External Field Strength, Resolution and Sensitivity

As we’ve previously discussed, the development of NMR technology has largely been geared towards improving magnetic field strength.  The original electromagnet spectrometers (i.e., iron core wound with copper wire) were found to have a maximum field strength of 2.35 T (100 MHz for 1H),[i] which was largely insufficient for the structural elucidation of some complex molecules (e.g., carbohydrates, saccharides etc.).  This spawned the era of superconducting spectrometer technology and the development of high field instruments that has enabled unparalleled molecular characterization.

Why?

1) The first, and immediately obvious answer is RESOLUTION! Not ‘resolution’ in the sense of the peak width at half height, of course, but in the more general sense – the separation of resonances.

Chemical shifts are typically reported in parts per million (ppm) so that spectra taken at different fields can be compared.  That is chemical shifts in ppm (δ) are independent of the field strength the spectrum was taken at.  Chemical shifts in Hz (υ), however, (from which the chemical shifts in ppm are generated) are directly correlated to external field strength.

These values can be easily interconverted:

If you picture a typical 1H spectrum it ranges from 0 – 10 ppm.  (This is, of course, a simplification as there are obvious exceptions in both the downfield (e.g.,aldehydes) and the upfield regions (e.g., metal hydrides)).  Regardless, for a typical 10 ppm spectrum – on a 60 MHz spectrometer this means that the chemical shifts are spread out over a 600 Hz spectral window.   This sounds more than respectable – until you realize that on a 400 MHz instrument this translates to a 4000 Hz spectral window.

e.g., iso-propanol ((CH3)2CHOH) will have three resonances)

There is a noticeable difference in the spread of the chemical shifts at higher field, so these will be better separated.  Greater separation becomes increasingly important with more complex molecules, especially if they are greatly coupled.  Generally a spectrum is considered ‘first order’ (i.e., it can be solved upon inspection) if the difference in chemical shift in Hz between two peaks (A and B) is bigger than four times the coupling constant between them (∆υAB > 4JAB).[ii] The smaller this separation is, the less chance you have of being able to fully interpret your spectrum with visual inspection.

2) The second, and probably less obvious, answer is SENSITIVITY.

In an external magnetic field the nuclear spins of proton will either align with the magnetic field (+1/2, α) or against it (-1/2, β) such that there will be a slight thermodynamic preference for the lower energy state and a Boltzmann Distribution will observed (i.e., Nα > Nβ).  The energy differences between the two states (∆E) is governed by the equation:where h = Planck’s constant; γ = magnetogyric ratio; B0 =  external magnetic field strength

As B0 is the only variable in the equation, this alone governs the energy separation between.  The greater the energy difference between the two states, the greater the thermodynamic preference for nuclear spins to align with the magnetic field and the more sensitive the NMR experiment is.

So what does this mean?

60 MHz instruments have less resolution and sensitivity then high field instruments inherently.  However, that does not mean that they cannot provide useful gaining information!!  For really complex molecules, there is no replacement for a high field instrument….resolution is fixed by field strength.  Regardless, as shown in the case of isopropanol, for basic organic characterization, the more robust and accessible 60 MHz benchtop spectrometer has more than sufficient resolution for structural elucidation, reaction monitoring, QA/QC!!!

Reduced sensitivity is easier to work with – this is why lower field instruments recommend running samples at slightly higher concentration then you may be used to!

If you’re curious as to whether or not this will work for your application – please see Will 60 MHz work for you?

[i] Jacobsen, N. W. “NMR Spectroscopy Explained: Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology” Wiley-Interscience: USA, 2007, Chapter 2, pg 40-45

[ii] Reich, H. J.; University of Wisconsin, 2010, http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-10-ax-ab.htm [Viewed June 18th, 2013]