I second Huaping Mo's suggestion. In fact, the temperature calibration
procedure for Bruker's VT unit relies precisely upon the temperature
sensitivity of spectral lines of the calibration standards. (You might want
to check out the procedure documentation to see how it relates to your
issue, and to find some starting references). In general, even within a
single compound, the temperature sensitivity of individual lines will have
differing degrees of temperature sensitivity. You can try running your
sample with VT on, waiting a few minutes each time for stabilization, to
see if that fixes the problem; or, if you really want to know more
decisively, you can take a couple of runs at temperatures 5 or 10 C apart
to see how big the temperature sensitivity is in Hz/C. (Keep in mind you're
measuring the spectral shift of your sample relative to the lock line.)
This is an example of those "well-known" phenomena that have been known so
long they're often forgotten, notwithstanding a vast history in the
literature.
Regards - Gerry
On Mon, Jun 8, 2015 at 7:34 AM, Huaping Mo <hmo_at_purdue.edu> wrote:
> Dear Atilano,
>
> We recently dealt with a very similar issue as yours. While we are in the
> process of putting together some theoretical analysis and solutions, based
> on the symptoms you described, I believe a very likely cause is the
> temperature fluctuation within the sample, either due to ambient
> temperature change or VT temp control. You may find on some days the
> spectrum looks perfectly fine and on other days it does not look as good.
>
> Best regards,
>
> Huaping Mo, Ph. D
> Purdue University
>
> ----- Original Message -----
> From: "Atilano Gutiérrez Carrillo" <agrmn_at_xanum.uam.mx>
> To: ammrl_at_ammrl.org
> Cc: "Atilano Gutiérrez" <atilanonmr_at_gmail.com>
> Sent: Friday, June 5, 2015 2:15:46 PM
> Subject: AMMRL: LOCK STABILITY PROBLEM / DILUTED D2O SAMPLES
>
>
>
>
> I have the following problem:
>
> I have a dilute solution of an organic solid sample solved in D2O, that
> requires at least 512 scans in order to have a good 1H spectra with an
> acceptable S/N ratio. Acquisition takes more than an hour. However, D-lock
> is not enough stable along the acquisition time. Signals asymmetrically
> broadens as acquisition proceeds. For instance, the 0.00 ppm TSP methyl’s
> signal, at the end of the acquisition could be as broad as 2 to 3 Hz with
> more than one maximum. It seems that signals shift up field at every single
> scan. It is worth to mention that field homogeneity is good. If acquisition
> is halted at any time and a single scan spectrum is acquired, the 0.00 ppm
> signal lineshape is almost perfect (0.7 Hz wide at HHFW) . Thought ,its
> chemical shift would have changed by some hundredths of Hz.
>
> Acquisition is taken place in Bruker AVANCE-III 500. I already played with
> the lock parameters, mainly, those relate to the LOCK LOOP (gain, time and
> filter), but without any success.
>
> I am kindly asking your advice to solve this apparently field drifting
> problem. What might be the best lock parameters?. This problem happens only
> on diluted D2O sample solutions, that require a long time acquisition.
>
> Regards.
> Atilano Gutiérrez Carrillo
> NMR Lab.
> UAM. Iztapalapa
> Phone (525) 55- 5804-6541
>
>
>
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Received on Sat Jun 13 2015 - 09:43:01 MST