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.
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.)
11.
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