There appear to be at least two forces in the universe that can cause
asymmetric spinning sidebands:
Probes: Chmurny and Hoult have a paper (Concepts in MR, 1990, 2,
131) that discusses asymmetric SSBs. Turns out they are
due to probe effects, in particular, coil leads (usually).
Shims: It's a 3rd order problem." Meaning z2x, z2y, x3 and/or y3
The spike is probably from Q-modulation, and the broad, asymmetric
part is the true inhomogeneity SSB. Good web site explaining how
to shim these out at http://www.acornnmr.com/Sam/symptoms.htm
Our problem turned out to be a combination of the two. We had
cryo-shimmed with our newest probe, an early inverse VSP.
It was one of the first generation of Bruker narrow bore probes
to be used with a spacer in a wide-bore magnet. The spacer was
a big piece of metal that will probably last hundreds of years,
but unfortunately distorts the B0 field. This can be replaced
with a teflon spacer, but the previous owner wasn't notified.
Hopefully pro-active tech support will soon save us from beating
our heads over solved problems. At least we only threw time, not
a new set of shims at the problem :). Cyro-shimming with a 5mm
1H/13C probe resulted in 1% SSB which the rt shims were able to
take to under 0.1%. Line-width at 0.55% is 3 Hz.
Off topic: multi-vendor spectrometer
The world still doesn't favor mixed marriages. No one else admitted
to having such a system, although there was a system that had three
different consoles and three different magnets. After 6 months of
intensive consoling, our pair seems to be doing well together.
They still can't decide upon a name. Some days its Brarian, and
some days Vucker. Usually the latter on days that counseling is
required :). There is still stuff to do. We haven't decided whether
to use the BVT-1000, or the Oxford temperature controller.
The father of the bride says that she's capable of < 1 Hz at 0.55%.
The magnet is still drifting up 7 months after energizing. If it
keeps going, its possibly the most likely way for us to obtain a
Very High Field instrument :).
Thanks,
dave scott
iowa state university
515-294-4057
"Some drink at the fountain of knowledge...others just gargle."