Tuesday, September 12, 2017

Optimizing the Signal-to-Noise-Ratio with Spin-Noise Probe Tuning

We have all been taught to tune our NMR probes to maximize the pulse power delivered to our sample (or minimize the reflected power back to the amplifier).  This prevents damage to the amplifiers and minimizes the duration of 90° pulses at fixed power levels.  This is typically done with the spectrometer hardware (eg, "atmm" or "wobb" on a Bruker spectrometer). Tuning a probe in this way optimizes the transmission of rf to the sample, however, the NMR probe must also detect signals from the sample to be amplified and sent to the receiver.  The "receive" function uses a different electronic path compared to the "transmit" function.  Since the electronic paths for the "transmit" and "receive" functions are completely different, they are expected to have different tuning characteristics.  A probe optimized to transmit rf to the sample is not necessarily optimized to receive the rf NMR signal from the sample.  As a result, one may not be getting the optimum signal-to-noise-ratio with a probe tuned and matched in the conventional manner.  The question then arises as to how can we tune  an NMR probe optimized to detect and receive the NMR signals from the sample.  This can be done by measuring a spin noise spectrum of the sample - using no rf pulses whatsoever.  It has been shown1 that a probe is optimized to detect and receive the NMR signals when one observes an inverted spin noise NMR signal from the sample.  Since the spin noise signal is measured without any pulses from the "transmit" function of the spectrometer, it depends only on the electronic path of the "receive" function. To tune a probe for optimum "receive" function, one must adjust the tuning frequency and matching of the probe followed by the measurement of a spin noise spectrum until an inverted spin noise signal is observed.   The figure below illustrates an example of this using a 2 mM sucrose solution in 90% H2O/10% D2O.
The proton channel of a 600 MHz cryoprobe on a Bruker AVANCE III HD NMR spectrometer was tuned and matched at 10 different frequencies using the "atmm" function of the spectrometer.  The tuning offset frequencies were measured using the "wobb" display of the spectrometer.  For each tuning offset frequency, a spin noise spectrum of water was measured using 64 power spectra collected in a a pseudo 2D scheme and summed to produce the spin noise spectrum displayed.  The spin noise spectrum for the probe optimized for the "transmit" function is highlighted in pink and the spin noise spectrum for the probe optimized for the "receive" function is highlighted in yellow.  For every tuning offset frequency, the 90° pulse was measured with the "pulsecal" routine of the spectrometer which uses this method.  As expected, the minimum 90° pulse is obtained for the probe tuned to optimize the "transmit" function.  With all pulses optimized, a 1H spectrum of the sample for each tuning offset frequency was measured using excitation sculpting as a means of solvent suppression (pulprog= zgesgp).  The sucrose signal at ~ 3.9 ppm is displayed in the figure.  The maximum signal intensity (highlighted in yellow) is obtained at a tuning offset frequency of -695 kHz corresponding closely to where the spin noise spectrum is inverted (-895 kHz).  The noise levels in the spectra were found to vary somewhat at higher tuning offset frequencies.  As a result, the maximum signal-to-noise-ratio (highlighted in yellow) was observed at a tuning offset frequency of -488 kHz.  This represents a 21% improvement in the signal-to-noise-ratio compared to that observed for a probe tuned in the conventional manner (highlighted in pink).  The degree of solvent suppression using excitation sculpting was also found to deteriorate at higher tuning offset frequencies.  In conclusion, one can obtain spectra with higher signal-to-noise-ratios by using a tuning offset frequency other than zero.  One expects the specific optimum tuning offset frequency to be probe, instrument and sample dependent.  This phenomenon is described much more elegantly in the reference below.

1.  M. Nausner, J. Schlagnitweit, V. Smrecki, X. Yang, A. Jerschow, N. Müller.  J. Mag. Res. 198, 73 (2009).

6 comments:

Unknown said...

Dear Glenn,
thanks for the very interesting phenomenum.
I guess this happens because transmiter and reciever have slightly different impedence.
Have you try to change Matching of the probe?
Do you think it will help to find best efficiency for the reciever channel.

Alexey

Glenn Facey said...

Alexey,
I have not tried to change the matching of the probe. It may very well have an influence on the efficiency of the receiver channel.

Glenn

Unknown said...

I know it's typical to show signal to noise as the parameter - but in this case, isn't the noise constant in all cases? Those spectra and the SNR values don't seem quite correct. Couldn't you just put the signal level instead of the signal to noise, for better consistency.

Glenn Facey said...

Andrew,
Thank you for your question. In this case the noise levels in the 1H spectra were slightly different for each measurement. This is why the SNR was used. Ultimately it is the SNR that is most important to an experiment.
Glenn

Marina said...

Dear Glenn,
I have a question about matching and tuning the probe. I work on INOVA VARIAN 400 spectrometr and we have Qone tec probe. Usually for collecting 13C NMR spectra we use the second channel and 1380 tuning and 0967 matching parameters on the probe. However sometimes our supervisor uses another parameters for 13C spectra: second channel, but ~1420 tuning and ~0010 matching. And 13C spectra are complitely good. He doesn't explain why he does it. But i'm intrested - does it mean that it is possible to use different combinations of matching and tuning to find the frequency for the same nucleus? Or what can be other reasons that he does like this?
(Sorry for my English, it's not my native language)

Glenn Facey said...

Marina,
First I should say that I have never used a Qone tec probe. Tuning and matching capacitor settings are very sensitive to the sample in the probe. If the probe is perfectly tuned and matched, the settings will likely be different from one sample to another. Although you could dial in specific values of matching and tuning elements to get you close to the frequency of 13C, you will have to change them for optimum tuning and matching for every sample. Could it be that your supervisor optimizes these values for his samples? Tuning and matching are not independent of one another.

See these posts:

https://u-of-o-nmr-facility.blogspot.com/2008/07/tuning-problems-for-samples-of-high.html

https://u-of-o-nmr-facility.blogspot.com/2008/04/tuning-and-matching-nmr-probe.html

Glenn