Polarization transfer methods for quantitative analysis of flowing mixtures with benchtop 13C NMR spectroscopy

Abstract

Benchtop NMR spectroscopy is attractive for process monitoring; however, there are still drawbacks that often hamper its use, namely, the comparatively low spectral resolution in ¹H NMR, as well as the low signal intensities and problems with the premagnetization of flowing samples in ¹³C NMR. We show here that all these problems can be overcome by using ¹H-¹³C polarization transfer methods. Two ternary test mixtures (one with overlapping peaks in the ¹H NMR spectrum and one with well-separated peaks, which was used as a reference) were studied with a 1 T benchtop NMR spectrometer using the polarization transfer sequence PENDANT (polarization enhancement that is nurtured during attached nucleus testing). The mixtures were analyzed quantitatively in stationary as well as in flow experiments by PENDANT enhanced ¹³C NMR experiments, and the results were compared with those from the gravimetric sample preparation and from standard ¹H and ¹³C NMR spectroscopy. Furthermore, as a proxy for a process monitoring application, continuous dilution experiments were carried out, and the composition of the mixture was monitored in a flow setup by ¹³C NMR benchtop spectroscopy with PENDANT. The results demonstrate the high potential of polarization transfer methods for applications in quantitative process analysis with benchtop NMR instruments, in particular with flowing samples.