Wednesday, August 18, 2010

Can Zero-Filling Correct the Baseline?

I want to show you a proton spectrum that has puzzled me during the last weeks. It contains something that's quite typical and something that I can't explain. I have processed the spectrum in two different ways, with zero-filling and without it. The spectrum without zero-filling is black, the spectrum with zero-filling is green (the number of points is doubled).
This detail is the bottom part of the TMS signal (magnified to show the ringing effect). Where does the ringing come from? TMS is a small symmetric molecule and its protons have a long relaxation time. Their signal persists at the end of the FID. When we add the zeroes after the signal, a step is created. The FT of the step is the ringing that we see. The spectrum without zero-filling doesn't contain the step, so there is no ringing. The period of the ringing is exactly 1 point. In simpler words: odd points are positive, even points are negative. Things are not so simple, actually, because the rule is reversed on the two sides of the peak. This is something I have always seen, I don't know if it's a constant rule or something that's merely more probable than its opposite.
Without zero-filling, we have only half of the points. They correspond to the maxima on the left of the peak and to the minima on the right of it. Any program for automatic phase correction is fooled by asymmetric peaks like this. Even humans are often fooled. They think that the spectrum is "difficult to phase" and don't recognize that the peak is asymmetric. Asymmetry and ringing are two sides of the same coin. Without zero-filling we have asymmetry, with zero-filling we have ringing. In the first case it is difficult to recognize that the signal is truncated, because it appears much larger than it actually is.
Up to this point I can explain everything. It's all familiar to me. There is another effect that I can't explain at all and appears when I observe the whole spectral range. The baseline of the normal spectrum is wavy.
The baseline is perfectly flat in the other case. This is the first time I see such an effect: can zero-filling correct the baseline?
This spectrum was acquired on a recent Jeol 400 MHz instrument. I wonder if the digital filter has anything to do with the latter effect.


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