Department of Chemistry, University of Alberta         April 2005
NMR News 2005-01
News and tips from the NMR support group for users of the Varian NMR systems in the Department
Editor: Albin.Otter@ualberta.ca         http://nmr.chem.ualberta.ca

There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.


Other content of this NMR News is no longer meaningful and has been removed May 2010.

Contents 

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m400 spectrometer: usage during the summer months

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larger sweep widths in EZ NMR (1D and 2D)

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personal identification for instrument use

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FAQ 2005-01.1: length of NMR experiments: how do parameters affect it?

 

m400 spectrometer: usage during the summer months
The Varian Mercury 400 MHz spectrometer (m400 in W1-19) was purchased to provide undergraduate students with an instrument to measure NMR samples themselves rather than submitting them to the NMR Service.

Approx. mid-April to mid-September, the instrument is available for for research samples. Your NMR lab key will also open W1-19.

The capabilities of m400 are basically identical to i400:

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H1 1D and 2D

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C13 (direct detection, e.g. APT)

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C13 (indirect detection, e.g. GHMQC, GHSQC and GHMBC)

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P31

m400 does not have a waveform generator and hence will not allow selective experiments like the 1D-TROESY. The sensitivity of C13 and P31 are identical to i400 but H1 is weaker at 150:1 compared to 250:1 on the i400. The lower sensitivity is of little, if any, consequence for most applications. Only with dilute samples and/or NOE-based techniques there might be cases where a move to a spectrometer with better H1 sensitivity is required. On the other hand, m400 is very user-friendly by means of its robot operation and fully integrated into the EZ NMR system. In other words, spectra acquired on this spectrometer will be available throughout the NMR network like all the others. This includes remote processing on the d601 and ibdw servers. The difference in H1 sensitivity is accounted for in the robot's experimental setup.

Larger sweep widths in EZ NMR (1D and 2D)
By popular demand, the default sweep widths in the proton domain have been extended from 10 to 0 ppm to approx. 11 to -1 ppm. This range was implemented in such a way that the acquisition time and the overall experimental time are not affected. To maintain the digital resolution, the number of data points was increased accordingly, resulting in slightly larger file sizes of the raw data which is completely unimportant in view of the ever increasing size of hard disks and faster CPUs for data processing. What and how parameters affect the overall experiment time see FAQ 2005-01.1 below. Note that the acquisition time at is related to the sweep width and the number of data points np by this simple equation:
 

acquisition time = number of data points / (2 x sweep width)    or:    at = np / (2 x sw)

This entails that when the sweep width and the number of data points are increased by the same amount, the acquisition time stays the same.

Personal identification for instrument use
Every user has to enter his/her username from the on-line reservation system in order to start up VNMRJ successfully. Failing to do so will close VNMRJ immediately. The information thus provided is logged with a time stamp to give better control on who does what and when. Be aware that you are responsible for your login name and password. If you share it with somebody else you will be held responsible for what happens to a system while it is used under your name. If you think you should change your username and password please do so by sending an e-mail to the editor. You cannot re-register yourself - this is not allowed in the reservation program.

FAQ 2005-01.1: length of NMR experiments: how do parameters affect it?
How long an NMR experiment takes and, particularly, what makes it shorter or longer are quite common questions. This is different from Experimental time and sample concentration which was discussed earlier. It is best to discuss this for 1D and 2D spectra separately. While in VNMRJ, you can at any time type dps (display pulse sequence) to visualize what sequence is about to be run or is already in progress. The pictures shown below are all dps command outputs. The wavy green line in the Figures (FID) is the acquisition time and the parameter label at is not displayed. Note that white numbers are in seconds, yellow in milli-seconds and light blue in micro-seconds.

a) 1D spectra
 

1D pulse sequence

duration of experiment: (d1 + at ) x nt
for nt = 16: (3.0 + 2.0 ) x 16 = 80 sec or
1 min 20 sec

Note that pw is 3.5 microseconds, which is completely irrelevant in the context of minutes and
seconds even if repeated many times.

APT pulse sequence

duration of experiment: (d1 + at  ) x nt
 for nt = 1024: (0.1 + 1.9 ) x 1024 = 2048 sec or
34 min

Since values in light blue are in microseconds while those in yellow are in milliseconds, they can be ignored even when the pulse sequence is carried out 1024 times. The error for ignoring both 7.0 msec delays is only 14 seconds.

APT in the EZ NMR setup has no limit on nt (it runs until stopped by the user).
nt=1024 is used here as an illustration only.

b) 2D spectra

GCOSY pulse sequence

duration of experiment: (d1 + at) x nt x ni
 for nt = 1 and ni = 256: (1.5 + 0.5) x 1 x 256 = 512 sec or
8.5 min

Like in the 1D cases, all light blue and yellow values are too short to affect the overall time significantly. Doubling of nt from 1 to 2 will increase the signal-to-noise-ratio by 40%. Increasing ni form 256 to 512 will double the digital resolution in F1 but has no effect on the s/n.
As each change doubles the duration of the experiment, both taken together (nt=2, ni=512)
make the experiment 4 times longer (ca. 34 min).

gHMQC pulse sequence

duration of experiment: (d1 + at) x nt x ni
for nt = 1 and ni = 256: (1.5 + 0.25) x 1 x 256 = 448 sec or 7.5 min

Like in all cases above, the light blue and yellow values are too short to affect the overall time significantly. Doubling of nt from 1 to 2 will increase the signal-to-noise-ratio by 40%. Increasing ni from 256 to 512 will double the digital resolution in F1 (C13-domain) but has no effect on the s/n.
Each change doubles the duration of the experiment so both taken together (nt=2, ni=512)
make the experiment 4 times as long (ca. 30 min).

NOTE that for gHMBC nt = 2 is the default to avoid artifacts, hence the experiment runs ca.
double as long as the gHMQC.


Footnote
This is the only Mercury spectrometer in the Department. All the others are Inova or DirectDrive systems and
as such capable of more advanced NMR. Mercury spectrometers are designed for routine NMR applications.
However, "routine usage" stretches quite a bit and very many types of experiments can be done.


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