this but I'm not sure my analogy will help you:

The bee in this analogy is the radio wave and changing the honey it is carrying amounts to changing the frequency.

[/quote]

o.0

Holy crap that worked. 

That is really interesting.  For some reason my brain immediately went to encryption technology:  You can only read this message if your collection of atoms has the EXACT same spin qualities as mine.

Offline calura

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Re: Nuclear Magnetic Resonance
« Reply #9 on: May 23, 2013, 08:52:30 PM »
Holy crap that worked. 

It really did!  Thanks stephako :)

It really wasn't that hard to grasp, but you know how you get half way through a paragraph and your brain starts going 'hey what's that over there' and you forget everything you just read?  That was happening to me until I read about your bears.  It also helps to know what it's useful for (though maybe that's just an engineer's way of thinking), and for that the Wikipedia page is really good.

Offline stephako

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Re: Nuclear Magnetic Resonance
« Reply #10 on: May 24, 2013, 02:00:44 AM »

o.0

Holy crap that worked. 

I'm as surprised as you are :D

That is really interesting.  For some reason my brain immediately went to encryption technology:  You can only read this message if your collection of atoms has the EXACT same spin qualities as mine.

I don't think nuclear spins are very practical for encryption purposes. When you do it on real systems you have stray magnetic fields (especially if you are transporting it) that are not easy to shield and the matter interacts with the container. Imagine honey pots showing up randomly on either side of the river or some of the logs getting caught at the river bank and turning or logs bumping into each other.
We have a group at our department that produces spin polarized 3He for medical purposes. I think they can store it polarized for a couple of month but they had to put quite some effort into developing special containers for it.

... but you know how you get half way through a paragraph and your brain starts going 'hey what's that over there' and you forget everything you just read?  ...

Oh I know the feeling, from almost every paper I read...
Null results, open questions and a bit of my writing: JUnQ