Comparison of Overhauser DNP at 0.34 and 3.4 T with Frémy’s Salt

M.-T Türke,M Bennati

doi:10.1007/s00723-012-0362-5

Abstract

Dynamic nuclear polarization (DNP) is investigated in the liquid state using a model system of Frémy’s salt dissolved in water. Nuclear magnetic resonance signal enhancements at 0.34 and 3.4 T of the bulk water protons are recorded as a function of the irradiation time and the polarizer concentration. The build-up rates are consistent with the T 1n of the observed water protons at room temperature (for 9 GHz/0.34 T) and for about 50 ± 10 °C at 94 GHz/3.4 T. At 94 GHz/3.4 T, we observe in our setup a maximal enhancement of −50 at 25 mM polarizer concentration. The use of Frémy’s salt allows the determination of the saturation factors at 94 GHz by pulsed ELDOR experiments. The results are well consistent with the Overhauser DNP mechanism and indicate that higher enhancements at this intermediate frequency require higher sample temperatures.

Highlights

Dynamic nuclear polarization (DNP) of the Overhauser type (OE-DNP) [1, 2] can provide a valuable tool to enhance the sensitivity of nuclear magnetic resonance (NMR) experiments in the liquid state [3,4,5].A polarizer molecule carrying an unpaired electron spin is inserted into the liquid-state NMR sample under investigation
Nuclear magnetic resonance signal enhancements at 0.34 and 3.4 T of the bulk water protons are recorded as a function of the irradiation time and the polarizer concentration
2 Overhauser DNP in Liquids In Overhauser DNP, the enhancement of the NMR signal e is derived from the steady-state solution, i.e. dhIzi=dt 1⁄4 0, of the Solomon equations [17], where the nuclear polarization is described as: dhIziðtÞ dt

Summary

Introduction

Dynamic nuclear polarization (DNP) of the Overhauser type (OE-DNP) [1, 2] can provide a valuable tool to enhance the sensitivity of nuclear magnetic resonance (NMR) experiments in the liquid state [3,4,5]. In recent years, detailed mechanistic studies have provided insight into the specific parameters needed to achieve optimal enhancements of liquid-state NMR signals, such as polarizer type, polarizer concentration, microwave power or the magnetic field at which the polarization step is carried out [6,7,8,9,10,11]. The results allow us to rationalize quantitatively our enhancements at 94 GHz in terms of the Overhauser mechanism

Overhauser DNP in Liquids

Experimental

Results and Discussion

DNP Build-up