I have previously blogged about direct Covariance NMR as a technique to increase the digital resolution of the indirect dimension in 2D homonuclear experiments. As is always the case, there is some price to pay: Covariance NMR introduces some unexpected resonances, in special when the number of t1 increments is small making the covariance exhibit poor statistics (JACS, 2006, 128, 15564–15565). Some of these extra resonances are true spurious peaks whilst others correspond to multistep or RCOSY-type correlations (see MRC, 2008, 46, 997-1002)The picture below shows the regular 2D FFT spectrum of Strychnine:
And this is its standard direct Covariance counterpart where I have highlighted some of the extra resonances.
While some of these additional resonances can be beneficial because they provide kind of TOCSY correlations, others are just pure artifacts which make the analysis of these experiments unreliable (it’s difficult to know in advance whether a peak is a real correlation or just an artifact).We have recently incorporated into Mnova a new filter which eliminates these artifacts very efficiently. For example, this is the result obtained after automatic filtering of the Covariance NMR spectrum:
It can be observed that extra peaks have been eliminated without giving up the resolution advantage.
All these new processing capabilities will be available in the next version of Mnova (5.3.0, to be released in a few days), although we have a pre-release candidate available to anyone interested. Just contact me and I’ll be happy to send it out.