NMR meets MUSICIANS

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3. Auditory FID Presentation - History and Now

The idea to listen to NMR FIDs is by no means new. Early after the introduction of the pulsed Fourier transform method operators started to tap the analogue audio signals which result from mixing the RF signal of the probe with the reference signal created by the pulse oscillator. At that time (early seventies) an acoustic extention device was offered commercially by spectrometer vendors.

However, the method sooner or later became forgotten, the reason presumably being the following: At the time mentioned, one-pulse experiments were dominating, some two-pulse experiments (T₁, T₂ measurements) were carried out non-routinely. Phase cycling as is done nowadays for 2D-NMR experiments was not carried out at that time. Hence, one FID sounded like the other, and I can well imagine that the monotonous "ping" was considered to be boaring instead of contributing information.

Things have changed significantly since then. Of course, "one-pulse" experiments are still carried out routinely. I found that the monotonous "ping" of each individual FID may still contain a number of informations:

• Approximately how many signals are present in the spectrum?

• Are the signals close to or far from the carrier?

• Which coupling constants are present?

• Is the shim ok (rapid or slow FID decay)?

In our lab, acoustic FID monitoring has proven to be highly useful for the detection of spectrum artifacts.

First, we regularly switch our spectrometer from solution to soild state measurements. Solid state FIDs decay much more rapidly (for ¹³C order of some 10 msec) than solution FIDs, resulting in much higher linewidths of solid state signals. An example of a solid state FID is here. We noticed that under CP/MAS conditions in some cases certain spinner rotors lead to formation of spikes in the FID (probe arching?), resulting in degradation of signal to noise. These spikes may be observed optically in the FID, however, only at the beginning of a long-term measurement. In an already summed up FID these spikes will no longer be noticeable. On the other hand, these spikes may be easily detetcted acoustically when listening to each individual new FID: there is a typical "cracking" or "sparkling" sound leading to a "dirty" sound of the FID noise instead of a "smooth" sound created by undisturbed FIDs.

A second case where acoustic FID monitoring has proven to be highly useful is 2D NMR. As will be familiar to the reader, phase cycling is employed in 2D NMR in order to eliminate unwanted coherences. Usually, a minimum of 4 scans per t₁-increment with different pulse phases is recorded. When listening to an individual FID in the stereo mode it's quite easy and interesting to actually hear the change of the pulse phases: this manifests either in a ping-pong like change in the stereo channels or in effects reminiscent to "flanger" sounds in music. When listening to successive pulses in a 2D experiment, after some training one intuitively co-adds the sounds in a four-beat manner.

In two cases where I setup new pulse sequences this four-phase change suddenly was interrupted: two successive FIDs sounded exactly equal! An inspection of the pulse program revealed an error in writing the phase cycle. Presumably, without acoustic monitoring this error would have never been detected; artificial results would have been obtained and possibly even been published.

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August 20, 1996