Friday, November 03, 2006

Reduce the complexity!

Apparently 2D phase correction is a problem with 4 variables. In many cases it can be treated like 4 small adjustments, all with a single variable. Often there are two variables and 2 constants. A simple trick is to store your old spectra. Before processing your latest NOESY, retrieve and observe an older NOESY of yours. Some of the coefficients can be carried over. For beginners it's also advisable to extract the first increment and process it, at least in the case of homo-nuclear spectra. You will discover that some sequences make all peaks positive, other (e.g.: ROESY) make the peaks in the two halves of the spectrum point into opposite directions. Aim for symmetry! Whenever you correct the phase, both in 1D and 2D spectroscopy:

  1. enhance the intensity by an order of magnitude
  2. look only at the tails of the peaks
  3. aim for symmetry

When the first increment is OK, proceed with the two FFT performed on the whole matrix. It may happen that the optimal phase correction of the matrix differs, by a few degrees, by what seemed optimal for the first increment.
By correcting the first increment, you set the phase along the direct dimension. That is really variable and unpredictable. Along the other dimension, the indirect one, I often find that it's enough to set the first order phase correction = 180º (for a NOESY, also set the zero order phase correction = 90º). If this is not your case, set both corrections to zero, set the "pivot" at the center (wrong term, it's a cursor that freezes the phase at a given point of the spectrum). It should be enough to optimize the first order phase correction.

Mono-dimensional Correction

It is possible, and advisable, to correct the phase without even touching the control for zero-order correction. Start with an automatic phase correction. Enhance the peaks 10 times. Set the "pivot" on the more-in-phase peak. Optimize the first order correction, focusing on a peak far from the pivot. Enhance the peaks twice (every time you do so you'll notice a residual asymmetry). Move the pivot on the most symmetric peak and continue adjusting. After a while, every peak will be perfectly symmetric, even if you continue enhancing the intensity. Adopting this method, correcting the phase will be simpler than adjusting the volume of your speakers!


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