Because it wouldn't make sense otherwise, NMR instruments use receiver systems equipped with digital filters since a relatively long time ago. The advantages of such digital filters (generally designed as low-pass filters and applied together with oversampling and decimation methods) are many fold, ranging from higher quality spectral baselines to SNR and effective dynamic range improvements, enhanced reduction of potential sources of folded signals, etc
It´s not all about advantages though … I’m sure most of you are already very well aware of the pesky problem that is infamously known as group-delay artifact in Bruker (and Jeol) data which has plagued the NMR community since these companies switched to digital receivers. In short, the FID resulting from the digital filter does not start at time = 0 but only after a long and slowly rising oscillation of length G (G = Group Delay).
Some empirical procedures to correct it were presented on the internet but they are palliative and do not resolve the problem completely, particularly when apodization is applied.
Typically and depending on how the FID is processed, the spectrum might exhibit smiles (baseline artifacts pointing up) or frowns (baseline artifacts pointing down) at the outer regions of the spectrum as depicted below:
The ultimate solution
These small artifacts are in general not a big problem as one could use a spectral width large enough so that the peaks of interest in the spectrum will not be affected by these artifacts (although some processing algorithms such as backward Linear Prediction could be somewhat problematic with the Group Delay). In any case, we did not feel very comfortable with present solutions to this problem. A few months ago, I went for dinner with Stan and right after it, the power of the red wine and above all, the Galician octopus inspired Stan in such a way that he managed to understand the engineering drawback and proposed a new correction algorithm which we implemented together in Mnova just a few minutes later (whilst still under the influence of the wine :-) ).
Basically we have now a new pre-processing algorithm that corrects in a totally automatic way any Bruker FID corrupted by the group-delay artifact, producing a normal and physically correct FID so that the smiles will not be seen in the f-domain spectrum. The performance of the new algorithm is illustrated in the figure below:
This enhanced correction is available in Mnova since version 6.1.1 onwards, although it is not the default processing method for the moment. In order to activate it, it is necessary to select it via Processing/Group Delay menu command.
I guess the take home from the story is never underestimate the power of red wine and Galician octopus :-)
