Friday, 21 May 2010

Bruker Smiles

The above title is not aimed to mislead and it does not refer to Bruker’s state of mind: Quite simply, the purpose of this post is merely technical and the relation between Bruker and its smiles will be apparent in a moment… Just keep reading…

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 :-)

Thursday, 20 May 2010

Back in the blogosphere

After a 3 month-long hiatus, I'm back in the blogosphere. I’ve been travelling and working very hard on several exciting projects, but my entries have consequently suffered.

Although my workload has not decreased, I am not planning to travel for the next few weeks. Hopefully I will manage to blog on a more regular basis from now on, especially now that there are many things we have been working on lately that I hope will be of interest to the NMR community.

For now, I’d just like to point out a very interesting blog entry written by my friend Stan on his well-known NMR blog. One of the tools in Mnova for which we are most proud of is GSD (Global Spectral Deconvolution) which, as any other fitting process, requires the definition of a line shape model. GSD uses a Lorentzian model and some of the reasons for this choice have been elegantly exposed on his blog.

Why spectral lines are Lorentzian