Basic Anasazi Proton NMR Tutorial
Instrument Optimization
Note: if there is a sheet of paper attached to the monitor which says 'Experiment in Progress' or something similar, contact your laboratory instructor before proceeding.
Check that the NMR and computer are switched on and running. Most often the screensaver will be displayed. Note that it says, "FILL OUT THE RUN LOG!" You must make an entry in the Run Log when you are operating the NMR. Include the date, time, your name, where you're from and what sample you ran or what you did on the instrument. If you find any problems with the instrument, note them there. The Run Log is meant to record everything that gets run or done to the instrument, and is very useful for tuning and maintenance purposes. It must be kept up to date.
Check that both the PNMR and NUTS programs are running on the computer. This can be seen
by looking at the Windows95 task bar at the bottom of the screen. If these programs are not
running, or only one is running, or multiple copies are running, shutdown and reboot the
computer.
INSTRUMENT OPTIMIZATION:
In the sample rack on top of the magnet there is a sealed NMR tube containing 5% ethylbenzene (ETBZ) dissolved in carbon tetrachloride with 1% tetramethylsilane (TMS). This tube is used as an instrument performance testing sample. It's always a good idea to run ETBZ before running your unknown sample to make sure that the NMR is performing properly. If the instrument gives you a good spectrum with ETBZ, but your unknown sample runs poorly, there's something wrong with your unknown and you probably need to prepare it again. On the other hand, if ETBZ runs poorly, there's something that needs tending to in the instrument.
Load the ethylbenzene test sample.
Click on the PNMR window and look at the data acquisition settings list. It should look something like this:
If necessary, set the data acquisition parameters to the default values displayed here. If everything is OK, type:
ZG
and press the Enter key. The computer screen will switch to acquisition mode and will display the FID during acquisition. Watch the PNMR window, and wait until the message window in the top right corner of the screen disappears before attempting to enter any further commands into either PNMR or NUTS.
Click on the NUTS window. Note that there may or may not already be a spectrum displayed in the window. If a spectrum is displayed, it is from the previous acquisition. To read in your data, use the A1 link (press the 'a' key and then press the '1' key but don't press the Enter key).
Your ETBZ spectrum is now displayed. If the peaks are pointing 'down', autophase the spectrum using the AP command (press the 'a' key and then press the 'p' key but don't press the Enter key).
ADJUSTING THE OFFSET
Look at the spectrum and verify that the TMS peak is within -5.5 to -6.5 PPM. If not, you need
to adjust the OFFSET on the instrument control panel below the desk (slides out like a drawer).
Increasing the OFFSET moves the TMS peak to the right, and decreasing the OFFSET moves the
TMS peak to the left. Change the units digit of the OFFSET control by two, either up or down as
necessary, and re-acquire the sample. Continue adjusting the OFFSET and re-acquiring the
sample until the TMS peak is within the target range.
CHECK THE SHIM SETTINGS
Use the ZOOM mode to examine the TMS peak, looking for splitting near the apex. If the shimming is OK, there should be no peak splitting evident. But, if the TMS peak is split, like the one shown below, you'll need to run the basic shimming routine.
For a description of basic shimming, click here. Exit ZOOM mode by centering the cursor in the
NUTS window and pressing 'Enter'. Press Alt_V_R to return to full screen view.
RUN THE ETBZ TEST SAMPLE AGAIN
Re-analyze the ETBZ sample. Use the A1 link to look at the spectrum. Examine the TMS peak again using ZOOM mode. If the TMS peak is OK, use Ctrl-F1 to process the data (press and hold the Ctrl key, then press the F1 function key, then release both keys). This starts a macro that tries to automatically process the spectrum, integrate the peaks, format the printout, and allows you to enter your name, the date, and the sample type before printing the spectrum. After entering your initials, etc., click the OK button. This should sent the output to the printer. If you see another window asking you where to send the output (usually it's a printer dialog box) click OK, and the printer should start to print.
Look at the printout and verify that the TMS peak appears at 0 PPM. A good ETBZ spectrum should look something like this:
Note that integration and peak picking parameters are discussed in detail in the NUTS program section and not critical for optimization using ETBZ.
That's all there is to it! You have now verified that the NMR is operating properly and ready to run your samples.
If you can't get a spectrum like this from ETBZ, you need help from an instructor.