I sent this yesterday AM and haven't seen it posted yet?
P. Bruce Deker (PhD)
attached mail follows:
Hi all, seeking to confirm or knock down some theories I have:
When sweep width for 31P is increased from default 100 ppm to desired 400 ppm on a Bruker QNProbe (pneumatic switching between 31P, 13C, 19F on the same inner probe coil (gold ball point pen spring appearance coil) with 1H on the outer coil) in a 1990 ARX 500 magnet system the sample temp is observed to rise 5-7degree K. The amount of temp rise can be minimized by increasing the interscan delay which gives time to dissipate the heat with the cooling air. The power level of the pulse and the 30 degree pulse width which was calibrated at the appropriate power level to get the specified 90 degree of ~ 7-9 usecs is not changed so sample heat up does not appear to be caused by more power going through the probe? Ostensibly what is happening is that since the frequency range is opened up more of the pulse is absorbed by the sample. On that specific probe the heteronucleus coil is just mm away from the sample tube with a very thin glass dewar between them. While _at_ 100 ppm SW the o1 (or tof) is barely visible at the center of the spectrum, however _at_ 400 ppm SW the o1 "peak" is much larger relative to sample peaks and noise. My belief is that since pulse duration (2.5usec) and power of the pulse are not changed in the paramer set there should be no significant damage to the probe by increasing SW. However my concern is that the elevation in temperature near the coil may cause arcing of the 31P coil? Please confirm, refute add insights.
On a 1995 Varian broadband probe model # 99337500; 31P (inductance stick) , 13C (no stick), 15N (cap stick) are on the inner coil which is a helmholtz in shape and is made of foil affixed to the heavy glass dewar, no temp rise is noted on increasing to 400 ppm 31P SW but the tof "peak" is larger. However air flow with that probe is realy high. The temp sensor is not that close to the coil so I still have the same concern with possible localized high coil temperature and arcing. Any insight or experience with this probe concerning heteronucleus coil arcing with increased SW?
Thanks much.
Regards,
Bruce Deker
P. Bruce Deker, Ph.D.
NMR Facility Coordinator
Univ. of Vermont, Cook A-214
Burlington, VT 05405
pdeker_at_uvm.edu
802-656-3125
Received on Wed Jun 13 2012 - 04:48:09 MST