Bruker BioSpin Solid State NMR, User Manual

Page: 99 / 328

User Manual Version 002 BRUKER BIOSPIN 99 (327)
6
Practical CP/MAS
Spectroscopy on
Spin 1/2 Nuclei
6
Once good setup parameters have been obtained to observe
13
C and get good S/
N on glycine, it should be easy to also observe
13
C-CP/MAS spectra on other
samples and on nuclei different from
13
C. Nevertheless, sometimes one comes
across samples where it is difficult to observe
13
C. This chapter deals with strate-
gies to optimize acquisition parameters for
13
C and other spin ½ nuclei.
Possible Difficulties 6.1
Usually,
13
C spectra are easily acquired. Several sample properties may however
make observation difficult:
1. Low concentration of
13
C in the sample.
2. No or too few protons in the sample.
3. Long proton T
1
.
4. Long T
I-S
.
5. Short proton T
1 ρ
.
If a nucleus different from
13
C should be observed, there are additional potential
difficulties:
6. Unknown chemical shift.
7. Unknown Hartmann-Hahn-condition.
8. Unknown relaxation properties (proton T
1
, T
1 ρ
, T
I-S
).
Possible Approaches for 13C Samples 6.2
1.Collect as much information about the sample as possible. Do not accept samples
for measurement with unknown composition. Request information about:
- possible hazards (upon a rotor explosion)
- concentration of the nucleus to be measured
- structural information about the molecular environment of the nucleus of in-
terest:
- mobility (rigid environment: expect long T
1
and repetition delay),
- proximity to protons (can one use cross polarization)