CryoMAS

Doty CryoMAS Probe – The Cryo Probe for Solids

Signal to noise improvement by cooling the rf coil and tuning elements
(previously found only in liquids cryo probes) now for solids MAS.

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A Doty CryoMAS Probe

  • Factor of 5 increase in S/N for room temperature samples by cryogenically cooling the rf coil, rf circuit, and preamps
  • 3 mm spinner for MAS up to 20 kHz
  • H/C/N triple resonance tuning
  • Independent control of sample temperature from -140oC to +80oC with N2 spinning gas
  • Automatic sample eject
  • Cryogen-free operation with closed-loop GM cryo-coolers; equipment costs similar to current cryo probes for liquids
  • For wide-bore magnets up to 600 MHz

S/N Comparison

These spectra were taken at 300MHz with a Doty 3mm MAS probe
and a Doty 3 mm CryoMASprobe.WEB-cryoMAS_SNR_comparison-1

In the spectra above, a rotor containing 1% boric acid solution was used to acquire 11B spectra at 7 T with 32 scans using a conventional 3 mm Doty CP/MAS probe and a 3 mm Doty CryoMAS probe. As shown in the traces, the signal-to-noise ratio from the cryogenically cooled MAS probe is more than 3 times higher than in the conventional MAS probe for the same number of scans on the same sample. Use of a cold, ultra-low-noise preamp also contributes to the factor of 3-4 gain in sensitivity that can be expected from the Doty CryoMAS probe.

 

The goal of this major, long-term development effort has been to achieve an order of magnitude gain in S/N in widebore Magic-Angle-Spinning (MAS) NMR probes using cryogenically cooled RF coils. This allows the sensitivity gains of cryogenic cooling to be extended to spectroscopy on samples that are rigid solids, as well as inhomogeneous mobile systems (such as liquid crystalline samples).

As with liquid sample cryogenically cooled NMR probes, the CryoMAS probe achieves improved S/N from the combined effects of reduced noise temperature and improved Q, in spite of a reduced magnetic filling factor. An even greater improvement in S/N, 8X, will be possible with the CryoMAS probe, versus a standard liquids probe yielding a 4X S/N enhancement. In addition to improved S/N for samples near room temperature, the CryoMAS probe allows independent control of sample temperature from -140oC to +80oC (with spinning gas) while the RF coils and capacitors are kept at 23 K.

Doty CryoMAS wide bore,
3 mm H/C/N MAS NMR probe.

Web-CryoMAS1

 

 


The engineering challenges of developing a CryoMAS probe have been substantial but not insurmountable. The probe currently includes a triple resonance (1H/13C/15N) RF circuit, a 3 mm insulated ceramic spinner that allows independent maintenance of the sample temperature, a vacuum insulated coil and capacitor region that allows cryogenic cooling to 25 +/-2 K, cryogen free operation with commercial closed loop GM cryo-cooler, and reliable automatic sample eject. The CryoMAS probe has demonstrated a spinning speed of 20 kHz, moderate RF power tolerance, and factor of 4 gain in S/N on room temperature glycine with cryogenic cooling of the sample coils and circuit.

We continue to advance CryoMAS performance at higher fields by developing a series of thermally conductive, high-Q, high voltage, ceramic capacitors, that are used to provide a pathway for cooling the sample coil. All commercially available ceramic capacitors failed when cold, in vacuum, and at voltages well below what they would handle in air. The novel capacitors developed at Doty are designed to withstand RF breakdown and repeated thermal cycling, required in a cryogenic NMR probe for solids-type NMR pulse sequences.

In related NMR probe development, we have recently demonstrated sample temperatures down to 86 K (-188oC) with nitrogen gas for the Doty DI-4 (Drop-In 4 mm) spinner on a narrow bore MAS probe See the 86K sample spectra. The design of the CryoMAS probe will make spinning at low sample temperatures even easier.

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