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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.