Cryoprobe safety and operating notes
Our 600 and 800 MHz cryoprobes have much in common, even though they
are produced by different vendors. For safe operation:
Read and be familiar with the details of the manufacturer's
requirements and recommendations for working with the probes.
The allowed high power pulse widths and decoupling fields are set up on
nonpolar test samples. The operating power is capped at this level and
the pulse widths on real samples float, depending on the salt
concentration in the sample. The Bruker "power check" feature
will prevent the pulse sequence from executing if the high power limits
are exceeded. The cryogenic power limits in BioPack will warn you if
you are exceeding the limits, but do not stop the experiment.
The amount of time that carbon and nitrogen decoupling is on must be
limited to avoid putting too much heat into the decoupling coil. The
vendors express those limits differently but the end result is almost
the same. All standard parameter sets on both instruments come up with
a pre-acquisition delay (d1) of 1 second and acquisition times (aq or
at) between 70 and 90 milliseconds. Under these conditions, the
decoupling is limited to a duty cycle of 8% or less. This ratio cannot
be increased. You cannot make d1 shorter, and if you make aq/at
longer, d1 must be correspondingly longer. The safe decoupling power
also may drop as aq/at gets longer. The safe course of action is
to stick with the standard parameters.
For sequences that use simultaneous decoupling of carbon and nitrogen,
the power on each channel must be reduced by half. (This limitation
applies to all probes, warm or cold.) This corresponds to reducing the
power level by 3 dB and increasing the decoupling pulse width by 40%.
In the Varian software, this should be done for you automatically; in
the Bruker software, it must be done by the operator. It is helpful to
use adiabatic decoupling at the lower power in order to recover as much
decoupling bandwidth as possible. Adiabatic decoupling on both channels
is standard in the current BioPack software. Instructions for
converting to adiabatic decoupling for carbon in the Bruker TopSpin
software are available in the NMR Guide--from the top menu bar, choose
Help, NMR Guide, and search "adiabatic decoupling." The pulse
shape and decoupling sequence are precalculated and are available on
the corresponding menu lists. Because the nitrogen chemical shift range
is limited, decoupling bandwidth is less critical there. Our standard
nitrogen garp decoupling pulse is 180 usec and increasing it to 250
usec with 3 dB less power is usually well tolerated.
One additional point with the Varian cryoprobe: note the probe heater
power before the experiment starts (typically about 5 watts). The pulse
sequence should not drop this value by more than 1 watt in the first
few minutes. If it does, stop the experiment and determine where the
excess heat is coming from.
One additional point with the Bruker cryoprobe: the sample depth is
strictly limited. Probe damage will result if the bottom of the tube
goes below the 21 mm mark. We set up the depth guage and our shims
around a sample position of 19 mm.
For optimal spectra: