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Education Article

  • Published: Jan 10, 2014
  • Channels: NMR Knowledge Base

Table of Contents

1. Filling magnets without worries

2. NMR tube washer

3. Acid free Chloroform

4. Sealing of Shigemi tubes

5. Superconducting Magnet Helium Gas Exhaust Valve

6. Unsatisfying multitasking behaviour of SGI workstations


1. Filling Magnets without worries

As anybody will agree, filling supercons with nitrogen is a time consuming and boring activity. One has to stay with the nitrogen vessel, because walking away from it leads to chilled ceilings and cracked lino on the floor. One can buy automatic filling systems based upon the Pressure-Temperature characteristic of Xenon. Because our magnets are (fortunately) not very close to one another, this would involve a lot of hardware and money, especially if one also wants to use the same system for filling smaller dewars for low-temperature experiments. Therefore, we chose a much cheaper solution that we built ourselves.
The setup shuts off the nitrogen supply as soon as liquid nitrogen is ejected from the outlet of the magnet. It consists of a normal composite resistor (NTC) as temperature sensing element, a cryogenic valve and a box containing the hardware. The cryogenic valve and the hardware travel with the vessel, every magnet or Dewar has its own sensor.

The biggest problem one encounters is to measure the liquid nitrogen in a stream of nitrogen gas at the magnet exhaust. Temperature-sensing seems impossible as both are almost equally cold. In principle, it is possible to put a known amount of heat in the sensing resistor by running a known electrical current through it. In this way one senses in fact the specific heat of the gas phase compared to that of the liquid phase. In the present case the calibration would be very tedious, however, as the flow rate of the gas differs each time. Moreover, the exhaust of the magnet will not immediately change from gaseous to liquid nitrogen.

We found a very easy way around this problem, however, by building, a kind of "phase separator". In fact this is not much more than a piece of bent copper pipe with a hole in it and closed on the end with a Teflon chamber containing the sensing resistor. Fig. 1 roughly shows the design. 

The nitrogen gas coming from the magnet blows into the pipe at point A and escapes from it at point B. The velocity of the gas causes a slight overpressure in the pipe past point C so there will not be any flow of gas past this point. This prevents substantial cooling of the resistor by the nitrogen gas. The situation changes drastically, however, at the moment a few nitrogen drops leave the magnet. They will obviously not escape from the pipe but are flung onto the sensing resistor in point D due to the centrifugal forces. Thus the sensor is cooled to near liquid nitrogen temperature very fast.

The hardware for the device to shut down the nitrogen supply is quite straightforward as can be seen from Fig.2. 

As a sensing resistor we use a resistor of nominal 1000 ohm at room temperature, going to over 1350 ohm at liquid nitrogen temperature. We implemented a test for open wires (> 1200 ohm) and short circuited wires (< 900 ohm) before the device will start at all. The "cold condition" is set to 1100 ohm. When the wiretest is O.K., hitting the push-button will open the valve on the nitrocen vessel, and filling of the magnet will commence. As soon as the sensor cools down and its resistance exceeds 1100 ohm the valve will close. 

The device is also equipped with a timer which allows us to keep dewars filled for low-temperature experiments. The timer will reopen the valve when it reaches its preset value, restarting the filling sequence all over again. We set it typically to a few hours to keep our Dewar for low-temperature MAS operation filled.

We added a safety procedure in case the valve freezes and does not close anymore. If the sensing resistor stays cold for more than one minute, it means the valve is frozen. This condition causes the (safety) gas valve of the nitrogen vessel to open, taking the pressure off the container. We have been using this device for quite some time now and it performed reliably. In fact, the safety valve never had to be activated. We hope that this idea will make life a little easier for all those responsible for keeping magnets and Dewars filled. 

For specialists an electrical scheme is available. 


2. NMR tube washer

By Karsten Alstad Sørbye (kasorbye@broadpark.no)

Cleaning NMR tubes can be a tedious and awkward procedure. The available commercial apparatus are expensive and, as most of them are involve treading the fragile NMR-tube over another glass tube, they have a tendency to damage NMR-tubes. 

The glass apparatus are also prone to receive irreparable damage themselves. Here is an apparatus which can be assembled in less than half an hour from parts commonly to be found in an organic chemistry laboratory. As it is made of plastic, the apparatus does not break easily, nor is it prone to damage the NMR-tubes. We have found that these characteristics more than make up for a somewhat wilted appearance.

This apparatus and minor variations of it have been widely used at our institution. As it is so easy to assemble (15 minutes assembly time with a bit of practice) those of us who have particularly difficult compounds have several such apparatus lined up for washing with a different detergent or solvent in each apparatus.


Required materials

  • At least 40 cm tubing of polypropylene or other plastic (must have diameter smaller than the inner diameter of your NMR-tubes).      
  • 1 washing bottle of simple plastic (pp or pe)      
  • A septum      
  • One large suction flask with a ground joint to fit the septum. 


Assembly

  • Cut the tip off a wash bottle at cut "A" in  figure 1; a bit above the commencing taper.      
  • Cut the end at a point where the taper is slightly smaller than the diameter of your plastic tubing at cut "B" in  figure 1.      
  • Cut away the bottom off the wash bottle      
  • Cut a long taper at the end of your plastic tubing, cut "C" in  figure 1, so that it is possible to thread the narrow end through the tip you cut from the washing bottle. Use a pair of pliers to pull the tube through the washing bottle tip . This is done in order to ensure a tight, non leaking fit. When the tube is well through the tip, the tapered part of the tube can be cut off and discarded.      
  • Assemble this with the rest of the washing bottle as shown in  figure 2. Thread the bottom end of the plastic tube, marked with "to vacuum" in the figure, through a septum and fit the septum onto a suction flask. When a mild vacuum is applied , pour acetone or other solvent into the apparatus, thread your NMR-tube over the top end of the tube, and voilà, solvent rushes through the tube, hopefully removing the remnants of your last sample. 

This is my "make it yourself in 15 minutes" automatic NMR-tube cleaner. A "must" for anyone who is tired of fiddling with Pasteur pipettes and NMR tubes, and who cannot afford to by a fragile and expensive commercial gadget for rinsing NMR tubes. I think anyone who has been working with NMR analysis will be able to appreciate this little Gizmo which I assembled from standard plastic parts found in most organic chemistry laboratories. The apparatus is a plastic version of a glass apparatus which I saw in Padova, Italy during an ERASMUS exchange study in the labs of Professor Gianfranco Scorrano. After about two years of wanting to make a plastic version an idea crystallized which then only took 15 minutes to realize. Feel free to make as many as you like. This is "FREEWARE" so feel free to show it to anyone who is interested or make as many as you want, but please write me if you find it useful (see address below). 


3. Acid free chloroform


Problem

Usually CDCl3 contains traces of HCl. This is true even if you fill pure acid free CDCl3 into your tube and expose this tube to light. The HCl may damage your sample or produce unwanted effects (i.e. restricted rotations in the case of azomethines). 


Solution

Put a few milligrams of silver oxide into the bottle and stir for about 3 hours. 
You may even put a tiny amount of silver oxide into sealed tubes to avoid the production of HCl within your tube. 

Contact Gábor Tóth by email: g-toth@ch.bme.hu


4. Sealing of Shigemi tubes
 


Old Solution

I believe many of you are using a Shigemi microtube. A drawback of it is that it is rather poorly sealed for most cases between outer tube and insert. Parafilm is far from satisfactory. A joint cap for 5mm tube made of Viton rubber is designed to temporarily hold the insert to a position.

However, it is not only unsuitable for preserving a sample for a long time but too costy to make it. I was reluctant to make different size caps because of the cost. Instead, I suggest using an epoxy glue for sealing. It softens on dipping in hot water. Thus it is not difficult to reuse the microtube when required. 


Better Solution

Now, I have a better idea. Use a piece of polyethylene tube which shrinks with heat (sorry, I don't know the name in US; in German this would be "Schrumpfschlauch"). Any size of tube could be obtained at an electronic shop. Cut it to a piece of 1-1.5 cm length, slide into position, and heat it with a heat gun for a minute. It fastens enough to make an air tight seal.

Contact Seizo Takahshi by email: t_seizo@jwu.ac.jp

 

5. Superconducting Magnet Helium Gas Exhaust Valve


Disclaimer

This information is posted for the general interest of the NMR community. We do not guarantee the efficacy or applicability of any modifications presented here, and we do not take responsibility for any direct or consequential damages that result from the correct or incorrect application of these methods. None of these modifications is in any way endorsed by any equipment manufacturer, and use of these methods may void any equipment warranties extant.    Common method to refill heliumOne of the main difficulties in doing the liquid helium fills on a superconducting magnet is arranging for the gas output generated during the fill to be exhausted appropriately. Like many people, I used to do this by removing the pressure relief valve on the "dogbone" prior to the start of the fill, and then either putting a plastic cap over the opening or a rubber stopper or wadded-up Kimwipe in the opening, and then quickly removing that when the fill started. When I came to UIC, I discovered that someone before me (if anyone reading this knows who's idea this is, please let me know, as I would love to give him/her the appropriate credit) had come up with a much better idea. Several visitors to our facility have commented on this idea, so I thought I'd share it with the community.    Usage of a ball valveThis better idea involves installing a ball valve on the exhaust of the magnet. The two older Oxford magnets here do this using what appears to be a custom-made adapter that incorporates the standard boiloff outlet and pressure regulating valve. When we got our Oxford 300WB magnet from Amoco, I installed a ball valve on this magnet using off-the-shelf components, as described and pictured below. After this, Professor Frydman and Professor Crich installed the same setup on their own Magnex and Oxford Magnets. 

The setup on our Oxford 300WB magnet is as follows. On the exhaust port of the "dogbone" a vacuum tee is installed. This is a standard high-vacuum component. Then, on one port of the tee the standard boiloff pressure regulator and flowmeter are installed. On the other port, a vacuum-connection-to-pipe-thread adaptor is installed (this also is a standard piece) and the ball valve is threaded (with teflon tape) onto the adaptor. 

closed valve  closed valve 
Here is the valve setup during normal magnet operation


During normal magnet operation, the ball valve is closed and the boiloff exhausts through the pressure regulator and flowmeter as normal. During a helium fill, nothing is done until after the transfer line is inserted into the magnet fill port. Immediately thereafter, the ball valve is opened to allow the gas from the fill to escape. When the fill is finished, the ball valve is closed immediately after the transport dewar is depressurized, and then the transfer line is removed and the fill port plugged. The valve greatly reduces the risk of air being drawn into the magnet, since the magnet doesn't have to be opened any more than just when the transfer line is inserted and removed. In addition, the magnet is kept pressurized right up until the transfer line is inerted and also when it is removed, which reduces the ingress of air during the insertion and removal. 

valve during refill 

valve during refill 

Here is the valve setup during a helium fill


Parts used


The parts used were obtained from Cole-Parmer (vacuum line pieces) and from Lakeview Valve and Fitting (Whitey ball valve). 

The vacuum line pieces are (Cole Parmer):    

  1. aluminum vacuum tee NW-25 #4-31400-32        
  2. aluminum NW-25 to 1/2" male NPT adaptor #H-79750-10        
  3. NW-25 clamp #H-31400-42 (2)        
  4. NW-25 centering O-ring #H-31402-63 (2) 

The valve is (Lakeview):   

  • Female 1/2" NPT Ball Valve, Low-Temperature capable, #B-L-63TF8 

(Whitey offers a low-temperature option which I chose but may not be necessary.)


6. Unsatisfying multitasking behaviour of SGI workstations


Observation

You have a background job running on your Octane or O2 workstation with low priority. You try to do interactive work with any graphics oriented program (i.e. X-WIN-NMR). The behaviour of your machine is unexpectedly slow, and that's even if you have enough memory. 


Reason

The size of the non-interruptable time slice for background processes (30 ms for O2; 100 ms for Octane as factory defaults) is too large.


Possible additional problem

Processing NMR spectra during a running acquisition may damage this acquisition.


Solution

Reduce the time slice for background processes. 

  • Become Supervisor   
  • start  systune -i   
  • select parameter  slice_size   
  • set this parameter to  1   
  • restart your machine to make the changes permament 

Contact Rainer Haeßner by email: Rainer.Haessner@ch.tum.de

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