Abstract
Low field (LF) NMR spectroscopy (sometimes also called time-domain NMR) operates in the frequency range of 2–25 MHz. Being a bench-top instrument, it represents a simplified and cheaper version of a traditional NMR spectrometer. Nevertheless, the instrument can provide useful information about relaxation behavior and diffusion behavior. Moreover, the instrument can also be used in connection with at-line quality control for quick analyses of fat, water and protein. Unlike conventional chemicalphysical analytical methods that may be time consuming and thus costly, the NMR technique is rapid and has low operation costs. Often, no sample preparation before analysis is needed, and chemicals that are potentially harmful to health and environment may be avoided. Unlike conventional NMR spectrometers equipped with strong superconductive magnets, LF NMR instruments have a relatively weak permanent magnet. Consequently, the instrument cannot resolve different spectral components in the frequency domain, i.e. the LF NMR technique can only deal with time-domain information. Data are collected by programming and running specific NMR pulse sequences such as free induction decay (FID) [1], Hahnecho [1], Carr-Purcell-Meiboom-Gill (CPMG) [2,3], and solid-echo [4]. The choise of NMR pulse sequence depends on the desired type of information and on the physical and chemical properties of the sample. Presence of solid and liquid phases, their mobility, and rigidity are among the most important parameters to be taken into consideration. Assessment of proton relaxation behavior is a frequently used application of the LF NMR technique. Two types of relaxations can be identified, longitudinal (spin-lattice or T 1) relaxation and transversal (spin-spin or T 2) relaxation. The longitudinal relaxation is the re-establishment of nuclear magnetization along the main magnetic field direction after the system has been excited from energetic equilibrium by irradiation of radio frequency (RF) energy. The transversal relaxation is observed as the loss of net magnetization in the plane transversal to the main magnetic field direction and is a result of the loss of the phase coherence within the ensemble of nuclei in time. The transversal relaxation times are more rapid to measure than the longitudinal ones. Typical data acquisition times for pure aqueous solutions are about 3 and 40 min, respectively. Due to this, and because transversal relaxation data may contain more information, sometimes only transversal relaxation data are reported. For instance, transverse relaxation may be preferred since it is more sensitive to protein unfolding (denaturation) than longitudinal relaxation [5].
