Signal
to noise improvement by cooling the rf coil and tuning elements
(previously found only in liquids cryo probes) now
for solids MAS.
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.
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.
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 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 4/02/2011
To view a poster displayed
at The CRYO-NMR conference in Southhampton UK 2009,
click below
"Preliminary Experimental
Results from a 1H /13C /15N CryoMASTM Probe
for Solids NMR at 500 MHz"
