FFC NMR relaxometry is more commonly used to investigate molecular dynamics by studying the relaxation rate of the proton nucleus (1H) in materials. Frequently the different types of water molecules in a given material or substance can be investigated. It is indeed an important complement to the conventional 1H-detected FFC approach, to be able to investigate content and /or to characterize compounds which include other important NMR-sensitive nuclei, or so-called “heteronuclei”.
A digital receiver with direct detection from 500 kHz to 90 MHz permits acquisition of some important heteronuclei, including deuteron (2H), carbon (13C), fluorine (19F), and lithium (7Li), thus allowing the exploration of the field dependence of the spin-lattice relaxation time constant 1/T1 down to low Larmor frequencies (10 kHz) where other conventional NMR experiments present severe signal-to-noise ratio degradation.
The possibility to perform this kind of multi-nuclear analysis extends the potential of FFC NMR relaxometry and may prove important for characterization of certain materials or substances and to unlock molecular dynamics information of other key nuclei.
Measurement of deuterium relaxation rates could be important in understanding the amount of deuterium enrichment and exchange processes between deuterium nuclei and protons, which are often important for chemical mechanism studies.
The figure shows heavy water (D2O) relaxation rates in fuel cell membranes, which were measured at three different field strengths (15.3 kHz, 153 kHz and 3 MHz): the deuteron T1 decay acquired on the sample shows an evident multi-exponentiality in particular at low field. The relaxation data at the three different fields were evaluated with discrete and continuous methods, using a two component traditional multi-exponential fitting as well as by means of a Laplace inversion algorithm (Upen algorithm) in order to evaluate the curve of distribution of T1.
Thanks to Advanced Industrial Science and Technology (AIST), Japan, for providing the samples for this study.
Molecular dynamics information on the 13C nucleus could be particularly important in studies of biomolecules, such as proteins, as well as for some carbon-containing synthetic materials.
The figures demonstrate the capacity of FFC to investigate the relaxometric behavior the 13C nucleus at low magnetic field strengths:
a) A 13C NMRD profile of a solution of the organic molecule, urea, in water.
b) The spin-lattice relaxation rate of a 13C-enriched sample of a carboxylic acid was measured by acquiring a 13C NMR signal at the magnetic field strength of 2.35 mT (equivalent to 0.1 1H MHz) and at the temperature of -120 °C.
Lithium (7Li) and Fluorine (19F)
Lithium is an important component of batteries in electronics industry. The fluorine nucleus is often found as part of the organic counter-ion of lithium-based electrolytes for batteries. The possibility to study the relaxation rates of these important nuclei could aid the studies for new battery electrolyte and electrode materials.
The figure shows proton (1H, in black), lithium (7Li, in blue) and fluorine (19F, in red) NMRD profiles belonging to the same sample of an electrolyte solution for a battery system.