Single-Sided NMR in Foods

Abstract

A relatively new approach in NMR is the relaxing of restrictions on the magnetic field homogeneity for the static field B0 as well as for the radio frequency field B1, known as single-sided or unilateral NMR. This realization of an NMR device implies cost reduction of the equipment as well as the possibility to non-destructively apply NMR techniques to samples larger than the probe. However, this approach has also physical consequences. The magnetic field gradients are in an order of magnitude where almost every pulse becomes selective thus leading to spatial selectivity which is given by the Larmor frequency and the bandwidth of the tank circuit. In addition, these single-sided NMR devices have an inherent sensitivity to diffusive or convective processes as the field gradients are large. For example, the device used for the measurements presented here exhibits B0 field gradients in the order of 10–20 T/m, leading to an full width at half maximum, FWHM slice thickness of about 2 mm with a quality factor of the circuit of about 70. TheB0(r) can be constructed such that shape and size of the sensitive volume corresponds to the sample to be measured and is therefore to be tailored to the desired properties. In case of homogeneous food samples, the sensitive volume should be chosen as large as possible, whereas small and sharp volumes are preferred in examinations of local properties. Examples are the investigations of skin cancer and of fat content in living salmon [1].