100 (327) BRUKER BIOSPIN User Manual Version 002
Practical CP/MAS Spectroscopy on Spin 1/2 Nuclei
- conductivity, dielectric loss (expect tuning and RF-heating problems if sample
is dielectrically lossy or even conductive)
2. Collect information about the sample first by running an “easy” nucleus:
Feasibility of cross polarization parameters is the required key information, be-
cause it decides the steps to follow.
If the sample information which you have collected shows that a
13
C CP/MAS
experiment should be feasible (sample contains more than 20% protonated
carbons), load a reference cross polarization data set (S/N test spectrum of
glycine), spin the sample at the same spin rate, set contact time (p15) to 1ms,
wait 1 min., do one scan. There should be a visible signal.
From there on, optimize the required repetition rate (d1), contact time (p15),
number of scans (ns), spin rate (masr) and Hartmann-Hahn adjustment until
the signal is optimum. In very few cases, the decoupler offset (o2) may require
readjustment.
If no
13
C-signal is found, the reasons may be:
- incorrect setup (recheck reference sample)
- concentration lower than expected
- unusual relaxation properties (long T
I-S,
long proton T
1
, short proton T
1ρ
).
3. Then the most important information about the sample (proton T
1
, proton T
1ρ
)
can be obtained by looking at the protons in the sample. Set up for proton ob
-
servation, set swh to 100000-500000, rg to 4 and pulprog cpopt (if not found
in the library, copy the pulse program in the appendix), p3 and pl12 for
p3=p90. Set spnam0 = ramp.100, sp0 = power level for HH, p15 =100 us. Do
1 scan and fourier transform/phase correct. Using popt, optimize d1 for maxi
-
mum signal.
Note: CP/MAS probes usually have a substantial proton background sig-
nal. Do not be misled by this, it will not behave like a regular signal:
- it will grow steadily with longer pulses
- it will not show spinning sidebands
- it will cancel when a background suppression pulse program like aring is
used with a full phase cycle.
4. Knowing the required relaxation delay, the following step is to determine the
cross polarization (contact time). On protons, we measure the time constant
T
1ρ
. Using popt in the previous setup, vary p15 between 100 µsec and 10 ms
(even 20 ms at reduced power, if a long T
I-S
is expected, as the distance be-
tween nucleus of interest is long or the mobility is high, leading to a small het-
eronuclear dipolar coupling between nucleus of interest and protons). This
measurement will tell you how long the contact time p15 may be. A value of
p15 giving 50% of the initial proton signal amplitude will still give a 2-fold en
-
hancement on
13
C. If the proton signal is below 50% at 1ms spin lock time or
even less, a full cp-enhancement cannot be expected.
5. Now we know the minimum relaxation delay and the maximum contact time.
With these parameters used as d1 and p15, the measurement is just a matter
of patience.
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