"Shuttle Device for Use in a Shared Commercial NMR Instrument Version II "
(December 2004)
by A. G. Redfield, Brandeis University

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Section ST. Shuttle tube and stop tube.

Cleanliness. The shuttle tube does not have to be super clean but if there is any chance that dirt has gotten in, the assembly should be taken apart and the inside of the glass washed out with good water and then ethanol, and dried by pushing a dry paper towel through it with a clean rod. The shuttle and O rings on it should be likewise cleaned. This is especially important if there has been glass breakage. The symptom of such dirt is: the shuttle gets stuck. It only takes 1/2 hour to clean things but it is annoying after a run is set up. Perhaps as a matter of policy we should disassemble and clean before every block of running.

The shuttle tube components are made simpler than before because they no longer carry the O-rings of the debounce system, these are now on the shuttle body; and I based the support system on the smallest diameter heavy wall brass tube that would clear the glass tube (1.25 outside dia., 1/16" wall). I chose a heavy tube because I thought it would be less prone to vibrations after the sample lands back in the center, and that they would be better damped. Perhaps so.

The current "new" design (6/2004) looks, on the other hand, more complicated than before because there are two window sections on it that expose the glass directly (see Fig ST-1), the support in these sections being provided by 1/4" dia brass rods (lower window inside the magnet) or 5/8 " dia. brass rods (upper window, above the magnet). These sections will allow me to put saddle coils around the glass and run a coax up to the top if needed, to apply rf to the sample at any desired stop position.

The saddle coil will be crudely made to slide up and down the glass by hand, when the support assembly is removed from the magnet. The lower window will allow me to map the magnetic field by nmr, by allowing me to try to saturate a thin water sample, mostly deuterated (to get a long proton T₁) in a field cycling manner, during a time of 1 sec or so when it is at the upper stop position. Loss of NMR intensity after the cycle will tell me that the sample was saturated, and was is in resonance with the rf frequency, and provide a point on a field vs height determination. I have not done this yet and it will be laborious. When it is done I can dispense with the lower window. The upper window allows the same thing above the magnet. I could also do T₂ measurements at up to 1000 gauss there, by shimming the field there moderately well, and I could even do coherence transfer between ¹³C and protons for example, at low field. I can see a potential use for this but have lots else to do. Optical and microwave irradiation could also be applied through the window to do various DNP experiments (not planned as of 12/04).

The windows also allowed me to put an optical sensor temporarily on the glass tube to investigate the speed and bounce of the system. It is an ultra-bright LED facing an inexpensive visual light phototransistor. They were mounted with tape on a light metal ring that I put around either window by taking the support rods apart. I established that the system can now move the sample up to 1 cm/msec but that it bounces a single bounce at both the top and bottom lasting around 30 msec (more at higher speeds), the height of the bounce being about 1.5 cm. I can't now say how serious will be protein denaturation problems at the highest speeds. I could not break either 5 or 8 mm tubes at the highest speed. But there is not much improvement in overall speeds for vacuum above about 2 psi (.133 bar) or pressure above 4 psi (.266 bar). Perhaps the flow of air is limited by turbulence in the solenoid valves (an estimate of the Reynold's number in the valves supports this idea) and greater speed could be attained with 3 or 4 valves in parallel in place of the 2 I now use. We may work on improving this flow in the distant future.

At the end of this section I will also talk about the tube extension and field bucking modification to do T₁ experiments down to 20 Gauss (reported in a poster, ASBMB meeting Boston '04) to get sensational new results on membranes (see PNAS Dcember '04).

Alol the PEEK machining has now been described and we now proceed to discuss, very briefly, the individual parts of the assembly shown if Figs. ST-1, ST-2, and ST-3 which are the main parts of the shuttle tube.

Figs. ST-1, ST-2, and ST-3 are schematic/assembly drawings of the shuttle tube support system, and ST-4A, ST-4B, etc are mechanical drawings of the parts from bottom up. Several parts have longitudinal grooves cut in them after being turned on the lathe, indicated on end views. The grooves are intended to provide a low-resistance return path for air flow. These do not have to be very precise as long as the remaining sections of the outer diameters are not touched. As in MRC, Fig. ST-2 and others may be confusing since the two sides are not identical: the cross section on the right side is through the connecting screws, and on the left it goes through the grooves, with dotted lines where material is cut to make them. (the assembly diagrams are very useful to check dimensions).

The window sections held together with brass rods were straightforward to design and I will not say much about them. I was afraid that the construction with brass rods would be flimsy, and the glass tube would break, but it seems good.

As before the support is pretty trivial: it supports the glass tube at the bottom on a Buna-N O-ring (in a well at the top of part (3), fig ST-4B), and it supports the glass tube at the top with very light force downward via a plastic ring (part (9) fig. ST-4H) that compresses the bottom O ring very slightly. The bottom end piece(part 1), fig.ST-4B) has to center the bottom end of the glass tube on the probe correctly, and for a Varian probe the top end of the probe also has to be centered by part (1), fig ST-4A. The top end of the support system has to be centered on the magnet bore and this is now done with a plastic ring (part (5B), fig. ST-4D) that is taped on the brass tube just below the point where the tube comes out of the magnet. The entire support tube, windows, etc are still supported from the top of the magnet, and simple adjustment screws at the top allow us to change its height to center the sample in the probe vertically. We are trying to avoid having to change this adjustment very much, by standardizing the length of the NMR tube etc. (above, a big success). These screws are now located at the very top of the shuttle tube support assembly, and they are, for the time being, sitting on top of a Helmholz coil system that we place there even when we don't expect to use it.(see Fig ST-1 for its approximate location). An extension tube can be added at the top of the system to allow us to move the sample up to 20 cm above the magnet for very low field work (see also Fig.ST-5A and ST-5B)

Details.

Starting from the bottom, we no longer use separate adapter rings rings stuck in each probe top to center the probe (as in MRC). Part (1) Fig. ST-4A has two surfaces (Outside diameter (OD)1.430 and 1.563 inch) that mate respectively with the upper inside of the probe, and the inside of the shim coils, and thereby center the probe in the magnet. This piece is made of Kynar, which is cheaper than PEEK but harder than Delrin. We like to use plastic to mate with the 500 magnet to avoid damaging it. We hope that this piece is the only one that would have to be modified for a Bruker, because their probe is already centered in their room temp shim coils assembly, supposedly, and only a single bearing surface may be needed.

Part (3), fig ST-4B carries part (1) and on the other end it mates with the glass shuttle tube (see Fig ST-2) and centers it precisely on the outside of a pedestal that pokes in to the precision-ground inside of the bottom of the glass shuttle tube. It has a 0.560 inch hole drilled through it, through which the nmr tube adapter can easily pass. The horizontal annular area between the glass tube and this hole is where the O-rings on the lower end of the shuttle-body land (see Fig. ST-2).

These parts might be modified as follows: Decrease the length of Part (4) and the nmr tube adapter to get a shorter, more rigid adapter. Decrease part (2) and increase the length of part (4) to allow space for more O-rings on the shock absorber. Increase the length of either part (3) or part (4) and open up the top of part (4) to be able to use a larger diameter shuttle tube. The current glass shuttle tube size is the largest that could extend below the room temp. shim coils.

Parts (4) and (6), Figs. ST-4C and ST-4E, are connected by the ¼ " dia. brass rods (RCA), stiffened by the well-fitting countersink holes. The brass tube (5A) fig ST-4D is soft-soldered into part (6), after it is hard soldered into part (7), fig. ST-4F, and after the alignment ring (5B) is slid on to it. You should cut the tube and rods last after you understand what follows, and you may want to change their lengths. Also, the face of part (7) (fig. ST-4E) should be refaced in the lathe after the hard soldering step. This seemed likely to be the most-strained joint so we hard-soldered it, but that may not be necessary. In any case all solder joints and rods must be made with as much care as possible to preserve alignment.

Finally, rods (RCB), fig. ST-4E connect part (7) to the top, part (8) fig. ST4G. The top should not be screwed to the 3/8 " rods, except for preliminary tests, until the cap is assembled on the glass tube (below) and the rods (RCB) are cut to length. Don't make these rods until you understand the following. And don't attempt to understand what follows without a copy of Figs. St-1 to ST-3 in front of you1!!

In the first place, early on you should put whatever probe of yours that you will use for field cycling in the magnet, in the normal way required by Varian (which is weird) and adjust the probe height with Varian's upper stack (their mysterious name for a nearly useless tube. "push up, push down"). (If you have two probes as we do it should not matter which probe you use, they are standardized (I hope). Then tighten the Allen screw tightly at the bottom, that clamps onto the probe to determine its height, and NEVER CHANGE IT AGAIN. Now the probe is in a good place we hope. Now remove the "upper stack", of course first REMOVING THE SAMPLE that perhaps was in there. Now assemble the shuttle tube support up to part (7), and be sure you PUT THE O RING in on the top of part (3) to cushion the glass tube when you put it in. It is an ordinary Buna-N ring, ¾ " I.D. x 1/16 " thick. Put the new assembly, carefully and gently, in the magnet hole from the top, leaving your probe in the magnet, (with no glass tube or shuttle etc) as far as it will go. It will probably go until part (7) hits the top of the magnet (that is, the top of the smaller aluminum flange the stays there always), or almost does. This means that the assembly is not too long. If it does not go in that far, make sure nothing unexpected is too big in diameter; if not, it is to be hoped that the bottom of the assembly, Part (1), is resting on the plastic part of the probe at the top, inside the top of the probe shell. (That is why you have to be careful!) Normally the bottom of part (1) is designed to be at least ½ cm above this point, and there should then be a few mm between the bottom of part 1 and the top of the magnet for air to get out and for the cap screw caps that connect to rods (RCB) fig. ST-4F. If the support seems too long at this point you have to shorten the ¼" rods (STA) or the 1.25" dia. brass tube. In any case you probably want to measure the distance from the inside plastic surface of the probe to the magnet top carefully using a clean plastic or aluminum rod as a probe, and a marker or piece of tape to mark the top. Then if it seems that the assembly is too short by the above criterion you have to throw away something, perhaps the ¼" rods and make new ones etc.

Now we come a diversion, namely the plastic cap (part (9), fig. ST-4H) for the top of the glass tube. It protects the tube end from chipping, and holds the tube in place. It is a good idea to order 2 or more glass shuttle tubes from Wilmad, and when you do, ask them to make them all with the same length. They will probably do so for free and then it is likely that, if you make identical plastic caps for the glass tubes, they will be interchangeable after being assembled.

What follows is needed especially if you want to connect glass tubes to other tubes (in our case, a shorter brass tube, see below, but in other cases such as for a larger magnet, another glass tube.) The outsides of their tubes are not precision ground, at least not the ones I could afford. So if you try to butt-join them it will be hard to avoid a shoulder on either side that would hang up the shuttle tube. My solution is to have a hole of slightly larger (.06 mm) diameter perfectly centered on the precision inside diameter of the glass tube. I have not yet designed a coupling between glass tubes, but the following was done to allow me to join a glass tube to a short brass extension tube (see below) to allow me to get the sample a few cm above the top of the magnet (to be described later). The cap (9) fig. ST-4H was made with a precision inner upper surface on a lathe, concentric with its precision outer reference surface. The inner lower surface is made a very small amount larger in inside dimension (perhaps 0.75 mm= .003 inch in radius) larger than the glass tube. Then I devised a way to fix this cap so that this surface is almost perfectly aligned with the inner precision surface of the glass (below). As a result, the outer surface of the plastic is perfectly aligned with the inner surface of the glass. It could then be coupled to another glass tube capped in the same way with ordinary couplings made on a lathe, to align the two caps.

In the design here, the upper inner surface of the plastic cap is specified to be very slightly larger than the inner diameter of the precision glass tube. The diameters of the shuttle body's bearing surfaces are conically reduced slightly at each end, to be able to negotiate the small shoulders (0.02 mm or 0.0007 inch) between the glass and the plastic. [In fact this is the next design. Currently the plastic inside surface and the glass are the same nominal diameter and I have had trouble with the shuttle sticking. I had to hand-sand the plastic with very fine sandpaper. I hope, and am pretty sure, that the new procedure described here will work better. Let me know.]

I assemble this by having made a Teflon "epoxy tool" fig. ST-4F (bottom), that mates the inside of the glass and the plastic together. The upper and lower outside diameters of the epoxy tool are very slightly smaller than the inside diameters, respectively, of the plastic cap part (9), and the glass tube, and the epoxy tool mates perfectly with these surfaces to align them The inside diameter of the lower hole in the plastic cap (fig. ST-4H) has to be made sufficiently larger than the outside diameter of the glass tube so that this is possible with as small a clearance as possible, as needed because as already explained the glass thickness is not precisely uniform. Then the glass tube, with the plastic cap stuck on it via the epoxy tool, is inverted and spot-epoxy (household, high viscosity)is applied to 3 spots in the space around the outside of the glass where it exits the plastic, while the parts are held firmly but gently together in a standard lab holder. After this sets I remove the "epoxy tool" and apply Epotek low viscosity epoxy which fills the remaining small space between the plastic and the outside of the glass to make a strong joint. I do not do this assembly in one step of cementing, because the low viscosity epoxy might flow around and interfere with removal of the epoxy tool. See fig. ST-4H. The household epoxy is too viscous to do so, but it establishes the alignment, and the immediately-following low-viscosity epoxy gives strength.

Back to the main assembly: Support the assembly made so far in a secure clamp to hold it vertically. Now you insert the heavy glass shuttle tube with its cap attached (see above) and put it in all the way, seated on the O-ring and surrounding the pedastle at the top of part (3). Gently hold on the top piece (8) and estimate how long the 3/8 spacer rods RCB should be. Cut them carefully to equal length with a little length to spare. Then screw them in place loosely, connecting parts 7 and 8. If the parts don't fit you made the rods too short; toss them out and make new ones.

Now measure how much extra space there is above the plastic cap which is on the glass tube. Now the hard part comes: you cut down the 3/8 brass rods until there is no excess space between the top of the plastic cap and the shoulder (0.6 " above the bottom of part (8)) to which it mates, but almost no force on the glass tube transmitted down to the O-ring on the top of part (3). This completes the shuttle tube assembly.

Now you have to make a support for the shuttle tube to hold it at the correct height in the magnet. If you did not make a Helmholz coil for work at very low fields you could make a "dummy" thing that resembles it; or you could have redesigned the part (7) so that it has a flange like the one at the top of part (8), with screws through it to adjust the height of the shuttle, which would rest on the top of the magnet. [In fact we used to do this (see MRC) but we always put down a thin sheet of aluminum to protect the magnet flange from being scratched.]