Device for Use in a Shared Commercial NMR Instrument Version II "
by A. G. Redfield, Brandeis University
[Sections like the following
one that refer to "very low fields" or "Helmholz coils" do not describe
anything related to MRC. They are new capability used to generate the
data in our last paper in PNAS, Dec. 2004]
The Helmholz coils
are each about 16 cm in average diameter, about 2 cm thick radially
and each 4 cm longitudinally. They were made by Dmitri Ivanov for his
quadrupole resonance experiment (J. Magn. Reson. 166:19-27 (2004)),
and each has about 345 turns of #16 wire. They were would on a lathe
using a wooden coil form 4 cm long and about 12 cm outside diameter,
having gaps and screwed together to >20 cm diameter end pieces of wood
so that the parts of the form could be pulled off the coil without sliding.
The wood was first treated with wax to resist epoxy bonding. Wire came
from a spool on an improvised stand next to the lathe, and it was tensioned
while winding by a rag held in the hand, or pieces of Delrin clamped
lightly on the wire. After each layer was wound epoxy resin was generously
smeared on the entire layer, before winding the next. The result is
rigid and conducts heat well so that an air fan could increase its power
capability, and we did point a fan rather crudely at the coils when
Dmitri used them.
The coils are supported by
3 plates outlined on fig. ST-8 (top),
separated by spacers which are shown informally in the sketch at the
bottom of the same figure. The coils rest on the two lower plates, contacting
the four wings on each plate only, for vertical support, and held in
place radialy by the four rods. Other than the wings, the coil surfaces
are exposed for the sake of cooling. A small muffin fan hung in mid-air
helped cooling for Dmitri's setup but we have not used it yet in this
device. The rods are ¾ inch in diameter and the central one is threaded
at each end for 10-32 screws. The upper rod is drilled out with a ¼
inch hole and the purpose of the upper plate was simply to provide a
convenient surface to support the shuttle tube assembly. The coils are
held in place by several plastic cabling ties. The supporting plates
and rods have pieces of Scotch-brand thermoplastic glass electrical
tape attached to them using a soldering iron briefly applied to melt
their adhesive, so that the coil does not contact the support structure
directly. The inside diameter of the coils was only slightly smaller
than the outside diameter of the top aluminum flange of the Oxford 500
magnet, so I only needed to offset the holes in the upper two sets of
rod by 1/8 inch to do it as shown. Alternatively separate sets of holes
could be used for the four feet. Probably you will redesign the coils
so I do not give detailed designs.
The lower support rods (fig.
ST-8, between the magnet and the coil) were Delrin drilled out to
¼ at the top, and hollow, with holes large enough for a cap screw head
at the top. They fit the outside of the aluminum plate at the top of
the Oxford 500 magnet snugly, and thereby align the magnet radialy as
well as supporting it vertically, resting on a black plastic (?anodized
aluminum?) surface to which the top plate appears to be screwed. Important!
The lengths of these rods should probably be determined once the shuttle-tube
assembly is complete. Their length should be such that the shuttle tube
assembly sits at the correct height for maximum sensitivity of NMR detection
(that is, with the sample at the sensitive region of the probe) when
the 10-32 screws with larges knurled tops (McMaster Carr) were screwed
in with ½ to 1 cm projecting out the bottom of the flange of part (8)
("HEIGHT ADJ SCREW", fig. ST-1). These
screws would have to be adjusted if the NMR tube is not epoxied into
the NMR tube adapter at the usual heath that you establish. The height
adjustment is again discussed in a few pages.
For our setup we have designed
the top cap (8) to have the adjustment screws as high as possible, so
that the Helmholz coil can be rather high, and so that its center is
about 14 cm above the top of the top aluminum plate of the magnet where
the field is slightly less than 0.03 Tesla, dictated by the fact that
the Helmholz coils and our 50 V 10 A supply could reach this field,
and buck it to allow us to get to nearly zero field. [Do you wonder
why not zero? If we went close to zero we would have to verify that
there is no transverse field component at the magnet center.]
The solid wires of each coil
are connected to separate short 2-wire cables that are tied down with
tape to the support assembly. Then these cables go to a temporary screw
terminal barrier strip where the are connected in series, and from there
via a few feet of multiple stranded wire cable to a 24 pin AMP "circular
plastic connector" type 23-24 which hangs on the cable (more trouble
to order than to make. Do not bother to get a crimping tool, solder
the wires.). The solid wires must not flex in ordinary use! This makes
a compact unit that can be lifted on and off the magnet when moving
in or out. This magnet produces more than 320 G using a 50 v 10 amp
Kepco supply which is left turned on, and whose output connection is
on and off with a solid state relay. A reversed power diode across the
coils protects everything. See Fig. EC.
This is the fringe field 13.5 cm above the magnet. The gradient from
the main magnet is about 10 G/cm there. For Dmitri's work we could get
the field homogeneous to about 1 Gauss over 1 cm by running unequal
currents through the two coils. We did this by connecting two cheap
Tektronix supplies carrying opposing currents to the two coils. You
can connect several current- regulating supplies to the same place but
you have to put in series resistors or series fixed voltage supplies
in series in order that the supplies can operate on a positive voltage
within their range. We will do this if we want to measure R2 or do polarization
transfer at low field. However, it is unlikely that we will do this
except perhaps for R2 of 31P. We probably
cannot get much above 1000 Gauss with this magnet, and in most cases
T1 is too short at this field to allow us to do any experiments.
It would be nice to have
a temperature sensor that will turn off these coils. Otherwise in case
of an electronics failure the 500 magnet might be damaged when the magnet
goes up in flames.
The extender tube
(fig. ST-5A) can be put on the top
when we need it, to get to very low fields by raising the sample to
14 cm above the top of the magnet and using the field of the coils to
buck the remaining ~300 gauss there to nearly zero. Because it is hard
to machine with precision we made it slightly larger in diameter than
the glass. Probably 0.804 inside diameter would work better to avoid
problems. It is very straightforward, mating at the bottom part (8)
in a precision manner, and shaped at the top like the top of part (7),
so that the stop-tube-clamp can be put at the top of the extender tube
instead of on the top of part (7). It is awkward to have this tube attached
if it is not needed, and I designed it with trapped screws where it
attaches to the top of the shuttle tube, which can easily be tightened
by hand (Fig. ST-5B). Only the lower
half of the inside of the brass tube has to be well machined because
the upper half does not actually contact the shuttle. It would be hard
to machine an extender tube much longer than this one unless you have
an unusual lathe, and instead a glass tube coupled with plastic caps
as described above would be needed.
The stop tube, fig.
ST-6, is very simple. We have 3 of different length, overall 20,
38, 81 cm. This is a good assortment to cover the full range of fields.
The shortest is for use at very low field with the extender tube. A
short shoulder around the outside of the lower end piece (fig.
ST-5A) with small clearance to the glass centers the stop tube so
that the end of the shuttle tube that holds the O-rings will not hit
the shoulder of the stop tube that contacts the O-rings when the shuttle
tube hits the stop. This shoulder should be short, as shown, so that
the glass cannot be easily broken when the stop tube is not lifted out
The upper end of the stop
tube is simple cut off with a small bevel, at the top. It is coupled
to a rubber tube that goes to the solenoid manifold by a vacuum quick-disconnect
as described in a later section. It happens that the tube slides into
this fitting too far, so we have a stainless steel collar semi-permanently
attached about ¾ inch from the top of each tube to prevent this from
happening. This has to be put on the end after the clamp (just below)
is put on the tube.
The clamp assembly,
fig.ST-7, holds the stop tube in place to determine the lower field-value.
There is an identical one on each stop tube. It gave a lot of trouble
because it must be easy to move the stop tube when the clamp is released,
but it must hold it tightly enough to resist the thousands of poundings
during cycling. No matter what the design, it is good to put a piece
of paper tape on the stop tube just above the clamp when you start a
new run so that you can look in on the spectrometer every so often and
see if the tube is moving upward during the run, at least if you are
not confident about this point.
We list and explain the components
of the clamp starting from the bottom, in the order in which they have
to be placed on the stop tube. The first component is, when in place,
just above part part (8), fig. ST-4G
(and above the plastic cap (9), fig ST-4H,
which it MUST not touch), is a ring (Delrin or aluminum) about 1.2 "
OD, 5/8 " ID, ½" long (not shown)) that sits in the small well at the
to of part (8) and centers the stop tube. It is machined to slide easily
on the stop tube and fit easily into the well. It is essential that
this end of the stop tube be moderately centered so that the shuttle's
upper end will enter the stop tube smoothly. After this is an optional
1/16 thick O-ring 5/8 or ¾ inside diameter, as a cushion. Next is the
part (A) fig. ST-7 which is in fact
two nearly identical pieces of aluminum or brass that fit around the
5/8" stop tube with the central hole illustrated made large enough to
slide on the tube without much resistance, as already mentioned. The
gap between pieces allows the clamp to be closed tightly on the tube.
One side of the clamp is closed by a 10-32 screw with a tight lock nut
and a spacer chosen to hold the clamp slightly open (all not shown).
The spacer consists of washers or thin shim stock, and they are threaded
onto the screw after it is inserted through the un-threaded hole of
one side of a clamp, and before the screw is threaded into the other
side of the clamp. The screw on the other side is closed with the wrench
(S2) that can be tightened without great effort. It has to be long enough
to clear the various components nearby. Next on top is another O-ring
for a cushion, and finally a top plate (B) that holds two knurled screws
(S1) in two small tubes that extend below it. The part (B) is put in
with these tubes pointing downward and is oriented as shown in
fig. ST-7 so that the flat plat is in contact with the O-ring. The
length of the small tubes is such that the lower ends of the tubes almost
touch the upper surface of part (8), and can be screwed into holes tapped
into that plate to secure the clamp system. The idea is that these screws
can be tightened to slightly compress the O-rings and firmly capture
the clamp, without preventing the clamp from being loosened. The screws
S1 and plate assembly trap the screws so that they do not get lost.
Finally, a small slot was cut in one of the clamp pieces to make it
easier to close while permitting a looser fit when unclamped.
for pulling out samples is shown on fig.
ST-8. It consists of a plexiglass fitting cemented to a standard
US ¾ size sweat-solder Tee. To the side arm of the Tee is permanently
attached a small valve, and a rubber tube is permanently attached from
the other side of the valve to the vacuum reservoir on the Tower (below).
The other arm of the Tee is not used and you can probably use an elbow
instead of a Tee. The valve is normally closed. To remove a sample the
stop tube has to be removed by unscrewing the knurled-knob screws that
go through part (B) page ST 7, and pulling the stop tube out being careful
not to break the shuttle tube. Then the bottom of the sucker assembly
is pressed into the 1.2" diameter shallow well normally occupied by
the lowest part of the clamp assembly, in the top of the shuttle assembly
(see part (8) fig. ST-4G), and the
valve is opened. The sample rises and you may see the top of the shuttle
enter the plexiglass piece. You then gently lift the sucker, with the
sample stuck to its bottom (leaving the valve open, of course). Once
it is raised about one inch you grab the shuttle with your fingers and
lift it and the sucker out, and eventually close the valve on the sucker.
I install the shuttle tube assembly before the probe and, obviously,
after the Helmholz coils are in place. Then insert the probe, as usual
taking things very slowly. For the 10 mm probe there is no problem.
For the 5 mm probe you have to worry about a vestigial ridge that previously
aligned the probe's gradient connector. [these lines were re-routed
by Varian to come out the bottom of the probe on a connector, at no
cost, after the probe was delivered. Be sure to ask them to do this,
apparently it is easy except there was some indication that they did
not do a great job of by passing these leads against RF interference.
Almost certainly they could omit the ridge I am talking about, it appears
to be made of epoxy]. For use with this probe I put in a matching groove
(on part (10), fig. ST 4A, and by
rotating the entire shuttle tube assembly the ridge and shoulder can
be lined up. Varian's ridge can also fit into one of the slots in part
(1) that is put in to let air pass. In either case you have to be very
careful not to force the shuttle assembly to rotate and break something
or deform the thin upper aluminum tube on the probe. [Better get Varian
to leave out the ridge! Bruker does not have such a problem].
Remember, do not loosen the
probe at the bottom the way you normally do for a Varian probe. Set
the probe height up once, then do not change it (assuming you have your
own probes). By not having the shuttle tube and probe in direct vertical
contact, as Varian does, I hoped to avoid transmission of vibration
to the probe when the shuttle lands at the bottom.
Adjustment. The only
adjustment is to have the height of the shuttle tube such that the sample
is centered in the sensitive region of the probe, when it is not being
sucked up. By sealing the NMR tube in in a reproducible way I hope to
avoid making this adjustment all the time, but it will be necessary
initially and occasionally thereafter. The height is changed by screwing
in/out the three knurled screws at the extreme top. Adust one, then
adjust the other two to equalize the force on them, changing height
about 2-3 mm at a time. Keep track of what you do by measuring the distance
between the top surface of the Helmholz coil assembly and the bottom
of the flange that holds these screws. Lock on the sample, optimize
the shim carefully including the phase, keep track of the lock gain,
and keep to lock power as low as possible while the lock gain is near
maximum. Note the lock signal size, change the height, optimize shim
settings and lock phase, try again. Very tedious, find the highest lock
signal. (It might be better to use the proton or phosphorous signal
instead but we have not done so.)
If you keep track of the
height as described below, then if you do not get the sample position
just right, as determined by measuring the distance from the sample
center to the bottom of the nmr tube adapter, you can compensate for
this error by changing the height of the tube at the top. Be sure to
remember that you did so, and change it back for the next sample. Then
the tedious adjustment can be avoided or at least speeded up. (This
works for us).
You may find that the vertical
screws that go through the top of part (8), to adjust the height of
the shuttle tube as just mentioned, end up at an inconvenient heigh,
so that the screws have to be screwed all the way in, or all the way
out, or nearly so. (In the former case spare blocks of metal can be
used as extra spacers). If so, you can change the wat you cut the NMR
tubes to length and cement them together, or else shorten or lengthen
(re-make) the rods RCB. Sorry.
Clamping the stop tube
at the correct height. The following applies when not using the
Helmholz coil to get to very low field. Before you run, measure the
overall lengths of the three stop tubes or refer to your notes if you
have already done so. Pick a likely stop tube and call the overall length
S. Before dropping each sample+shuttle in, measure the length
N from the liquid sample center to the top of the upper O rings
on the shuttle. Measure the height C of the Helmholz coil assembly
from the magnet top flange to the support assebmbly top (note that this
is 9mm less than the actual height of the coil support in our design
because the legs doe not sit on this flange.
For each R1
run, first decide what Tesla you want to get. Refer to the graph that
tells you what the depth D is below the top of the top plate
of the magnet, to get this field. You want to calculate the height H
to set the top of the stop tube above the Helholz coil assembly top.
By drawing a diagram on a piece of paper you will find the relation
D=N+S-C-H, or H=(N+S-C)-D. So you calculate (N+S-C)
when you put the sample in, and write it down, and subtract D
to get the height H for each Tesla value. Now with the meter
stick and a drafting square you try to clamp it at this height, as tightly
as possible. The put a piece of paper adhesive tape on the stop tube
to easily see, during a run, if it is moving as a result of the shuttling.
If the height H is calculated to be negative, or positive but
too small to get two cm or so that is needed for the clamp assembly,
then you have to use another stop tube of a different length, or you
may not be able to reach the desired field. If H is calculated
to be too high for convenience, or the stop tube will not pull out that
far, you may have to use a longer or shorter one.
Running at very low field
(below about 300 G.) Use the shortest tube, which will permanently live
on the upper end of the brass extender tube. It has a commercial stainless
collar for a clamp. It is supposed to be set so that the sample is sucked
up to be centered half way between the two helmholz coils. We set this
by eye, in the magnet, by unplugging the electronics from the wall power
(which opens the normally-open vacuum solenoid valves) and connecting
the vacuum line to the wall, and opening the suction supply valve on
the wall (and setting the vacuum regulator, if necessary to a couple
of inches vacuum). If the sample is not in the right position half way
between the Helmholz coils, the clamp has to be removed from the brass
extender tube, and the clamp loosened with a hex wrench and reset to
make the position correct.
To set the field, a parameter
"cur" is set in the sequence FCLtonep, for example, to a number 1 to
255 according to a table that I supply, of cur vs field. Not that setting
cur to zero turns off the field cycling entirely. You can do experiments
with several values of cur, and one at 11.7 T, in que'd experiments.
This completes description
of the parts that were made by a professional machinist. Most of the
rest was assembled by A. R. with only hand tools and a drill press.