For 11B MAS in Bruker MAS probes, I have used the "DEPTH" method (D. G.
Cory and W. M. Ritchey, J. Magn. Reson. 80, 128, 1988). This tends to
select signals from regions inside the coil, and it works on the basis
of the rf field amplitude. It is useful also to suppress background
signals of 1H, 19F, and 13C, which are in the Bruker MAS probes. DEPTH
typically reduces the background by a factor of 10 to 100, depending on
where the background material is located in the probe.
My implementation of DEPTH for the MSL goes something like this:
START, D1 [F1 @PLS1 RGATE] ; 90 DEGREE PULSE
D2 [F1 @PLS2 RGATE] ; 180 DEGREE PULSE
D2 [F1 @PLS3 RGATE] ; 180 DEGREE PULSE
D3 [STA RGATE] ; PRE-ACQUISITION DELAY
D0
++PLS1
++PLS2
++PLS3
GOTO START
BEGIN LISTS
PLS1, +X -X +Y -Y
PLS2, +X +X +X +X +Y +Y +Y +Y -X -X -X -X -Y -Y -Y -Y
PLS3, +X
RLS, +X -X +Y -Y -X +X -Y +Y
END LISTS
; NS = MULTIPLE OF 16
; DUMMY SCANS (DS) MAY IMPROVE THE BACKGROUND SUPPRESSION
DEPTH works best when the 90 and 180 degree pulses are accurately set.
This complicates things for quadrupolar nuclei such as 11B, since the
effective rf amplitude (or "nutation frequency") depends on the size of
the quadrupole interaction. Therefore you may find that the optimum
pulse length settings are sample-dependent. Also the DEPTH sequence may
distort the relative intensities in the spectrum. At least, these
effects are theoretically posssible.
-- Anthony Bielecki Applications Scientist Bruker Instruments, 19 Fortune Drive, Billerica, MA 01821 Telephone: 508-667-9580, ext 145 Facsimile: 508-667-0985