Simulating a COSY experiment with 3 spins
Pulse Program: COSYSH
For this example we selected a 3-spin (1H) system, with the chemical
shift positions at 4.2, 2.8, and 3.3 ppm
Input the following values for the J couplings between these spins: 1-2=30Hz, 1-3=20Hz, 2-3=40Hz.
Note that R1 relaxation rate has been increased to better illustrate the phenomenon of axial noise.
You can use the GraphMode panel for spin setup:
Don't forget to first select the pulse program cosysh from the list of programs and then click
the XLATE button to update the program parameter panel with the parameters specific for this
Set: p0=p1, d1=2s
Several parameters in this panel (e.g. l3, tdaq, swaq, etc dependent on the values of td1, td2, sw2) could display values
different from those shown here. These parameters will be updated automatically after the setting
of experimental parameters during the next step below.
td1=128, td2=64, ns=1, p90=10 usec, PL=0 dB, SWp1=2.2 ppm, SWp2=2.2 ppm
Select States for proper processing in the f1 dimension.
This is the resulting COSY spectrum for 3 spins:
If the spectrum you obtained does not look like this one, go back and check
all settings including J couplings.
If you want to achieve a better resolution in the f2 dimension, go back to the
Experimental Setup panel, set td2 = 128, and rerun the simulation.
You will then get a spectrum that looks like this:
Comments: These spectra are a result of a single scan experiment, without phase
cycling. Therefore, axial noise can be seen (peaks unlabeled in the f1 dimension
that appear along the abscissa at f1 = 3.5 ppm, indicated by the pointer in both spectra).
Increasing the number of scans to 8 (full phase cycle, see below) will suppress the axial
noise. (Note that the noise appears at f1 = 3.5 ppm because this pulse sequence uses States
processing for t1 frequency sign discrimination. Sequences relying on
TPPI and States-TPPI processing will displace "axial" noise to the edges of the spectrum.)
The axial noise is suppressed when a full phase cycle is achieved. In order to
do this, the number of scans (ns) has to be increased to 8 in the ExpSetup Window,
all other parameters remaining unchanged:
The resulting spectrum does not contain the axial noise:
Here is a 3D-view of the resulting COSY spectrum:
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This manual page was designed by Peter Nicholas. Last modification by David Fushman 12/26/2003.
(c) Copyright 2003 by David Fushman, University of Maryland.