Tuesday, November 07, 2006

Empirical Approach

How to get a flat base-plane in multidimensional NMR? Many years ago they told me to multiply the first point of the FID by 0.5. Many colleagues know it. You can find the rationale in the literature. I too could find it, just was too lazy for studying. To justify my laziness my arguments were:

  • The literature is very old. If what they say is right, manufacturers, who read it before me, are already accounting for it and the acquisition software already pre-multiplies the first point.
  • OK, the first increment of a 2D doesn't look pre-multiplied. But my spectrum still needs a baseline correction after FT, so it makes no difference.
  • The literature predates phase-sensitive 2Ds, predates 2D baseline corrections, predates fast computers. Probably it's not valid for today's spectra. Probably they had to avoid baseline correction, because their computers were too slow, and invented this poor man's alternative.

Another questionable practice is the baseline correction performed on the transformed increments, soon after the FT along the direct dimension. It must be automatic (normally you don't even see the imaginary part) and the spectrum is still complex. How can an algorithm find a region of pure baseline into a crowded dispersion spectrum? How can you trust into it, without a visual feed-back? You don't have this problems with first point pre-multiplication. It's so pure, you know. Like homeopathy, it can't hurt. This is why I used the latter, like everybody else. Mestre-C still applies it by default on all spectra (1D included, I mean).
With the Mac Quadra, running at 25 MHz, it was possible to transform a matrix in less than two minutes. Finally I could optimize my processing parameters by trial and error. I pre-multiplied by 0, 0.25, 0.5, 0.75 and by 1.25. Judging from the final result (after complete processing, base-plane correction included), my first spectrum preferred no pre-multiplication. My second spectrum, a week later, preferred no pre-multiplication. The third spectrum did just the same. I declared my experimentation period terminated. I haven't kept the results with me. From that day I simply stopped pre-multiplying and my spectra were not so bad. Since the introduction of the digital filter by Bruker, the first point is zero or near-zero, so in that case you can even multiply by 10, if you are in the mood.


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