2D phase correction with iNMR (for brukerists)

processing a 2D spectrum can sometime become a puzzle. It is necessary, therefore, to learn some theory.

DEFINITION
The dimension with the highest index is called "direct", all others being "indirect".

THEOREM
Half of what Bruker says is false.

COROLLARY
The other half is simply misleading.

DEMONSTRATION
Bruker calls "f2" the indirect dimension and "f1" the direct one. Being this in contrast with the definition, the theorem is demonstrated. Demonstration of the corollary is left to the reader as an exercise.

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Now the practice. First thing first: you should determine if yours is a problem of phase or a poor choice of FT parameters.

RULE 1
If your FT parameters are wrong, you get all the signals doubled, or they fall at the wrong chemical shift, or both things happen. If you get the correct number of peaks in the correct chemical shift order, your parameters are correct. Spend time on correcting the phase.

RULE 2
With the classic NOESY, TOCSY and ROESY experiments it is possible to bring in phase all signal simultaneously. When the pulse sequence become longer, and if you add a pre-saturation step, phasing all peaks may become impossible. In such case, ignore outlier peaks and phase the rest.

RULE 3
The option "Swap Sides" is normally ON. The only exception I remember is represented by Bruker spectra in "States-TPPI" mode, which require "swap sides" only along f2.

RULE 4
The option "Real FT" is normally OFF. The only exception I remember is represented by Bruker spectra in pure "TPPI" mode, which require "Real FT" only along f1.

HINT 1
If you have troubles with the phase correction, use a sine bell shifted by 90 degrees along f2 and a squared sine bell shifted by 90 degrees along f1, and do not use neither LP-filling nor zero-filling. When you have found the right phase parameters, you can add zero-filling or LP-filling.

HINT 2
If you know that a spectrum is phase-sensitive or "echo-antiecho", use the corresponding shuffling along the indirect dimension, but never along the direct one.

HINT 3
Learn the iNMR mechanism "Extract"-"Repeat" and use it to find the right FT parameters.

HINT 4
To become a wizard in phase correction, make practice with good-looking, good-behaving spectra. Each time you feel having reached the perfect correction, increase the amplitude factor (= press the plus key). Are all peaks still in phase?

the iNMR manual is on-line