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NMR-006: Valve Tubes and Air Sensitive Samples in NMR

Keeping samples sensitive to air or moisture from making contact with atmospheric oxygen or moisture requires special equipment designed for these samples. Otherwise, you must handle samples under an inert atmosphere, e.g. in a glovebox. When NMR studies of such samples are required, the sample tube must be compatible with the sample handling system you're using. In a glovebox, that might mean using standard NMR tubes with a serum cap, such as the WILMAD 521-S. Or it might require a tube that can be attached to a vacuum rack for vacuum transfer of a sample into the NMR tube. This report describes the diversity of products available for handling air or moisture sensitive samples, the advantages of each, and their application to NMR studies. The table on the last page summarizes the applications of WILMAD Valve NMR Sample Tubes.

Drying Samples
If your samples are sensitive to moisture, then you'll want your solvents dry. Resonance Report NMR-004 provides details on selecting and handling solvents for best results, including steps for drying the solvents. Molecular sieves, like Linde 3A or 4A, non-spherical in shape, work well with most NMR solvents. But their use with sample solutions could catalyze degradation reactions, so test these or any other drying agents to see that your samples aren't adversely affected before drying any NMR sample.

Freeze-Pump-Thaw Cycles
Freeze-pump-thaw (FPT) cycles can be used to remove residual oxygen, which is paramagnetic and decreases T2s, or other unwanted gases from the sample solution. You'll need a vacuum rack to do these, though. A WILMAD Tip-Off Manifold, such as 552-5, can help you build an NMR vacuum rack. A dry ice/slush bath (-56.64° C) is usually sufficient to freeze most samples in the tube. Liquid Nitrogen (-210° C) will also work, but using this stringent cryogen can lead to problems like oxygen condensation and tube failure even if you're careful about making temperature changes slowly. After freezing the sample, bring the pressure above the sample to 10-4 torr or less, close the stopcock above the tube, and let the solution warm to room temperature to complete a cycle. After three cycles, a tube can be flame sealed after refreezing the sample solution and assuring a moderate vacuum (<1 torr) above the solution. Valve tubes can be kept closed after the last Freeze-Pump-Thaw.

Sealed Sample Tubes
The most reliable method of preserving valuable samples for NMR is to seal the sample into an NMR tube. When samples are introduced via vacuum transfer and the tube is sealed in vacuo, you're assured the sample isn't exposed to air or moisture until the tube seal is broken. For longevity, no other method of conserving a sample is as reliable. That's why NMR Reference Standards continue to be made in sealed NMR tubes.

To make a symmetrical seal, it helps to have a sample tube that's constricted at the point of the seal. WILMAD constricted NMR tubes are tapered to a 1mm ID 25mm from the top of the tube. A microtorch works best, but you can even make a torch from copper tubing. Firmly seal one end of a 75mm length of copper tubing, punch 6-8 holes 0.5mm in diameter along one side, spaced about 8-10mm apart. Then, curl the tubing into a hook around a 25mm diameter pipe with the holes on the inside. When a gas/oxygen mixture is supplied to the tube, gas lighted at the punched holes will form a cylindrical array of flames that can be used to evenly heat the constriction. Adjust the diameter of the hook to your requirements. Using this 'torch,' symmetrical seals can be easily made. All samples should be frozen before applying heat at the constriction of the tube.

Taperlok® NMR Tubes
The first NMR tube made with a reusable valve was WILMAD's Taperlok® NMR Tube. This all-Pyrex valve is a specially modified 5/20 Standard Taper Joint that functions like a stopcock. Rotating the upper section (inner joint) to a certain position opens and closes the valve. When attached to a vacuum system (using WILMAD's Tip-Off Manifold, for example), the tube is ideally suited for T1 studies, where reproducible freeze-pump-thaw cycles are essential for getting consistent results.
This tube shouldn't be used with samples that generate any pressure during studies or storage. The joint can be easily separated by pressure build-up inside the tube, even with springs holding the joint together (in the version with hooks). But it's ideally suited for variable temperature studies that don't generate internal pressure because all components have the same coefficient of thermal expansion. Valves using plastic components may be strained or opened by changes in temperature, because their components change size with temperature at different rates.

WILMAD makes the Taperlok® joint from Quartz, too. You get the advantages of a valve system that doesn't mix materials of different thermal expansion coefficient for both Quartz NMR and EPR sample tubes, as well.

J. Young Valve NMR Sample Tubes
The most widely used valve tube in NMR today is this one. Like the gas-tight piston valves in laboratory glassware used with air-sensitive chemicals, the J. Young Valve is easy to use, seals reliably, and separates for easy cleaning.
And because the valve is made with O-Rings covered with Teflon® sheaths, the sample sees only Pyrex and Teflon®. You don't need to worry about material incompatibilities. After fusing the Glass Valve Adapter (GVA) under a stopcock on your vacuum rack, just slide the J. Young Valve into the adapter to position it on the rack. Open the stopcock and NMR Valve and you're now connected to a vacuum. Freeze-Pump-Thaws are easy. Even samples that generate moderate pressure can be studied with this tube. It's ideal for T studies. Vacuum transfers are as easy as air-sensitive sample studies. It's even been used in gas/liquid phase catalytic studies at relatively small pressures. Before using NMR tubes in pressure applications, refer to Resonance Report NMR-003. The J. Young Valve NMR Tube is recommended for use only over a ±40°C temperature range.

Pressure/Vacuum Valve NMR Sample Tubes
By combining a pressure rated valve with an NMR tube, the WILMAD Pressure/Vacuum Valve NMR Sample Tube lets you obtain spectra or study reactions of solutions under controlled gaseous atmospheres. Vacuum transfers can be made with plastic connectors and a Pyrex adapter that gets fused to a stopcock on your vacuum rack, both available from WILMAD. The PV tube can be attached to the regulator of a gas cylinder through Swagelok® fittings (1/8" OD tubing nut and ferrule), too, so you can add most any gas to an evacuated Pressure Valve NMR Tube. But before using any NMR tube in pressure applications, read the precautions included in Resonance Report NMR-003.
Sample compatibility is excellent, since only Teflon®, Viton®, or Pyrex are used in the PV Tube. Adapters for a vacuum rack add a Polyethylene component to the list, though. With the PV Tube, sample size is limited to solutions or low viscosity suspensions that can pass through a 0.8mm constriction without clogging. Powder samples, viscous liquids, or course suspensions will clog the valve. Use care when tightening the Pressure Valve. Teflon® threads can easily cold flow or strip. Replacements are readily available if the valve gets damaged. The Pressure Valve NMR Tube is recommended for studies only over a ±40° C temperature range.

Omnni-Fit Valve NMR Sample Tubes
To study the reaction of an air-sensitive sample with a compound that gets added in aliquots to your NMR tube between spectra, many other valve tubes and sealed sample tubes just won't do. But the Omni-Fit Valve is ideal. It seals with a double protection system that includes a plastic stopcock and septum. Just slip it on the top of a special 5mm NMR Tube. Because of the tight fit of the valve to the tube, the Omni-Fit tube has a 50mm medium wall section at the top.
To add aliquots of a reactant to the sample tube, rotate the stopcock to the open position with a small screw-driver so a syringe needle can be passed through both the septum and valve. Remember to dry the syringe carefully and flush it with dry nitrogen before drawing up the reactant so no moisture or oxygen is injected into your sample. Septa can be replaced even during experiments. Close the stopcock, remove the old septum, flush the top of the valve in a stream of dry nitrogen, then replace the septum cap with a new septum in place.

The components of the Valve and Adapters are made from Teflon®, Kel-F, Tefzel®, Polypropylene, and Pyrex, so chemical compatibility of the Omni-Fit NMR Tube is excellent. It's also the lowest cost approach to the study of air-sensitive samples available today.

You can do vacuum transfers through the Omni-Fit Valve with plastic connectors and a Pyrex adapter that you fuse under a stopcock on a vacuum rack. The Omni-Fit valve would be pushed off the tube by internal pressure, so it isn't recommended for samples studied at elevated pressures or over wide temperature ranges (±40° C). Because the valve is not axially symmetrical, the Omni-Fit Valve NMR Tube system is best used non-spinning, when possible.

Screw-Cap NMR Sample Tubes
Not truly a 'valve' tube, the WILMAD Screw-Cap NMR Tube from WILMAD provides many advantages which make it appropriate to discuss in this context. It's easy to appreciate the advantages of the Screw-Cap closures in NMR. You get the same security you have when samples are stored in a vial. Solid caps and open caps with septa are available for most NMR tube sizes. Injecting or withdrawing samples is as easy as with a vial, but you'll need a longer needle, available from Hamilton, to reach the bottom of an NMR tube.

The WILMAD Screw-Cap NMR Tube is the only NMR tube that lets you perform air-sensitive coaxial studies, since coaxial inserts (and vortex plugs) can still be inserted into these tubes as long as they're length is just a little shorter than the tube length. You'll need to load the tubes under an inert atmosphere, though. Because it's relatively easy to mount perfusion assemblies on an adapter that screws to the top of these tubes, the Screw-Cap NMR Tubes have also found occasional use in perfused cell NMR studies.

Omni-f

it is a trademark of Omnifit, Ltd.
Linde is a trademark of Union Carbide.
Taperlok is a trademark of WILMAD.
Teflon and Tefzel are trademarks of duPont.
Viton® is a trademark of DuPont Dow Elastomers.
Kel-F is a trademark of 3M Company.
Pyrex is a trademark of Corning.
Swagelok is a trademark of Crawford Fitting Co.