Dear all,
Thanks a lot to everyone who replied to the EPR question I posted upon
the request of Dr. Jianqin Zhuang (CUNY), especially to Prof John
McCracken, Dr.Timothy Burrow, Dr Shaoxiong wu, Dr. Shaw Huang, Dr.
Simon Sham, and Sand fro their suggestions.
Based on John's suggestion, Dr. Zhuang turned the knob back and forth
and cleaned the potentiometer pot. Then the auto tune is ok and the
frequency does not drift any more.
In case someone has the similar EPR problems infuture, a summary of
suggestions were attached as followings:
Problem From Jianqin (CUNY):
Recently, we have some problems when we use our Bruker EMX-300 EPR. In
tune mode, we can see the dip and adjust the AFC meter at the center. Once we
change to operate mode, the AFC starts to drift till the edge. It seems
like the AFC meter can not be locked with the right frequency during
operation. Does anybody have the same problem before and how to fix it?
Thanks a lot.
Jianqin
Suggestions:
1. The Bruker X-band epr bridge has two potentiometers that may be the source
of this problem. The slow drift of the lock offset off of center that you
are observing occurs because when you switch to operate mode, the microwave
source is not actually centered on the frequency dip. I know that the
display tells you it is centered there, but there is a potentiometer in the
microwave bridge tune circuit that is used to set the range of the source
frequency sweep for the "tune mode." On most models, this potentiometer is
mounted to the back panel of microwave bridge and you can adjust it with a
knob to make the "dip" display broad or narrow. Anyway, because the pot is
in the same position for most of its life, it gets a bad spot burned in it,
or maybe it just gets dirty, and it doesn't provide the correct value to
the sweep circuitry in tune mode. As a result when you switch to operate,
the source frequency is not where you think it is. This pot can be easily
cleaned by spraying with contact cleaner and turning the knob back and
forth a few times and then retuning. Eventually you will have to replace
it.
The other potentiometer that can cause this problem is the ranging
potentiometer in the source module. When you adjust the frequency of the
source, the machine keeps track of the frequency by reading the resistance
of a 10-turn wire-wound potentiometer that is in the source box. It is
connected directly to the tuning stepper motor via the gear assembly on top
of the source box. This pot also gets dirty and the AFC will eventually
cause the frequency to jump about during the measurements. It creates a
lot of noise and can also cause the source AFC lock to fail. You should
notice that the "Lock Offset" meter has a lot of unusual noise that it
displays - it almost vibrates. Once this starts happening, you need to
order a new potentiometer and replace it. This is not a difficult chore.
I don't know if you have a contact at Bruker. I find that Chuck Hanson in
the Billerica Office is an excellent source of information and he has been
very helpful to me in troubleshooting problems via email and phone.
Good luck!
2. I can think of two things that might be the problem.
First, the tune dip is made from a VCO whereas the operate microwave
source is from a Gunn diode or (on very old systems) a Klystron. If your
Gunn diode is not working or dying then that could cause your problem. If
you are in operate and the Gunn suddenly switches to Stand By, then a
thermal shutdown is happening and you need to look at your chilling water.
Second, you may need to adjust the AFC if the tuning dip is weak or broad.
On newer bridges, you can adjust the AFC through the "Options" button. On
older systems, there are some switches at the back of the microwave bridge.
Ours has a switch that has 1x and 1/8x. We normally use the 1/8x except
for salty samples that we use 1x.
I hope this helps!
3. I would try to manually adjust the iris to make the tuning dip sharper
first, then do autotuning.
4. You should first try a low dielectric sample, such as a blank
quartz tube or solid dpph sample.
If you do not have any problem locking with it, then other reasons and
instrument problem need to be investigated. It seems to me a high
dielectric material such as water in the cavity.
5. Do you have a manual for the instrument? Or call Bruker service center
6. First, most people in this forum are NMR people. Going back to you
question, what sample did you have? Did you check Bruker's EMX manual.
There is one page that talks about trouble-shooting.
Cheers,
Banghao Chen
***************************************
Chen, Banghao, PhD
Director of NMR Facility
Department of Chemistry & Biochemistry
Florida State University
-------------------------------------------------------------------------
Sent via webmail for Chemistry & Biochemistry _at_ Florida State University
https://webmail.chem.fsu.edu
--
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.
Received on Tue May 06 2014 - 04:56:32 MST