University of Ottawa NMR Facility Web Site

Please feel free to make suggestions for future posts by emailing Glenn Facey.



Friday, May 23, 2008

Acoustic Ringing

When a pulse is applied to an NMR probe in a strong magnetic field, the oscillating rf current in the circuit induces mechanical (acoustic) oscillations in metal parts of the probe. These mechanical oscillations in turn generate rf signals detected by the coil. The so called "acoustic ringing" is seen in the FID and (depending on the specific frequency and probe) usually decays within several tens to hundreds of microseconds after the pulse. It is more of a problem at high fields and low frequencies and a particular problem when wide spectral widths (short dwell times) are employed. The acoustic ringing may impede the observation of very broad lines and cause baseline and phasing problems when observing sharp lines with large spectral widths. An example of the latter case is shown in the figure below.

4 comments:

Anonymous said...

Hello Glenn:

I have some questions related to probe ringing AND the group delay.

Firstly, I am using Bruker Avance 300 to evaluate Be9 (~42.174 MHz). In the fid, I can clearly see the group delay.

In the "processed" spectrum (whose SW is incidentally as wide as allowed at this field), the extremes of the spectum contain "frowns", "smiles" or 1 of each sometimes.

Additionally, the "processed" spectrum has a rolling baseline which I am willing to bet is due to probe ringing.

So, the first question is:

Are the group delay, frowns(or smiles), and rolling baseline all related to each other?

Finally, if they are related, can (or rather should) I "simply left shift the data?

I am asking because I am having a hard time processing the data because the baseline is really rolling.

Thanks in advance for any help you can provide.

Glenn Facey said...

Anonymous,

Thank you for your entry.
To answer your first question - The "smiles" and "frowns" resulting from the group delay are not related to any baseline roll due to acoustic ringing.

Second question - You can left shift the data as it is but you must take into account the group delay when you left shift. In older Bruker software this is not displayed as one of the parameters. Alternatively, you could convert the digital to "analog-like" data by using the Bruker command "convdta". This will write your data in analog-like format in another experiment. If you look at this FID (with no apparent group delay), you can decide on the number of bad points to left-shift. Another solution my be to use a pulse sequence designed to suppress acoustic ringing.

Glenn

Anonymous said...

Hi,

Great post! Very informative.

My question is:

Can you still have ringing even in CPMG sequence? I thought refocusing pulses are supposed to cancel the ringing. Is this true?


Regards,
Zade

Glenn Facey said...

Zade,

Thank you for your comment. Echo sequences like CPMG do not cancel out the acoustic ringing. There is still ringing after each pulse, its just that in an echo sequence, the data are generally collected at a time after the refocusing pulse such that the acoustic ringing has stopped.

Glenn