Tuning the microwave cavity and bridge.

DO NOT GO NEAR THE MAGNET UNLESS THE ATTENUATOR IS ON 60 OR ABOVE AND THE

INSTRUMENT IS IN EITHER THE STAND-BY OR THE TUNE MODE ALWAYS TRIPLE CHECK!!

1. Turn on the main gas valve of the nitrogen tank, then turn on the in-line valve until a slight hissing of the gas is heard. (You only need slight positive pressure

to keep the cavity Oxygen free!)

2. Flip on the console switch (a silver toggle on the right lower rear of the console.) Let the console warm-up for 15-30 minutes, one hour for best results.

3. During warm-up: Turn on the frequency counter (top of the magnet)

Log into Log Book

Turn on the computer, select EPR icon from the chemistry window.

Go to file and change the path to the appropriate directory (current directories

located at c:\epr\XXX)

4. Turn on the main power supply unit by pressing the green ON button. If it does not power up Press the bottom white button and then the green ON button. If it still does not power up Try the red reset button inside the back panel. (Be careful of the high voltage in the back of the power supply.)

5. For RT EPR, select appropriate collar and place on cavity. Insert EPR tube so you can see the sample in the grating.

6. After instrument is warmed-up, switch the microwave bridge to the TUNE mode in order to adjust the excitation FREQUENCY. If the TUNE mode button does

not light up, the instrument is not sufficiently warmed-up, wait.

7. Find the dip. The microwave ATTENUATION should be at either 50 dB or 60 dB. The higher the power (ie the smaller the number on the attenuator) the

larger the signal you will see on the oscilloscope. There should now be a signal on

the oscilloscope. Adjust the FREQUENCY knob (Slowly) so the dip in that

signal is centered, looks like an M (typically 9.XX for organic radicals at RT in

quartz tubes.) If the dip is unsymmetrical adjust the SIGNAL PHASE knob/dial

until it is symmetric.

* This pattern is a display of the microwave power reflected from the

cavity and the reference arm power as a function of the microwave frequency.

The dip corresponds to the microwave power absorbed by the cavity and thus is

not reflected back to the detector diode. By centering the dip on the display

monitor, the microwave source is set to oscillate at the same frequency as the

cavity resonant frequency.

8. Fine-tune the microwave source frequency. Make sure the microwave ATTENUATION is set at either 50 dB or 60 dB. Put the instrument in the

OPERATE mode to fine-tune the frequency. Observe the Lock Offset meter and

adjust the FREQUENCY until the needle is in the red (center). Throughout the

rest of the set-up process frequently check the Lock Offset meter to ensure that

the lock has not been lost.

9. Adjust the bias level. Change the microwave ATTENUATION to 50 dB. Set the DIODE current to 210 by adjusting the BIAS screw. * Reminder: Keep an eye on

the Lock Offset meter. Sometimes the cavity will loose lock at 50dB (the needle

will run off to the right or to the left of the Lock Offset meter.) It will regain lock

at higher microwave power levels (More power means lower values in dB.)

10. Critical coupling of the cavity. Increase the microwave power by 10 dB (i.e. ATTENUATION settings of 40 dB or 30 dB.) If the Diode current moves from

210, click the IRIS buttons for the iris screw motor until the DIODE current

returns to the 210 . Repeat this procedure until you have reached an attenuator

setting of 20 dB. Make sure that the Lock Offset meter stays centered. If it moves, adjust with the FREQUENCY knob until centered and reset the DIODE

current to 210 if needed. Check the pencil marks on the iris adjust arm to ensure

that you are not over adjusting. Over adjusting could damage the screw.

Note: the iris buttons work in opposite direction. The up arrow means you are twisting

the screw down and The down arrow means that you are twisting the screw up.

11. After adjusting the iris at 20dB also adjust the SIGNAL PHASE knob to

maximize the DIODE current. If the DIODE current is now significantly higher

than 210 set the ATTENUATOR to 60dB and repeat the above steps.

12. Verify that you have achieved critical coupling by changing the microwave ATTENUATION from 20dB to 50dB with virtually no change in the DIODE

current. Repeat the coupling and bias adjustment if necessary. For power levels

greater than 20mW (lower than 10dB), set the ATTENUATOR to 0dB and once

again adjust the DIODE current. The current may drift because the high

microwave power starts to heat the sample. If this happens, wait a minute or two

and readjust the coupling.

13. Finally set the ATTENUATOR to the value you will be running your sample. 14. Set the center of the magnetic field by entering the value (typically 3475G for

organic radicals at RT) and then press the CF button (center field).

15. Set the sweep width by entering the value (550 G sweep width is 250 G on each side of the CF) then press the SW button (sweep width). Larger sweep widths

should be used for compounds for which little magnetic info is known.

16. You can check your SW and CF by pressing the RC button followed by either the SW or CF button.

17. Set the SWEEP TIME by turning the knob to an appropriate setting. 18. Set the GAIN, MODULATION AMPLITUDE, TIME CONSTANT, PHASE

MODULATION FREQUENCY. (Gain 10^5, MA 0.05G for solution phase and

1.25 for frozen, time constant 10x10^-2, phase 0, modulation frequency 100kHz.)

19. Set the number of scans on the data acquisition menu on the computer. Then select acquire data.

20. To interface the control panel with the computer, select SLOW button under ACCUMULATE on the control panel. Then start the scan by pressing the button

with single forward arrow on it (red light).

21. Remove the sample from the cavity. Bring the ATTENUATOR back up to 60 dB or greater and place the instrument in STAND-BY mode. Remove the sample.

If a reference scan is desired put in a reference sample. Return the instrument to

the OPERATE MODE and bring the ATTENUATOR back down to 20 dB. Set

the GAIN to its lowest setting. REMEMBER DO NOT CHANGE THE

FREQUENCY. (this would invalidate the reference.) The iris can be adjusted if

needed.

22. Shutting down the instrument. Turn the GAIN all the way down and the MODULATION AMPLITUDE off. Put the ATTENUATOR at 60 dB or above

and put the instrument in the STAND-BY mode. Turn off the console and the

magnet. TURN OFF THE NITROGEN!!

Written By: Candice Brannen