UMass-Amherst Electron Paramagnetic Resonance Facility

Electron Paramagnetic Resonance Facility at UMass-Amherst

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Facility Introduction

ROUTINE USE OF ELEXSYS E-500 ESR SPECTROMETER; Q-BAND

Stop and reverse the startup procedure, and inform the system manager if any step described below cannot be accomplished as described.

1. The Q-band cavity should already be installed to proceed further below. This should be done by the system manager. The description is given in User Tips (see menu to left), but this should not be done by the routine user.

2. Turn clockwise yellow lever on pipes behind magnet to turn on cooling water at a flow of >20 psi.

3. Turn on the main computer if necessary, and wait for it to boot up. Log on to your account and double-click the Xepr icon on the Linux desktop to start the main control program.

4. Press the power button (green, upper left) on the main console that stands to the left of the magnet. It will take about 30 seconds for all the hardware in the console to initialize, during which you can proceed with subsequent steps.

5. Flip the heat exchanger power switch to ON (you will hear the water circulation begin). Press on the ELECTR ON button on the extreme top left of the power supply, then press the POWER ON button next to it.

6. In Xepr, go to Acquisition > Connect to spectrometer. This will display a small panel that says Server name with the entry E500 (case sensitive). Click OK. This should bring up a monitoring panel that shows Standby, Tune, Operate, Frequency and other settings.

7. In Xepr, go to Acquisition > Spectrometer Configuration > Misc and set the Resonator to Q-band. Next, go to Acquisition > Spectrometer Configuration > Signal Channel. and make sure that Resonator 1 has been selected. Select the Standard Calibration Data file XXXX from the pull-down entry, and Apply it. Close the panel.

8. Click the Tuning button in the upper icon menu (it looks like a small copy of the standard dip), then activate the Tune button in the panel below this.

9. Set the Attenuation to 30 dB. (Left set of vertical arrows in the attenuator box change the setting by 10 units at a time and right set of vertical arrows change the setting by 1 unit at a time). Use the rear right thumbwheel near the top of the cavity to set the proper cavity length to find the dip and set it to the center of the Tuning display. With no sample in the cavity, the little vernier scale to the left of the rear right thumb-wheel should be set for about 24 to get the dip on scale. If you cannot find the dip, ask the system manager for help.

10. Prepare your sample for insertion into the cavity. Insert your quartz sample tube (no borosilicate!!) into the appropriate Mylar sample holder. Nonpolar solvents are easiest to use at room temperature. Toluene is a good one for Q-band. Dichloromethane is probably OK at lower temperatures. A concentration of 300 micromolar is typically best. The fit should be snug, but the tube should slide up and down around without feeling like it will break if pushed. Screw holder/tube into the sample insertion rod. About 5.0 cm of sample tube should project out of the bottom of the holder. Remove the Q-band cavity cover insert, and VERY CAREFULLLY, slowly insert the sample holder into the cavity while watching the signal dip. When the sample begins to enter the cavity detection zone, the dip will begin to move. At this point, you will need to initiate tuning.

11. You CANNOT use AUTOTUNE or the regular Xepr software tuning. Instead, you should continue to tune frequency with the rear right thumb-wheel. Continue to insert the sample slowly, a couple of millimeters at a time. Each time, the dip will move. Use the right-hand frequency wheel to keep the frequency centered. Lower the sample as far as it will go, then lift the sample rod ~5 mm (a red line on the sample rod will be seen in the graduated window atop the cavity). Lastly, rotate the sample rod until the dip moves to the highest frequency (furthest left on the screen). This is typically the best positioning for the sample. Tighten the collet to hold the sample rod in place. [If you have trouble tuning and you think you have a lot of signal in your sample, carry out the tuning steps 11-14 with the dip set slightly off resonance (i.e., set the dip a bit to the left of the dip point, so that the Offset will not be zero) – this will help get the tune better.]

12. On the computer, move the frequency slide-bar to center the dip. From this point on, any sample inserted into the cavity should move the dip back to the range where the slide-bar should work.

13. Back at the computer, in the Tuning panel, activate the Operate button on the Tuning menu. Lower the attenuation to 10 dB a few units at a time, making sure that the Diode Current remains 200 mA, and that the Offset is about zero. If you do not do this, the power supply unit may “scram” during the ensuing tune process, shutting off the magnet.

14. In the Tuning panel, set the attenuation to 25-30 dB, turn the Reference Arm “off”, and set the tuning monitors to “logarithmic scale”. Engage the Monitor button to get an expandable panel that shows Diode Current and Offset. Expand the panel until you can see it from the cavity (old folks probably will need young folks around to help see, anyhow). At this point, the Diode Current should be 0 mA – if not, lower the bias current to zero. Use the frequency wheel on the cavity to center the Offset. Lower the attenuation 1-2 units at a time until it reaches 10 dB. Each time you lower the attenuation somewhat, adjust the left front iris wheel on the cavity to maintain the Diode Current at zero. Adjust the right rear wheel as needed to maintain the Offset at zero. Eventually you should be at attenuation of 10 dB with the lock offset of zero and the bias current at zero.

15. In the Tuning panel, reset the attenuation to 60 dB: you will lose the lock at this point. Turn the Reference Arm on and off a few times, then leave it set “On”, and turn off the logarithmic scale setting. Raise the bias until the Diode Current reads about 200 mA. Reduce the attenuation to 30 dB. Adjust the Phase until the Diode Current maximizes. Again, adjust the bias until the Diode Current reads 200 mA. Use the rear right frequency wheel on the cavity to center the Offset. At this point, if you cycle between Reference Arm on and off, the Offset should not move, if the phase is good.

16. In the Tuning panel, reset the attenuation to 60 dB. Adjust the left front iris wheel to set the Diode Current to 200 mA. Each time you lower the attenuation somewhat, adjust the left front iris wheel on the cavity to maintain the Diode Current at 200 mA. Adjust the right rear wheel as needed to maintain the Offset at zero. Eventually you should reach an attenuation of 10 dB with lock offset 0 and the bias current at 200 mA. Return the Atten to 20 or 30. The cavity should now be critically coupled and ready to use. Close the Monitor and Tuning panels.

17. Click on the new experiment button (icon menu atop main display, star-burst with Exp inside it).

18. In the new panel, click the C.W. (continuous wave) tab. Set Abscissa 1 to Field, Abscissa 2 to None, and Ordinate to Signal channel. Temp unit, Goniometer, and Gradient Unit buttons should not be green (active) for routine use; deactivate them if necessary. Click Create. You will have to give a temporary name to the experiment; after this the panel will close.

14. Toggle on Activate button (rightmost in the picture shown). The other buttons from left to right are Stop and Pause (after the present scan is finished). If you toggle off the Activate button, ESR parameters are set only when you Run your spectrum (step 20).

15. Click the Parameters button in the main (upper) tool bar.

16. In the Acquisition Parameters panel, click the Absc Field subtab, and set the Center Field and Sweep Width parameters. Do not use the Setup Scanning procedure (Chapter 5.3 in the Basic Operations Manual). Using the button to set to sample g-value often is helpful here. For inorganic samples, sweep widths of at least 1000 gauss are likely to be needed.

17. In the Acquisition Parameters panel, click the Scans subtab and set the number of scans to make.

18. Click the Run button to acquire spectrum; if needed, click Stop (stops after current scan) or the Run button again (stops immediately).

19. To get a well-centered spectrum, execute Acquisition > Tools > Center Tool (horizontal double arrow in the tool bar beneath the spectrum), move the mouse to the cursor that shows Center Tool, left click, and pull the cursor to the position that you want to be the spectral center. You can also change the spectral width by pointing, left clicking, and dragging the left or right limits of the Center Tool tool. Click on the trash barrel in the tool bar above the spectrum to Remove the Center Tool. Click on the Run button to reacquire a well-centered spectrum.

20. Store your spectrum by clicking the leftmost, red spectrum in the upper tool bar, and choosing the appropriate data set. This means your spectrum will be available for as long as your Xepr session is open. It has not yet been saved to the computer, however.

21. Save your spectrum in a manner similar to Store (step 20). You will need to enter a filename, thereby saving your spectra as a pair of files to the hard-drive (*.DTA, *.DSC). Do not put spaces into the name (none of that Apple naming stuff). It is also possible to save ASCII or old Bruker format files for export.

22. When finished, click the Tuning button, change attenuation to 60, go to Standby, and Close.

23. In Acquisition, Disconnect from spectrometer, then Exit from Xepr (best not to save settings) and Logout.

24. Remove your sample tube carefully from the cavity, and replace the Q-band cavity plug.

25. In order, turn off the two buttons on the power supply, then the heat exchanger, then the main console, then the water. TURN OFF THE WATER. TURN OFF THE WATER. TURN --- OFF --- THE --- WATER!!!