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Wednesday, September 19, 2007

Exponential Line Broadening

If you have a spectrum with a poor signal-to-noise ratio or a spectrum containing a very broad resonance (due to slow molecular motion for example), you will probably be able to get higher quality data by using exponential line broadening. This is a very common data processing technique where the FID is multiplied by a decaying exponential function (the Fourier transform of which is a Lorentzian line with a full width at half height of LB). The larger LB, the faster the decay of the exponential. The product of the FID with the exponentially decaying function is Fourier transformed to give the NMR spectrum.Since the signal-to-noise ratio of the FID is worse near the end than at the beginning, multiplying by the exponential decay effectively improves the overall signal to noise ratio. Broad signals tend to show up better as broad lines have very rapidly decaying FID's and hence the multiplication of the FID by an exponential emphasizes the faster decaying components of the FID.

All things come at a price. Application of exponential line broadening will decrease the resolution in your NMR spectrum.

How do I use it intelligently?

In TOPSPIN or XWINNMR type:
"lb 0"
"efp"
"apk"

Using the cursors, measure the line width at half height of the narrowest line of interest. Input approximately 75% of this value as "lb". For example, if the narrowest line of interest was 1 Hz, set lb to 0.75.

type:
"lb=0.75"
"efp"
"apk"

2 comments:

Steve Clemens said...

Glenn,

It sounds like you are describing how to apply the matched filter.

Perhaps it's naive to ask, but why do you set the LB value to 75% of the matched filter and not its actual value?

Glenn Facey said...

Hi Steve,

Thanks for the comment. One can maximize the signal-to-noise ratio by using an LB equal to the line width of interest to presumably maximize the signal-to-noise ratio (matched filter). I perfer to use an LB of 75% of the narrowest line of interest. In doing this one gets most of the SNR advantage of the matched filter but with less loss in resolution. There is nothing magic about the 75%. It is just my preference. I often use higher or lower values of LB depending on the need for resolution.

Glenn