Thank you to Martine Ziliox, Ulrike Werner-Zwanziger, Jochem Struppe,
Kelly Moran, Charlie Dickinson and Jay Shore for the great help
regarding the REDOR setup on our Bruker DSX400. You all have been a
great help and with your input
I was producing REDOR data in no time.
Please see below for their direct answers.
Also thank you to Stefan Berger who was kind enough to send me a
preprint of relevant pages of his new book "200 and more NMR
Experiments", which will appear in Mid 2004.
The major problem, which I had, is that depending on the version of the
spectrometer's, SE451 it is not really possible/very inconvenient to
detect any signal in the F3 (y) channel. For a SE451 with channels for
H(F2) and X(F1) only, it can be achieved by setting both the F3 and the
F1 channel to the same frequency. Pulsing on F3 while detecting on F1
leads to signals with fluctuating phases. So adding up individual FIDs
after FT and magnitude calculation might be useful to measure simple
spectra feasible for adjustment procedures associated with the F3
channel.
However, it is best to characterize the Y-nucleus with respect to its
excitation frequency and 180 degree pulse length by using the F1
channel. The power output fed into the probe can then be measured with
an oscilloscope to provide a guide for a coarse setting of the power
output of the F3 channel. Fine tuning of the F3 channel can be achieved
by tuning the F3 output power or the respective pulse length using the
actual REDOR experiment (see below).
Kind Regards,
Hanno
*******************************************************************
Martine Ziliox:
My favorite way of setting up the 3rd channel is still the elegant
method that Tony Bielecki had devised at the time when having a 2nd BB
preamp was not possible. I suggest that you use doubly labeled Glycine
diluted to 10% in natural abundance Glycine to set up the experiment.
The N15 signal is at -6.35ppm from NH4Cl, but you don't really need to
look at the 15N signal unless you want to check out the performance of
the 3rd channel in terms of sensitivity. To find out the correct
irradiation frequency for 15N, first record a regular 13C CP spectrum.
Measure SFO1 of the carbonyl peak and multiply this number by 3.976245
to get SFO2 (the proton frequency) and by 0.4029213 to get SFO3 (the 15N
frequency). To determine the 180 degree pulse on the 3rd channel, run a
1D version of the REDOR sequence, with a 1ms recoupling time. Vary the
180 degree pulse length (paropt), monitoring the amplitude of the alpha
carbon line. The signal is lowest for the exact 180 degree pulse length.
I am faxing you the procedure as it was described by Tony 10 years back.
**********************************************************************
Jochem Struppe:
You have two possibilities: first, you need to determine the 180 degree
pulse using a double resonance experiments and an appropriate sample on
the 3rd channel.
The you bypass the preamplifier module, setup the redor experiment and
measure on an easy sample a redor without dephasing ( no pulses on f3)
the number of rotor periods should be chosen such, that with dephasing
pulses you would expect a strongly attenuated signal. Then you switch
on the 3rd channel use the power level -0.5 - -1dB ( about 0.5 to 1 dB
less power - to account for HPPR losses), use the measured pulse widths
for the 180 degree pulse and paropt pl3 or whatever you have on channel
3 in 0.1 - 0.2 db steps and look for the signal minimum - maximum
attenuation. Then you have the correct PI pulse.
Method 2 is: Use the Bloch-Siegert shift for measuring B1 field of a
channel by measuring the dephasing of a spin echo signal. In detail, you
make a spin echo experiment, e.g. on glycine CP-tau - Pi_pulse - tau
-acquisition, you ft and phase the spectrum. Then you apply a pulse
with a certain power level for a time T prior to the echo on channel 3
during the spin echo experiment:
Reference experiment:
F2 e.g. 1H p90- CP - Decouple
F1 e.g. 13C CP ---- tau ---- PI ---- tau ---- acqu
F3 e.g. 2H -----------------------------------------------------
(no pulses)
Shift experiment :
F2 e.g. 1H p90- CP - Decouple
F1 e.g. 13C CP ---- tau ---- PI ---- tau ---- acqu
F3 e.g. 2H ------------PulseT------------------------------
one pulse with duration T before the
echo pi pulse. You measure the additional phase correction you need for
a correctly phased spectrum. From this phase difference you can
calculate the B1 field in channel 3. Terry Gullion suggested this
method some time ago.
Charlie Dickinson:
We operated for years with 3 channels and 2 preamps with Bruker. While
the pi/2 pulse determined with the preamp in the line is different than
the pi/2 without the preamp, the differences are slight and we could
optimize for example the N15 pulses with the REDOR effect itself.
Annoying but it works. 3rd preamp would be a luxury.
*********************************************************************
Ulrike Werner-Zwanziger:
basically, we determine the 90 degree times by direct measurement
through the X-preamplifier. I hope your nucleus on a test sample allows
for that. What we found to be very important is to use the relevant
pathway in edasp. By that I mean, you have rout through the FCU's and
amplifiers, that you will eventually use, in the determination of the
pulse length. This is typically different from the standard
configuration for the y-channel.
***********************************************************************
Jay Shore:
I just use the channel with the preamplifier to measure the 90 for the
third channel and then set the third channel to the same pulse voltage
peak to peak and use the measured 90. So you just have to have a high
power attenuator or a bidirectional coupler to be able to measure your
pulse power.
************************************************************************
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The original question:
Dear All,
We are in the process of setting up REDOR-type experiments on our Bruker
DSX400 solid state NMR spectrometer. We have a somewhat minimal
configuration, which Bruker assured us would allow triple resonance
experiments:
- Triple Resonance Probe
- 3 FCU units
- 2 BLAX300 and 1 BLAH300 transmitters
- the old high power cabinet with two 1 kW tube amplifiers for 1H and X.
We have only two preamplifier channels, which makes all setup procedures
of the third channel quite complicated. Does anyone have a similar
configuration than we do? What is the setup procedure to determine the
pi-pulse length for this channel?
Thank you,
Hanno
********************************************************
Dr. Johannes Leisen
Research Scientist II
Georgia Institute of Technology
School of Polymer, Textile & Fiber Engineering
Atlanta/GA 30332-0295
phone: (404) 894-9241; FAX (404) 894-9766
e-mail: johannes.leisen_at_ptfe.gatech.edu
Received on Mon Mar 01 2004 - 10:45:37 MST