External address advance for the purposes of acquiring CPMG data is
possible on an AC series spectrometer. However, it cannot be achieved
without slight hardware modification.
The problem that I was running into was the status of F1 (whether it was
set to xmit or receive). This is normally toggled by a ttl level voltage,
and certain software commands apparently toggle F1 to the required xmit
or receive mode. In the CPMG, we had to ensure that F1 was on xmit prior
to the 180 degree refocussing pulses, then switched to receive prior to
taking the data point on top of the echo. Sincere thanks go to Mike
Geckle at Bruker Instruments in Wilmington DE for suggesting this as the
source of our problems.
We have used RCP4 to set F1 to receive mode, and the spare RCP3 to set F1
to xmit mode. The switching occurs using a pulse of 1 usec, so there is
minimal overhead here. All of the hard work was done by Stan Woodman
(Service Manager, Bruker Spectrospin Canada, Milton, Ontario), and he has
asked that if anyone requires details on the modification that you
contact him directly.
So, I am currently running CPMG's with the following pulse program:
;T2XAD.AUR
1 ZE
2 D0 ;RELAXATION DELAY
3 P1 PH1 ;90 DEGREE PULSE
4 D5 ;D5=DE/2 (STANDARD SETUP FOR ACQUISITION)
5 D7 ADC ;INITIALIZE ADC, D7=2 USEC
6 D5 PH0 ;SET DETECTOR PHASE
7 P8:X ;TAKE THE FIRST DATA POINT
8 P8:C3 ;SET F1 TO TRANSMIT MODE
9 (D2) (D3 P2 PH2):T:E ;ECHO SPACING D2, D3=D2-(2*D5+D7+2*P8)
10 P8:C4 ;SET F1 TO RECEIVE MODE
11 (D2) (D3 P8):X ;TAKE THE DATA POINT ON TOP OF THE ECHO
12 L1 TO 8 TIMES UPR ;LOOP FOR "UPR" ECHOES
13 RCYC=2 PH3 ;CYCLE FOR NS SCANS
14 EXIT
PH0=1
PH1=0 2
PH2=1 1
PH3=R0 R2
I've run this on our high temperature probe at 170 C, with a dead time of
around 20 usec, proton pi/2 pulse times of ~ 9 usec, collecting 1024 data
points with no problems whatsoever (knock wood). Tau spacing of 50 usec
doesn't seem to be a problem, though I have not gone shorter (something
tells me that this might present duty cycle problems...).
The only thing missing is the equivelant of the MSL RCPH command to set
the receiver phase so that the decay is maximum amplitude, right side-up
on the monitor. To fix this problem, we do what the pioneers did and
scrounge up a variety of lengths of BNC cables and try different length
combinations until we get what we want.
A sincere thank-you to all out there who took the time to respond. Now,
back to CPMG's...
Tim Bremner
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Z Y' : DR. TIM BREMNER, Inst. for Polymer Research |
| / : :: Department of Chemistry, U. of Waterloo |
|/ : : : :: Waterloo, Ontario, CANADA, N2L 3G1 |
-----o====>X' : : : : : ::::::::::::::::::::::::::::::::::::::::|
/| : : : : :: Ph: 519-885-1211 x2510 |
/ | : : :: Fax: 519-888-6179 |
/ | :: E-mail: tbremner@polymer.uwaterloo.ca |
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