1. Make sure your data files are in a directory on the computer on which you are working. The best thing to do is to transfer them from the network drive to your floppy diskette. Alternatively, if you know how to use FTP, you can bring over the file from the NMR server in Gilbert 238.
2. Open NUTS by clicking on the appropriate icon on one of the PC's in GBAD311 (ACORN ICON)
3. Open your data file. The raw data, or Free Induction Decay (FID) should come up on the screen. This is the interferogram.
Raw data: When you open the FID, it looks like this:
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This is an intensity-vs.-time plot. |
One minor adjustment to the data must be made. Type RD, and hit "OK" on the dialog box that pops up. This modification ensures that the data as produced by the spectrometer is converted to a mathematical form that Nuts is expecting.
4. Perform a Fourier Transform on the FID by going to "Processes"; "Transforms"; "Fourier Transform." In a second or two, the FID will be transformed from a time-based to a frequency-based spectrum.
Transformed data:Then, after you perform the Fourier Transform, you will normally see something like:
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The peaks in the left spectrum are upside down! |  |
The right spectrum is more typical. |
The peaks in the typical display are uneven from side to side and dip below the baseline. Since the goal here is to be able to accurately integrate the peaks, the peaks first need to be phased to remove the asymmetry as much as possible. This is what Phasing is all about.
5. Normally some phasing of the spectrum will be necessary. This is one of the more difficult skills in working up NMR data. The following directions will lead you through the processes of phasing and then integration of the spectrum.
Phasing:
A. Go to Process:Phasing, and select Quick Phase. You will get a pretty good spectrum from this (left), but if you expand the Y-axis (scroll bar on the right), you will see that the peaks are not really phased properly (right spectrum below): (The curvature in the baseline is normal; you will take care of this after phasing.)
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Typical result from Quick Phase. |  |
Expanded Y-axis reveals that there is still work to be done! |
B. Now, use Phase by mouse. This uses both the left and right mouse buttons. In sequence, click and drag the left button left or right to phase a peak on the left side of the spectrum. Go a little bit "too far". Try to make all peaks have the same shape. Then right click and drag to phase the right side of the spectrum. It is usually necessary to go back and forth several times to get a good phase:
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This spectrum is just right! |  |
This spectrum needs a better phase adjustment. |
Important! Hit "Enter" (or you'll lose the phase correction!).
Check the NUTS help files for more information on phasing.
Integration of Peaks:
Baseline Correction: Accurate integration will require a good baseline. To
correct the slight baseline curvature normally present, go to "Processes";
"Fit Baseline". A series of vertical red stripes, corresponding to what the
program thinks is baseline, overlies the spectrum. If by chance a real peak lies in one of
the red areas, mouse click on it to remove it from consideration. Use as many sections as
possible; normally the initial selection is OK.
Type "c" to begin the calculation; when the calculation is finished type
"a" to apply the correction and return to normal mode. Your spectrum should look
like this:
Establish a reference for the chemical shift axis (X-axis):
A. Set the reference peak by placing the cursor on the peak, left-clicking as you type "V". A dialog box for inputting the chemical shift will appear. Enter the correct chemical shift value for the solvent peak that you have selected:
| CHCl3 | 7.26 ppm | C6D5H | 7.15 ppm | CD2HOD | 3.30 ppm |
| CD2HCN | 1.94 ppm | CD2HC(O)CD3 | 2.04 ppm | CD2HS(O)CD3 | 2.50 ppm |
B. Set the spectrum limits using a similar dialog box. Type "ZO" (caps), then "f". (Or, select the "View; Frequency Limits" menu if you are in the ZO expansion subroutine.) The spectrum limits are for the total region to be displayed.
Integration:
From the ZO routine, you may transfer directly to integration under "File". Otherwise, look under "Processes" for "Integrate Spectrum". For each peak you wish to integrate, left-click to the left of the peak, then click and drag the cursor though the peak, and left click again. If you want to set a reference integral, left click on that peak and type "V" for the appropriate dialog box.
Saving and Printing:
When the spectrum is as you want it, save and print under the "File" menu. Note that expansions may be obtained with the Zoom routine and copied and pasted using the appropriate Windows functions, or (better) printed separately. If at any point you mess up, you can always return to the full spectrum with "View"; "Show All Reals". Or you may reload the original data and re-transform.
If you want to practice this on your own computer, a demo version of NUTS is available (with some sample spectra) from Acorn NMR: http://www.acornnmr.com/. You will not be able to copy or print from this version, but it will give you some experience with using the program to manipulate data.
These directions are excerpted from the CH 362 Web page written
by Dr. Kevin Gable.