Recently I posed three questions regarding cryoprobes to AMMRL. Here is
a summary of the responses that I (very quickly, I might add) received.
Question 1 related to the ability of the 15N coil to withstand CPMG-type
spin locks. Some institutions prefer to avoid putting this much stress
on the cryoprobe, but it seems that the probes hold up well provided
that one follows Bruker's directives to use a 180 pulse LONGER than 160u
and not extend the CPMG past 250 msec and use an inter-pulse delay of at
least 900u. Unfortunately, this nets a relatively weak pulse train
which may allow development of substantial HN 2-spin coherences.
It was suggested that one monitor (cryo) heater output and that a
noticable drop would be indicative of too much power. Bruker's
monitoring software only gives real-time readouts of probe and amplifier
power reserve, the rest is password protected, so it's not practical to
check this during a run.
Question 2 asked whether Bruker users kept the cryoprobe coils clean
with an equivalent of the Varian cryonoisetest routine (i.e., repeated,
fairly high power pulses to burn off contaminants). It's more commonly
spoken about with Varian systems (including at the 2005 ENC user
conference). The Varian philosophy seems to be that it's a good idea to
do this (but that it may not really help much) while the Bruker
philosophy seems to be that there is no point (but that it probably
won't hurt, either).
The upshot is that some people believe it's necessary on both vendors'
probes, while others believe that it isn't. It seems to be a good idea
on burn-in. Whether it is necessary as a continuous practice or not may
be system dependent, and is likely coil-material dependent. The optimum
solution is to keep your spectrometer busy, and to be aware that
performance degradation on (especially) 13C might be aided by a repeated
burn-in.
Question 3 asked whether other Bruker users have noticed a steady, rapid
(~4%/day) drop in preamp reserve power. It does appear to be fairly
common among systems that have been sitting around for extended (months)
periods between delivery and installation. Ours sat an embarassingly
long time while we hashed out siting details. The cause is likely
contamination of the helium compressor system.
Solutions range from a warmup/cooldown cycle to changing the adsorber in
the helium compressor to more extended helium compressor/system
cleaning. The extent of cleaning necessary seems to depend on how fast
the power drop is. Bruker is aware of our situation (it occurred
immediately after installation), and it looks like a fairly serious (3-4
day) cleaning process will be required.
Thanks to everyone who has shared their insights and experiences.
David Horita
-----------------------------
David A. Horita, Ph.D.
Department of Biochemistry
Wake Forest University School of Medicine
Winston-Salem, NC 27157-1016
Tel: 336 713-4194
Fax: 336 716-7671
email: dhorita_at_wfubmc.edu
web: http://www1.wfubmc.edu/biochem/faculty/Horita.htm/
Received on Tue Jun 14 2005 - 16:16:12 MST