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NMR-007: Coaxial Inserts in NMR Studies
NMR experiments that involve external reference/lock or
that compare two solutions require two well defined sample
volumes within the probe. WILMAD makes a variety of unique
sampling systems that create such distinct sample volumes.
This report will help you select the right insert or coaxial
system for experiments where two volumes are needed. Refer
to the table on the last page for a summary of the inserts
WILMAD makes and what experiments each can be used for.
Challenges in Coaxial NMR Sampling
In superconducting NMR spectrometers, cylindrical geometries
work best in most applications requiring inserts. While
spherical sample devices like WILMAD's 529-A series can
be used for some comparisons, it can be difficult to maintain
magnetic field homogeneity over the sample volume. It's
easier to make highly precise cylindrical shapes in Pyrex
than to blow precise spherical shapes, except for small
sizes (spherical bulbs <5mm OD). Consider saving spherical
sample inserts for microsampling applications. When determining
magnetic susceptibilities, modulation sidebands caused by
imperfect sample tubes can make precise measurements of
chemical shifts impossible. Only highly skilled manufacturers
can offer precision inserts of this type, so the number
of suppliers of coaxial inserts is severely limited.
Generally, coaxial systems made only of Pyrex will provide
a higher degree of precision than systems incorporating
Teflon components. Because Pyrex is rigid while Teflon is
soft and cold-flows, Pyrex can be made to tighter tolerances
than Teflon (see precautions in Resonance
Report NMR-010 about preserving NMR tube precision).
If you're performing Variable Temperature (VT) experiments,
all-Pyrex coaxial systems are preferred. With these systems,
components have the same coefficient of expansion and you'll
avoid breakage that can sometimes occur when you mix plastic
and Pyrex under VT conditions.
Inserts in External Referencing
External referencing has been replaced in recent years
by assignment of chemical shifts calibrated to the deuterium
signal of the spectrometer's lock channel. But when you
have a sample that you shouldn't dissolve in a deuterated
solvent, you'll need an external lock and a reference (internal
is pre-ferred). However, when your samples are too precious
to mix with a reference chemical that is difficult to remove
or reacts with your sample, then external referencing can
be employed.
In these rare circumstances, the external reference must
be present only in sufficient quantity to produce a discernable
signal in your spectrum. Generally, small amounts of the
reference are needed if the signal produced by the reference
lies away from the critical regions of the NMR spectrum
of your sample. WILMAD Product Number 516 is a coaxial system
that provides a tiny 50µ space between the insert
and outer tube, into which a few drops of concentrated reference
can be placed. Sets are available in sizes from 3 to 15mm
OD. Filling factor is reduced with this system, so use it
only when you aren't sample or solubility limited. If you
need a larger reference volume, e.g. to combine an external
reference with an external lock, follow the recommendations
for external locking, below.
The 529 microsample system can also be used with samples
studied in 5mm sample tubes. Place or seal a small volume
("16µl) of reference material into a 529-D capillary
and carefully position in a 5mm tube using the Teflon Holder
(529-B). Make certain the capillary is straight in the Teflon
Holder.
The 529-D is ideal for sample or solubility-limited situations,
since you get spectra with only a small reduction in filling
factor.
External Locking
The lock channel of an NMR Spectrometer requires a significant
signal to maintain Rf stability. Thus, coaxial systems like
the 516 series should not be used for external locking.
The spacing between the walls of these inserts and outer
tubes (50µ) is not sufficient to support a spectrometer
lock, even if 100% deuterated solvents are used.
The most commonly used coaxial insert, which will give
the best lock signal, is WILMAD's WGS-XBL Series inserts
(X = 4, 5, 8, 10, 12, 13, 15, 16, 18, or 20). These fit
standard WILMAD outer tubes snugly, just like the 516, but
each tapers to a smaller diameter stem in the lower 50mm.
For the most common 5mm size, the stem is 2.5mm OD. The
capacity of the stem, where reference material goes, varies
in relative capacity from 5%(for large diameter outer tubes)
to 16% of the sample volume. You place your sample in the
annular space. Add just the amount of sample that will fill
the annular space below the taper of the insert. For approxi-mate
volumes of the annular space of the outer tube (see WILMAD's
NMR Catalog).
If you seal your lock/reference solution in the insert,
you can switch the insert from one tube to another simply
by rinsing and drying the outside of the insert. If you're
performing VT studies of two solutions, then this is the
ideal coaxial system. Special insert sizes can be made if
the standard sizes aren't right for you.
WILMAD Product Numbers 517-519 can also be used for external
referencing and magnetic susceptibility comparisons, too.
Although confined to the 5mm OD tube size, these coaxial
systems offer a way of varying the ratio between the reference
and sample volumes without buying specially prepared products.
And they're ideal for VT experiments, because the components
are all made from Pyrex. It may be necessary to use a wire
hook to position or remove the Pyrex spacers, which fit
snugly into the outer 5mm tube. Avoid scratching the tubes
or positioning the spacers in the Rf coil of the spectrometer
probe.
The least costly approach to external referencing is WILMAD's
Teflon Sleeve Adapter Sets. Useful for probes that take
tubes between 8 and 20mm OD, these permit you to combine
standard NMR tubes to make coaxial sets. Because of the
machinability of Teflon, which is soft and easily cold-flows,
these sets are of moderate precision. Teflon Adapters should
not be used in VT studies. The lower adapter is easily positioned.
It has notches on the outer edge that permit you to position
it with a wire hook. Avoid scratching the tubes or positioning
the spacers within or near the Rf coil of the probe. Teflon
Sleeve Adapters for 5mm tubes are not available. Use WILMAD
product numbers 517-519, above, instead.
Measuring Magnetic Susceptibility
Because of the high degree of precision of WGS-XBL inserts,
you'll get the best resolution in susceptibility comparisons
using these coaxial inserts. The 517-519 Series of inserts
also offers the degree of precision and filling-factor needed
for these critical experiments, but WGS-XBL is easier to
use. We recommend that WGS-XBL inserts be used for all magnetic
susceptibility studies, unless there is an overwhelming
reason to avoid them. A reference solution can be sealed
into a WGS-XBL insert that's ordered 50mm longer with a
constriction. Inquire about prices.
Pyrex is a trademark of Corning.
Teflon is a trademark of duPont.
| Product Number |
Description |
Sizes |
Experiments |
| 516 |
Coaxial Insert |
3-15mm OD |
External Reference |
| 517-519 |
Coaxial Insert |
5mm OD' |
Microsamples, External
Reference, External Lock, Susceptbility Measurements |
| 529-A Systems |
Spherical Bulbs |
5 - 20mm OD |
Microsamples |
| 529-D Systems |
Capillary Tube |
5mm OD |
Microsamples, External
Reference |
| 529-E Systems |
Cylindrical Bulbs |
5 - 20mm |
Microsamples |
| WG-1132-1 to WG-1132-32 |
Teflon Sleeve Adapters |
8 - 20mm OD |
External Reference, External
Lock |
| WGS-XBL |
Stem Coaxial Inserts |
4 - 20mm OD |
Microsamples, External
Reference, External Lock, Susceptibility Measurements |
|
