The PFG steady-state stuff typically are PFG- pw90 - PFG element at the
start of the sequence. Pretty darn effective for any experiment for
artifact suppression but not a cure for too short d1 in terms of signal
response. My experience with Inadequate is that far and away the best
thing you can do is to not short-sheet F2 acquisition too much as there
is lots of signal living there you are free to filter with line
broadening if need be but you cannot get it back if you set acquisition
time too short. Same as time spent with recovery delay but actually
acquiring data. More about that later..
Also run phase-sensitive and set sw1 = sw because the DQ stuff can only
fold once and there is no ambiquity plus it immediately cuts experiment
time in half for any given T1 resolution.
Finally.. absolutely use a composite adiabatic refocusing 180!!! A
composite CHIRP of WURST are great. This can double signal in the real
world.
Regarding number of scans.. Remember the sequence is in effect a 450
degree pulse so with a simple one pulse 13C knowing the S/N for a
single 450 not a 90 is important. For that reason probe rf homogeneity
really matter. If the peaks of interest for any given combo on scans,
acq time, and d1 reach a S/N of ~ 110/1 when you run the full 2D
assuming F1 resolution of at least 8ms the 2D ft should show beautiful
anti-phase spots.
Best,
Ron
On 3/5/2015 7:58 AM, David Richardson wrote:
>
> Hi all,
>
> I need some advice. I have a professor regularly doing INADEQUATE
> experiments on natural products. We use a shegemi tube and have been
> getting pretty good data, but obviously the concentrations are low and
> the experiments long. He is a solid-state spectrocopist so 6-7 day
> experiments are not a big deal to him, but on my shared liquids
> instruments it has become an issue. We are having a dispute about
> using a lengthy d1 and making the experiment much longer than I think
> is necessary. We did a C13 T1 measurement on the most recent compound
> and found the longest T1 to be about 12 seconds, so he wants to run
> with a d1 of nearly 30 seconds. I think this is utterly insane,
> especially for an INADEQUATE. I tried to explain about the
> steady-state gradient pulses (see the attached picture – hopefully
> screenshots make it through the filters) that precede every scan in
> most of the modern advanced pulse sequences and that they exclude the
> need for a long d1 even when some of the spins have a long T1. He
> didn’t buy my explanation and still has his students run 6-7 day
> experiments; I am about to restrict his access to our 500 and make him
> use the 300 exclusively. I understand the steady-state gradient
> pulses (apparently called crusher gradients, as I learned in my
> reading yesterday) kill any remaining transverse magnetization, but I
> am unable to explain how. Does it actually return the system to the
> equivalent of thermal equilibrium? Even if not, I imagine any signal
> loss would be more than compensated for by running 10+ scans with a
> short d1 in the same amount of time he runs 1 scan with a long d1.
> Can anyone please help clarify this for me so I can better explain to
> him it is unnecessary. Also, if I am completely wrong and a long d1
> is still needed please tell me that too. I can understand if it were,
> perhaps, a DOSY or some other quantitative experiment, but not on your
> general 2D experiments.
>
> Thanks in advance,
>
> David
>
> David Richardson, Ph.D.
>
> NMR Instrumentation Specialist
>
> Chemistry Department
>
> University of Central Florida
>
> david.richardson_at_ucf.edu <mailto:david.richardson_at_ucf.edu>
>
> 407-823-2961
>
> http://chemistry.cos.ucf.edu/nmr/
>
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Received on Thu Mar 05 2015 - 09:55:08 MST