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Tuesday, June 24, 2014

2H NMR on a Bruker AVANCE Spectrometer

The acquisition of high resolution 2H NMR data on a Bruker AVANCE spectrometer is done differently than that for 13C, 31P or any other heteronucleus.  Most heteronuclear data are collected using a broadband amplifier, a broadband preamplifier and the high sensitivity coil of a broadband probe.  This configuration cannot be used to collect 2H NMR data as the broadband preamplifier on AVANCE spectrometers has a built in 2H stop filter.  There are at least two options for collecting 2H NMR data on a Bruker AVANCE spectrometer using a broadband NMR probe: one, requiring no re-cabling with low sensitivity and another, requiring some re-cabling with high sensitivity.  The low sensitivity option uses the 20W 2H amplifier (normally used for 2H gradient shimming), the lock preamplifier and the 2H lock coil of the probe.  Although convenient, since no re-cabling or reconfiguration is necessary, the sensitivity is low because the lock coil often has a very low filling factor and the 20W 2H amplifier has limited power.  This method can be used to observe 2H labelled compounds at high concentration where sensitivity is not an issue. The high sensitivity option uses the higher power  (300 W in my case) broadband amplifier, the lock preamplifier and the broadband coil of the probe tuned to 2H.  This method requires a bit of re-cabling and re-configuration but has a large sensitivity advantage.  It is suitable for cases where the deuterium is in low concentrations where sensitivity is an issue, for example to observe 2H at natural abundance or very low concentrations of 2H labelled compounds.  The figure below shows an example of both cases on a 500 MHz AVANCE spectrometer using a triple resonance (BB, 1H, 31P) probe.  The sample is neat tap water where the 2H is at natural abundance (0.015 %).  The spectra were collected with 90° pulses, 2 sec recycle delays, 1.8 sec acquisition times and 128 scans.  The pulse programs used were zg2h and zg for the low and high sensitivity cases, respectively.


For this NMR probe, there is a 26 times gain in signal-to-noise ratio between the two methods.  This will depend strongly on the type of NMR probe used as the filling factor of the lock coil compared to the broadband coil must be taken into account.  

10 comments:

Mike said...

Can I assume there was no tuning/matching involved when using the lock coil?

How did you tune the BB coil to 2H - via the 2H preamp or with some external setup?

Glenn Facey said...

Mike,

Thank you for your questions.
1. I did not change the tuning or matching capacitors for the lock channel accessed on the side of the probe. The lock channel of the probe was tuned several months ago using a CDCL3 sample.
2. Once the spectrometer was set up correctly (using cf or edasp) and cabled properly, I was able to simply use the wobb command to tune the BB channel to 2H via the lock preamplifier.

Glenn

Anonymous said...

Hi Glenn,

do you observe any improvement from using the lock preamplifier instead of the BB preamp?

best regards

dani

Glenn Facey said...

Hi Dani,

Thank you for the question. On our AVANCE spectrometer, the BB preamp has a built in 2H stop filter preventing us from using it for 2H observation. The purpose of the filter is to reject any signal from the lock channel.

Glenn

ohytos said...

Hi Glenn,
thank you for an interesting post.
However, when I tried this using our HPPR/2 2H lock preamp I found that it was impossible to get a good signal with the simple zg. I either had to use zg2h or to add LOCKDECK_ON and 2H_PULSE to the zg to enable 2H-switch in the lock preamp and then switch it to pulse mode.
Andrew

Glenn Facey said...

Ohytos,

I am not sure why you would have to resort to anything more complicated than a simple "zg" sequence. I made sure the routing was correct via "edasp" (I had to do a "cf" to force the change). I turned the lock nucleus off, turned the lock power to the minimum and ran unlocked.

Glenn

Pete said...

Hi Glen,

You should try using the lock coil again, but use pulse program zg2h - you might find the S:N increases somewhat! If you just use zg pulseprogram, the switch in the 2HTX is not switched and you get very low pulse power (some power gets through hence you do see a signal).

You will still get better S:N via the BB channel, but the difference may not be so large that you think it's worth bothering anymore.

Glenn Facey said...

Dear Pete,
Thank you for your comment. I did use the zg2h pulse program for the spectrum using the 2H TX board and the lock coil.

Glenn

Anonymous said...

Hi Glenn,
I was surprised the difference between the lock coil and the BB coil was so much, that's all. I guess I was also thinking your probe was TBI rather than TBO, which would have pushed things more in the direction of the lock channel being favourable.

For my SmartProbe I only see a factor of 4 difference, but I guess the fixed 31P channel of your probe is on the outer coil as well as the 2H, so I guess the 2H tuning circuit is less efficient?

Pete

Glenn Facey said...

Pete,

I think the large difference in sensitivity I see is due not only to the difference between the outermost coil vs the innermost coil of the TBO probe but also in using the 20W lock amplifier vs the 300W BB amplifier.

Glenn