Markus Cudaj

SEC-MR-NMR: Online Coupling of Size Exclusion Chromatography and Medium Resolution NMR Spectroscopy

Online coupling of size exclusion chromatography together with medium resolution nuclear magnetic resonance (SEC-MR-NMR) might be one solution to the problem of chemically sensitive detection in liquid polymer chromatography. By use of a combination of SEC with a table-top, specially designed 20 MHz NMR spectrometer, based on a permanent magnet, online (1) H NMR spectra of SEC fractions can be obtained. The integration of digital filters, mechanical shims and electronic shims led to substantially improved sensitivity and chemical selectivity compared to former TD (time domain) 20 MHz instruments. (1) H NMR spectra of PMMA and PS homopolymers as well as PS-PMMA block copolymers were of sufficient quality to enable detection and de-formulation of unknown polymer compounds. (1) H NMR spectra of acceptable resolution and S/N ratio were collected online during the chromatography. The SEC separation online with the NMR measurements performed well and resulted in the proof of principle of the SEC-MR-NMR combination.

Medium Resolution NMR at 20 MHz: Possibilities and challenges

Detailed studies of compositions, for example in food and oil industries 1-4 , are of great importance to guarantee a high and constant quality and well defined properties of a product. Many applications still need expensive analytical techniques like high resolution NMR (Nuclear Magnetic Resonance) to determine each component‟s concentration like the marginal amounts of acids, colorants or polyphenols 5 in wine analysis 6 or small amounts of special fatty acids in virgin olive oils 7 . Moreover high field NMR delivers valuable information for characterizing the botanical origin or for investigating the age of a natural product like vinegar 8,9 . Up to now, low field NMR instruments in the range of 1 – 60 MHz are almost exclusively applied in relaxometry studies 10,11 , which proved to be a powerful method, for example, to characterize the movement and redistribution of water within soils on the basis of T2 relaxation times of the water proton signal. However, there have been very few investigations on the ability of low field instruments delivering spectral information, concerning chemical shifts and J-coupling of the studied sample. In contrast to relaxometry methods for determination of water and oil content 12-17 , prerequisite for frequency resolved NMR proton spectra is a sufficient spectral resolution of the NMR spectrometer in the range of 0.2 ppm full width half maximum (FWHM) 18 and frequency or B0 field stability. The general feasibility of NMR spectroscopy with adequate resolution at low magnetic fields have already been shown by different groups 19-22 . The technique is named medium resolution NMR (MR-NMR) in the following. Apart from composition determination via MR-NMR, processes like fermentation, ripening or encapsulation of active agents are of interest. Reactant concentrations can principally be followed as a function of time allowing the study of reaction kinetics. Moreover, a question which has to be addressed is the way how raw data are processed; the possibilities include integration or the statistical chemometric approach. Whether or not the limited spectral resolution and sensitivity at low magnetic fields can be “compensated” more effectively by “classical” data processing (integration and subsequent linear regression) or multivariate data analysis (chemometrics) is studied on two models. The First published in: Magnetic Resonance in Food Science. An Exciting Future. Ed.: J.-P. Renou. 2011. ISBN 978-1-84973-233-8