University of Ottawa NMR Facility Web Site

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Wednesday, April 16, 2008

How Much Sample Do I Need to Get a Solid State MAS NMR Spectrum?

Synthetic chemists frequently ask how much sample they need in order to get a solid state magic angle spinning spectrum. The answer is not simple and of course depends on what nucleus is to be observed, the field strength, what type of NMR experiment is needed and at what spinning speed the rotor must be set. The signal-to-noise ratio is directly proportional to the quantity of sample, so in principle, the larger the rotor, the higher the signal-to-noise ratio. This would be true if all rotors were able to spin at the rate needed for the experiment. Unfortunately, this is not the case and we are often limited by how fast we can spin the sample. The smaller the rotor - the faster the maximum available spinning speed. Often, one wants to maximize the quantity of sample (large rotors) yet employ the fastest possible spinning speeds (small rotors). A compromise must be made. The picture below (courtesy of Victor Terskikh) nicely shows the sizes of the common Bruker MAS rotors and gives an idea of the volumes of solid needed to fill the rotors.
From left to right the diameters are 7 mm, 4 mm, 3.2 mm, 2.5 mm and 1.3 mm. All of these rotor sizes are available at the National Ultrahigh Field NMR Facility for Solids. the maximum spinning speeds from left to right are 8 kHz, 18 kHz, 23 kHz, 35 kHz and 70 kHz, respectively. At the NMR Facility on the Ottawa U campus, the 7 mm, 4 mm and 2.5 mm sizes are available with maximum spinning speeds of 7 kHz, 15 kHz and 30 kHz, respectively.

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