Comparison of 1H NMR Spectra Obtained at 42, 60, 82, and 300 MHz – Fish Oil Omega-3 Ethyl Ester Supplement Example

Process NMR Associates is currently developing NMR applications based on direct measurement of chemometric modeling on NMR data obtained on numerous NMR platforms. Our intention is to develop solutions that can be executed on any NMR platform. With this in mind we are currently developing a fish oil analysis application that can provide the EPA and DHA omega-3 fatty acid content of fish oil supplements manufactured by an ethyl ester esterification process. We have obtained data at 42 MHz, 60 MHz, 82 MHz and 300 MHz. The chemometric modelling yielded PLS models for all 4 field strengths that yield effectively the same result – DHA can be measured to +/- ~1.1 wt% and EPA can be measured to +/- ~2.2 wt% by a 40 second 1H NMR measurement. THe correlation is derived from a  regression of the 1H NMR variability with primary GC analysis values.

This analysis has been shown for the 300 and 60 MHz data in a previous post on this blog. The same analysis was also obtained, with similar results, on 42 and 82 MHz platforms proving that individual applications can be automated and provided at all relevant frequencies of NMR analysis whether on superconducting lab systems or permanent magnet benchtop systems.

At each field strength the relative lineshapes are pretty much the same (<1 Hz at half height). The field strength differences mean that the same spectrum is dispersed across frequency space proportionate to the magnetic field. Figure 1 below shows the frequency space spectra obtained at all 4 field strengths on the same sample.

Figure 2 shows the same spectra displayed on the usual normalized chemical shift scale (ppm). In these spectra the data is stretched in order to allow the chemical shift comparison of the data. IN effect the 42 MHz NMR is stretched by a factor of 7, the 60 MHz data by a factor of 5 and the 80.2 MHz data by a factor of 3.7. The effect of the relative size of J coupling compared to the frequency space occupied by 1 ppm is an interesting observation to see directly. In traditional NMR analysis the resolution of various peaks was always the driving force for increasing the magnetic field strength of NMR instruments. With todays powerful PC’s and advanced software information can be garnished readily from any of these spectra by way of global spectral deconvolution or multivariate statistics.It is no longer necessary to obtain baseline resolution in order to integrate a resonance and obtain quantitative information.

THough the data is more closely packed together in the low field spectra it can be acknowledged that the same information is present in all 4 spectra. Automation approaches can be developed that will allow accurate measurement of quality parameters, component quantification, or structural verification to be performed on data obtained from any of these NMR systems.

The development of readily deployable NMR benchtop systems at an affordable price point must surely lead to the development of NMR into a more widely utilized technique outside of the realms of scientific study and quality control that NMR has thus far been involved.

1H NMR at 42, 60, 82, and 300 MHz

Figure 1: Comparison of 1H NMR spectra obtained on an omega-3 fish oil supplement obtained at 42, 60, 82, and 300 MHz displayed in frequency space. These spectra show the native spectra dispersion obtained at the different magnetic field strengths of the NMR spectrometers

CHemical SHift Scale Normalization of 1H NMR spectra obtained on the same sample at 42.5, 60.3, 82.3, and 300 MHz

Chemical Shift Scale Normalization (ppm) of 1H NMR spectra obtained on the same sample at 42.5, 60.3, 82.3, and 300 MHz. In these spectra the absolute frequency of the spectrum is divided by the spectrometer frequency to obtain a normalized spectrum where the chemical shift in ppm of the various peaks in the spectrum are observed at the same position on the ppm NMR scale. In this normalized view the relative proportion of a ppm that the J couplings represent in the 1H multiplets can be plainly seen.