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NMR-003: Pressure Performance of NMR
& EPR Sample Tubes
The application of NMR and EPR techniques to investigations
in catalysis and the study of reactions involving gas and
liquid phase components has renewed interest in the pressure
performance of glass NMR and EPR sample vessels.
WILMAD glass sample devices should not be used in pressure
applications before the hazards involved are fully understood.
WILMAD does not guarantee the pressure performance of any
of its glass sample devices for NMR and EPR spectroscopy.
The nature of glass and the critical connection between
the handling of sample tubes and their pressure performance
makes it impossible for WILMAD to assure the performance
of its tubes in these applications. Indeed, the application
of glass vessels in pressure experiments is risky. WILMAD
does not encourage the use of its glass sample devices in
pressure experiments.
Glass Strength
However, there is an abundance of technical information
from the literature and this technical bulletin was prepared
to share some of this information with those preparing for
such experiments.
Engineering formulas for cylinders not longer than 300mm
predict the Pressure Maximum, P(max), of any rigid material
is:
P(max) = (Wall/O.D.) X K(t)
where O.D. and Wall are the outside diameter and wall thickness
of the tube, respectively, and K(t) is a constant related
to the Tensile Strength of the material from which the cylinder
is made. For most types of glass, the K(t) is approximately
20,000 p.s.i. (1406 kg/cm2). However, it is important to
understand that there are many unquantifiable characteristics
of glass which can radically affect the performance of glass
under pressure. These include interruptions in the integrity
of the outside surface of the glass, where imperfections
can reduce the Pmax by many orders of magnitude and cause
the glass to fail prematurely and catastrophically. The
majority of WILMAD NMR and EPR sample tubes are ground and
polished on the outside surface and the finish of these
NMR and EPR tubes appears quite smooth. However, the polishing
step is completed when a dimensional tolerance is reached
and the surface quality of these tubes can vary slightly.
It is important to appreciate that surface finish differences
on a microscopic basis, which can be viewed at 10X or 20X
magnification, can exist between tubes and can significantly
affect the pressure performance of these tubes.
Application Procedures
Thus, it is prudent in applying the engineering formula,
above, to apply a safety factor to account for surface quality
differences.
A Kt = 2,000 p.s.i. (140.6 kg/cm2) should be used. While
it has been reported that WILMAD glass sample tubes have
been taken to pressures beyond those predicted by the formula,
above, when using Kt = 20,000 p.s.i. (1406 kg/cm2), it is
wiser to obtain an empirical appreciation of the performance
of glass under pressure to support ambitious application
of glass at pressures above those obtained from this formula
when using K(t) = 2,000 p.s.i. (140.6 kg/cm2).
As many applications in NMR require the sample tube be
rotated at frequencies approaching 25 Hz, it is important
to test samples in a dynamic rather than just static mode
outside the spectrometer prior to performing any NMR experiments.
It is also important to protect yourself, those around you,
and your equipment from injury or damage at all times when
handling pressurized glass sample devices. If you suffer
a catastrophic failure of a sample tube under pressure in
a valuable spectrometer, you will not gain much research
cooperation from others whose work relies upon the same
NMR instrumentation. Proper shielding should be employed
outside the spectrometer at all times when devices are pressurized.
Contact your safety officer or director prior to performing
an experiments at elevated pressure to assure that all precautions
prescribed by your organization are followed. WILMAD can't
take responsibility for the results of failure of its glass
sample devices at elevated pressure.
Although the J. Young® Valve NMR Tube can be used at
small to moderate pressures, WILMAD offers a tube with a
more substantial closure in its Pressure Valve NMR Tube.
This Pressure Valve (PV) NMR Tube, is made with a heavy-duty
Teflon® Angled Needle Valve Assembly atop a standard
NMR tube. A modified flowmeter valve, the needle valve of
these 'PV' tubes can be attached to a Swagelok 1¦8"
nut and ferrule or to accessories available for attachment
to a vacuum rack. The valve can be closed by hand or using
a small wrench. Samples must be limited to those which can
pass through the 0.8mm orifice of the valve assembly. Axial
symmetry is achieved through the design and preparation
of the valve assembly. Although annealed, extreme caution
should be applied when using PV NMR tubes, as noted above.
Order the shortest tube possible to reduce the overall weight
of the tube, which is substantial with the valve attached.
Sample spinning or ejection can be difficult if your sample
is too heavy.
Alternate materials have been used in obtaining spectra
at high pressures, including Sapphire 1 and Stainless Steel
2. These materials should be considered when higher pressures
are contemplated.
1 C. Roe, J. Mag. Res. 63, 388 (1985).
2J. Jonas, et. al., J. Mag. Res. 42, 169 (1981).
J. Young is a tradename of J. Young (Scientific Glassware)
Ltd., London.
Teflon is a trademark of Dupont.
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