I discovered yesterday, when upgrading BioPack on one of our systems to
the current 10-30-08 release, that the full calibration process was silently
changing the probe file values for C13dofCO and C13dofN15 from those set with
BPbiopack2. This is due to the setoffset commands introduced in BPbiopack2a
about six months ago in an attempt to automate the selection of the dof
values through the use of the lock frequency on D2O. This behavior can be
eliminated by deleting the three lines calling setoffset() from BPbiopack2a.
While automated setting of the various dof's would be nice, the use of
the lock frequency is fraught with dangers. It would require an additional
temperature correction which in turn would require a corrected temperature
value (determined with methanol). Also, there should be a correction for
the effect of salt on the D2O line position (which in turn creates an
uncertainty as to the true salt concentration of the sample). We have had
very good results across our five systems using the DSS line in the
doped water sample to determine the absolute zero frequency for 1H and then calculating
the same absolute zero frequencies for the 13C and 15N channels and using those
to calculate the theoretical frequency for 120 ppm 15N and 174 ppm 13C.
Just thought everyone should be aware that if you have done a full
calibration in a recent BioPack that your referencing may not be what
you expect it to be.
Jack
--
------------------------------------------------------------------------------
Jack W. Howarth, Ph.D. 231 Albert Sabin Way
NMR Facility Director Cincinnati, Ohio 45267-0524
Dept. of Molecular Genetics phone: (513) 558-4420
Univ. of Cincinnati College of Medicine fax: (513) 558-8474
Received on Wed Dec 17 2008 - 14:00:07 MST