We
expect to deliver our first CryoMAS probe
in the spring of 2010
Factor of 5 increase in S/N for room temperature
samples by cryogenically
cooling the rf coil, rf circuit, and preamps
3 mm spinner
18 kHz MAS demonstrated
H/C/N triple-tuned circuit
RF coils maintained
at 25 K
Independent control of sample temperature from
-170oC 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
(Cryogens needed only for low sample temperature MAS)
For wide-bore magnets up to 600 MHz (750 MHz
in development)
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 will allow 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 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 -188oC to +80oC
(with spinning gas) while the RF coils
and capacitors are kept at 23 K.
Figure 1. Doty CryoMASTM wide bore,
3 mm H/C/N MAS
NMR probe.
The engineering
challenges of developing a CryoMAS probe have been substantial
but not insurmountable. The probe (Figure
1.) 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 18 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
(note the preamps were not cooled in this
test). The results are shown below in Figure 2.
Figure
2. Natural
Abundance 15N NMR spectra showing ~4X gain in S/N
with CryoMAS probe. Another
2X gain in S/N is expected with a cold preamp. For both spectra:
glycine sample at room temperature, 10 μs
15N π/2,
no 1H decouple or CP,
11.7 T, 7 kHz MAS.
RF coils at 294 K, 64 scans, S/N=49:1.
RF coils at 25 K, 16 scans,
S/N=96:1.
We continue to advance CryoMAS performance 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 85 K (-188oC)with nitrogen
gas for the Doty DI-4 (Drop-In 4 mm) spinner
on an OptiMAS probe (See the 86K
sample spectra
on
the OptiMAS page). The
design of the CryoMAS probe will make spinning at low sample
temperatures
even easier.
The spinning speed of the CryoMAS 3 mm spinner is projected
to be 25 kHz at development conclusion, as well as 70 kHz 1H
rotating
frame
field strength on all channels.
Page
updated 3/26/2010
To
view a poster displayed at The CRYO-NMR conference in Southhampton
UK 2009,
