Spinsolve 80 Phosphorus – Limit of Detection

The best-selling Magritek 80 MHz Spinsolve benchtop NMR is also available with the X-channel set to 31-Phosphorus.  31P NMR spectroscopy is routinely used by chemists to determine structure and measure impurities.  When looking for impurities it is important to know the lower limit of detection (LOD). The LOD is the lowest concentration of a molecule that can be distinguished from the absence  of that molecule.

In NMR it is the sensitivity that determines the  LOD for a particular substance, and the higher magnetic field of an 80 MHz magnet brings a number of advantages including increased sensitivity.  We thought it would be interesting to determine the LOD for tetramethylphosphonium chloride with different acquisition times.  We defined the LOD as an NMR peak with signal height that was 3 times the noise level, i.e. an SNR of 3.

In the graph above you can see a series of 31P NMR measurements made on tetramethylphosphonium chloride with different concentrations. You can clearly see that the peak height is directly proportional to the concentration and even below 8 mM concentration we can clearly detect this peak.  For a 5 minute NMR measurement (64 scans) we determined our LOD was 3.9 mM. If we increase the acquisition time to 20 minutes (256 scans) then the LOD is just 1.9 mM.  It is worth noticing also that increasing the acquisition time further has less benefit as the sensitivity only scales with the square root of the number of scans, so an acquisition time of 40 minutes has an LOD of 1.5 mM.

These results are summarised in the table below

Measurement time (mins) # Scans LOD (mM)
42.6 512 1.5
21.3 256 1.9
5.3 64 3.9