Hello all, I am attempting to take a TOCSY on a complex mixture, and was wondering what sort of considerations should I pay special attention to?
Currently, I am wondering if things like solvent viscosity, amount of material, amount of water in the solvent, ect... will effect the mixing time that I develop. What are good benchmarks when running a TOCSY, and how would I optimize the parameters for a complex mixture? Are there variants of the TOCSY pulse sequence that are more sensitive than others?
Using a Bruker 600 with a 5mm TCI cryoprobe.
Dear Joseph,
none of the items you list above has an effect on the choice of mixing time, Except maybe viscosity If the sample gets viscous enough you might loos more signal.
The choice of mixing time is given by the spin systems you look at. In general the longer you mix the further you propagate through the spin system. But there is overall intensity loss with long mixing times. Typically 60 - 80 msec are ok for small molecules.
Amount of materail and water will have an effect on your ability to see very small signals in your mixture. Large signals tend to have a bit of t1-noise and can also show some out of phase components. These artefacts can obscure small signals.
You should use a sequence that gives the best in phase signals such as dipsi2gpphzs with Keeler Z-filter. If water is still too large add some presat to it.
Clemens
I may also add that sometimes a 1D selective TOCSY (eg. Bruker pulse program selmlgp) can be really helpful if you have a particularly busy region of the spectrum that you'd like to deconvolve. By playing with the mixing times you can actually affect how far you propagate the magnetization from 3-bond (COSY-esque) at around 15-20 ms. And, since it's 1D, the experiment is pretty fast. See Glenn Facey's blog (U of Ottawa) for more info.
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