LN2-I Advanced Cold Gas Suppy System

Doty LN2-I: Enables Stable MAS Down to 90 K

Doty’s Liquid-Nitrogen-Injected (LN2-I) cold-gas supply system could revolutionize what is feasible with your current dual-stream XVT MAS probe – NB or WB, made by Bruker, Agilent, JEOL, or Doty.

The LN2-I solves three problems: (1) the fundamental instability of all previous methods of controlling flows of saturated vapor-phase streams, (2) getting the H2O content in the N2 entering the gas-cooling coils below 40 ppb, and (3) reducing heat leaks in the transfers. The patented Doty LN2-I system solves all these issues.

A controller is provided to deliver stable flows of pressurized saturated vapor streams to the probe for days of continuous spin-ning at amazingly stable speeds and temperatures down to 85-130 K, depending on the probe.

The above drawing illustrates the flows of one cold gas stream. The lines and coils are dou-bled to provide cold streams for both the bearing and the drive – from a single LN2 cryostat.

With the Doty LN2-I system, you may be able to run experiments stably for days if needed (your liquid N2 cryostat can be refilled without interrupting spinning) at lower temperatures and faster spinning speeds than you previously achieved in very short runs.

#95923 Liquid-Nitrogen-Injected (LN2-I) Cold-Gas System … $25,000
Includes: Ln2-I Heat Exchanger, Controller, 50 Liter LN2 Dewar, Exhaust Dewar, and Transfer Lines.
(US$ −Foreign prices higher, plus taxes.)

Briefly, How does the LN2-I Work?

It is impossible to stably control the mixed-phase stream (nitrogen liquid and vapor) from a gas-cooling system feeding a spinner assembly using conventional methods. However, it is easy to separately control both a cold vapor stream and a liquid nitrogen stream. So the solution is to cool nitrogen gas in one set of coils to a temperature that doesn’t lead to any condensation problems (say, 100-180 K), and then inject into that vapor stream the precise amount of liquid nitrogen needed to cool the combined stream to the desired temperature and balance all the heat leaks in the transfer from the LN2 cryostat to the spinner assembly.
LN2-systemTwo controlled room-temperature (RT) N2 gas streams go into the LN2-I for the bearing gas supply, and the mixed-phase stream (with the exact vapor fraction needed to deliver 100% vapor fraction at the desired temperature to the spinner bearing) leaves the LN2-I in a vacuum insulated transfer line. Likewise, two controlled RT gas streams enter the LN2-I for the spinner drive gas, and the needed mixed-phase drive stream emerges.For simplicity, the schematic drawing above illustrates the flows for just one of the cold gas streams – either the bearing or the drive. The lines and coils are simply doubled to provide cold streams for both the bearing and the drive – from a single LN2 cryostat.

Does it work for any temperature, or just near the saturated vapor temperature ?
Actually, not only does it work better than other cold gas supply systems at the low end, it also works better (much less LN2 consumption, better stability) in the range where they also work.

(A caveat is that we’re not yet sure how much of a benefit the LN2-I system will provide for 3-stream LT-MAS probes, but to our knowledge those have only been available for WB mid-field magnets, and they have their own issues.)

What is the low-temperature limit for stable, fast spinning?
Presently, we’re seeing 6.7 kHz at sample temperature under 102 K with an LXVT XC4 probe (extremely stable, for days, with periodic refills of the cryostat during the run). We expect to report NMR MAS data at temperatures down to 85 K in the near future.

Are there any moving parts that could fail? 
The only moving parts are in the standard, robust RT pressure regulators and valves in the gas control unit, that often last for decades.

Are there any small orifices likely to plug up with ice? No.

How compatible is it with automated control?
The degree of automation currently is limited, but the temperature and spinning speed stability are much better than what has previously been possible with automated controls at low temperatures. You easily set it, and it stays there. We find that with occasional minor adjustments it is easy to keep the temperature within 2 K and the spinning speed within 30 Hz (for many hours) on the Doty XC4, for example, for spinning in the range of 100-110 K at speeds in the range of 5-9 kHz with no automation.

With a Doty probe controller made during the past 10-12 years, one can achieve a factor of two improvements in temperature and spinning speed stability, but the minimum temperature at a given speed will be a few degrees higher. Within a year we expect to have a new controller that will permit even better stability at temperatures as low as is currently possible without electronic control, but it will require an upgrade in existing probes for full functionality.

What hardware comes with the LN2-I cold gas system?
1. An LN2-I gas control unit, which includes digital flow meters, flow-control valves, precision pressure regulators, digital pressure gages, the needed on-off flow valves, and a bank of moisture and HC traps to dry the source RT-N2 to the dew-point needed for extended operation (below 0.1 ppm H2O). (Specify high flow range, for 4-10 mm probes, or low flow range, for 2-5 mm probes.)

(The traps should last several years if the source RT-N2 is reasonably dry – below 5 ppm. They can be regenerated or replaced as needed.)

2. An 8-coil exchanger core assembly (with its four internal vacuum-insulated lines, etc.) that goes into the 50-L LN2 cryostat. (Specify high flow range, for 4-10 mm probes, or low flow range, for 2-5 mm probes.)

3. A standard 50-L LN2 cryostat (unless you don’t need another one),

4. Two vacuum-insulated flexible transfer lines with the needed internally dewared couplings.

5. Two 30-m rolls of HDPE tubing and a bag of connectors and adapters. (The only type of plastic tubing that will work is HDPE. Moisture diffusion through other types is much too high.)

6. A dewared fill tube and adapter for use with common transfer lines from pressurized 150-250 liter LN2 cryostats.

7. Instruction manual.

How soon will it be available?
Available now! Please contact us for more information and to order your system.

Why do you need improved LT-MAS? 
Magic Angle Spinning (MAS) Dynamic Nuclear Polarization (DNP) has recently demonstrated S/N gains of up to two orders of magnitude in many solids at ~100 K compared to conventional NMR-MAS. This has led to greatly increased interest in LT-MAS – both with and without DNP capability, in both NB and WB magnets, at low fields and high fields.


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