2005-02

Department of Chemistry, University of Alberta May 2005
NMR News 2005-02
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

	sample preparation
	automatic selection of plotter/printer in remote processing sessions
	data created by gennmr

Sample preparation
How can I make better NMR samples? This should be one of the most frequently asked questions. Unfortunately, it isn't and most NMR users do not care nearly enough about the quality of their samples and the implications for the resulting spectra. Based on extensive experience, a document was created by the people at the front of the NMR service: Glen Bigam, Lai Kong and Mark Miskolzie. It is available in HTML or PDF format - whatever your format preference: please read it and follow the recommendations.
Automatic selection of plotter/printer in remote processing sessions
More and more research groups use remote access from Macs and PCs in their labs to process their NMR data. So far it was necessary to select a plotter located in the research lab manually (a macro was created for each group to make this reasonably easy). Even better, Mark Miskolzie created a neat new macro which is used by the VNMRJ startup script; based on the remote username it selects the appropriate plotter/printer automatically. In cases where there are two or more printers in a research group, a decision was made to select one by default.
On the other hand, if the user is logged in at the d601 or ibdw keyboard then the directly attached 8.5 x 11 plotter is selected.

Time Limit on Data created by gennmr
The replacement of an old hard drive with a much larger new one, has made it possible to extend the "grace period" of data recorded in the general gennmr account from 120 to 180 days. Users of gennmr are reminded of this limitation every time they start VNMRJ. This change has no effect on any other accounts, gennmr is the only one with a time limit.

NMR Sample Preparation [1] (for a PDF version of this document click here)
To obtain high resolution NMR spectra it is necessary that your NMR sample is free of suspended material (precipitates, dust, etc.). Suspended material present in solution will increase the line width of the spectrum; broad spectral lines reduce spectral resolution and no amount of shimming can correct for this. Suspended material can easily be removed from an NMR sample by constructing a filter using cotton wool as a filtering agent [2]. Conversely, glass wool is a very poor filtering agent since glass wool does not filter out small particles, whereas cotton wool tends to filter out the smallest of particles. A filter can be made by placing a small amount of cotton wool inside a Pasteur pipette as shown in Figure 1.

Figure 1: Pasteur pipette with cotton wool

Note that the Pasteur pipette has been cut to facilitate handling of the cotton wool. Alternatively, the cotton wool can also be pushed down an uncut long [3] pipette with the help of a second pipette (this method was used in Figure 2 to position the cotton). The solute of interest can then be dissolved in a separate glass vial using less than the final volume of deuterated solvent (e.g. CDCl3, DMSO-D8, CD3OD, etc.) required to make your NMR sample. After the solute has been dissolved it can be transferred directly to a NMR tube by passing the solution through the cotton filter as shown in Figure 2.

Figure 2: Filtration through cotton wool

Figure 3: NMR request sheet depth gauge

Finally, the sample volume can be adjusted by adding the remaining solvent to the NMR tube so that a final sample volume of ~700 mL or a sample height of ~55 mm is reached (Figure 3), followed by vigorous shaking of the sample to effectively mix its content [4].

The suggested sample volume for our instruments is 700 mL. Trying to concentrate your sample by reducing the sample volume is not recommended. A short sample (i.e. volume less than 500 mL) can only be shimmed through tremendous effort and this time can be much better spent acquiring your data.

References

[1] Adapted from the Chem 241/341 sample preparation guide by Dr. Jason Cooke, whose students consistently submit some of the best NMR samples seen by the NMR Service Lab.

[2] Medical cotton is reported to be free of compounds that can be extracted by organic solvents (A.E. Derome, Modern NMR Techniques for Chemistry Research, Pergamon Press, 1987).
[3] 9” pipettes are sufficient (no need to buy the much more expensive 10.5” type)
[4] Failure to mix the contents of the sample tube results in a concentration gradient which degrades spectral quality.

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