To Spin or not to Spin

That is one of the most common questions we get here at Nanalysis.

People who are familiar with acquiring spectra on high-field NMR spectrometers are used to spinning their sample before locking, shimming and data acquisition.  Why?  Well, simply put, spinning ‘averages’ out the sample.  What do I mean?  I mean it’s a simple way of making a magnetic field seem more homogenous than it actually is.  Well, more homogenous on the two axes where you are not spinning (the off-axes – x and y).  Magnetic field inhomogenities are not evened out along the axis that the tube is being spun (the on-axis – z).  This is why when shimming on your sample you typically only shim along the on-axis (z) coordinates.

Bloch discovered this phenomenon in the early days of NMR.  At this time spinning a sample did give substantial improvements in observed line widths.  However, as the NMR technique developed and diversified, magnetic fields became inherently more homogenous and spinning therefore less important.  Nowadays, although slight resolution improvements are gained for small molecules, there is no appreciable resolution improvements observed for spinning samples of large molecules.

Even for small molecules data is often acquired without spinning.  Overnight data acquisitions, for example, where both 1-D and 2-D data is being collected (the typical 1H, 13C, COSY, HSQC type sequence), the spin is left off.  Why?  Well, the resolution loss for the 1-D spectra is negligible and it is found to be detrimental to 2-D data.  For 2D spin is found to increase t1 noise and disrupt the natural equilibriums we are trying to study.

The NMReady, like other benchtop systems, does not spin the sample.  Although we may see small improvements in our line widths by spinning, the reasons our spectrometers were not engineered with spin are threefold:

1)   Spinning requires moving parts.  Moving parts are more likely to break than stationary parts.  We envisaged an NMR system that was robust and portable.
2)   The use of electrical spinning methods creates signal modulation and noise in a spectrum that can affect the integrity and repeatability of integral data.
3)   So, if we wanted to spin samples we would require either accessible on-site air or a separate box capable of generating air.  As we wanted to manufacture a portable, easy-to-site NMR, this did not fit our mandate.