Device for Use in a Shared Commercial NMR Instrument Version II "
by A. G. Redfield, Brandeis University
ST. Shuttle tube and stop tube.
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
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.
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
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
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
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
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.]