In many works, Si/Al ratio was determined using 29Si MAS NMR spectrum. My question is, what are the important experimental NMR parameters to get a healty result? Randomly selected parameters (relaxation delay for example) or methods (CP, MAS or CP/MAS only ) seem to not yield correct results.
Hi,
As you talk about measuring Si/Al ratio, I think that the systems you study are aluminosilicate-based material, possibly partially hydrated. In this case, the most direct approach is to use direct excitation 29Si MAS (for resolution) spectrum with 1H decoupling (if OH or H2O are present). 1H-29Si CPMAS is not a quantitative method and cannot be used for anhydrous materials.
The most important parameter for the direct excitation 29Si MAS spectrum is the recycle delay (delay between two consecutive scans) which needs to be long enough to insure full longitudinal relaxation. You can measure the longitudinal relaxation time T1 and choose the recycle time as 5*T. Alternatively, you can acquired a set of direct excitation 29Si MAS spectra (same exp conditions) by gradually increasing the recycle delay until the intensity does not evolve anymore.
The recycle delay for 29Si (spin 1/2) is generally long, few minutes. It is usually longer for crystalline than for amorphous materials. It also increases with higher magnetic fields. However, the presence of a small amount of paramagnetic impurities (e.g. iron) can often depress the recycle delay to 20-20 seconds. Note that if your sample contains too much paramagnetic impurities (let's say more than 5%), the quantitative aspect may be altered.
Hope this helps
Michael
Hi,
Micheal the information you shared with us is very usefull, thanks for that first of all.It is very clear for me now. A few things more:
The sample I examimed was a clinoptilolite. I measured a 29Si MAS spectrum at 10KHz with and without hydrogen decoupling. The resultant spectrum showed almost no difference. I know MAS decouples homo and hetero nuclear dipol interactions to a certain level. I think only MAS without hydrogen decoupling seems ok for clinoptilolite samples since MAS seems to decouple enough.
With regards....
Özgür,
Provided that your 1H decoupling is efficient, you're right. In this case, MAS alone is sufficient to remove 1H-29Si dipolar interaction and you can use your MAS spectrum without decoupling. 29Si-27Al dipolar interaction is weak and easily removed by MAS at 10 kHz. Finally, even in static condition (no MAS), the 29Si spectra of non-labelled samples are barely affected by 29Si-29Si homonuclear dipolar interaction due to the low natural abundance of the 29Si isotope.
Regards,
Michael
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