Wood is a hygroscopic material that can reach an equilibrium moisture content when ambient temperature and relative humidity are constant. Moisture affects all properties of wood, as well as its preservative treatment. The hygroscopic behavior of wood can be attributed to the hydroxyl groups of its constituents. Since hemicellulose shows the greatest water affinity, it can be considered the main responsible for the ingress of water into the wood mass. Below the fiber saturation point, wood moisture is only stored in the cell walls. Proton Nuclear Magnetic Resonance (NMR) is a relative method used for the evaluation of moisture content distribution in wood and NMR relaxation is an excellent tool to study the hygroscopic behavior of different woods below the fiber saturation point. This work aimed to test the hypothesis of discriminating among softwoods and hardwoods of different botanical species and identifying further sub-clusters of woods based on the NMR proton spin–spin (T2) and spin–lattice (T1) relaxation times of their cell wall water in the hygroscopic moisture range. Importantly, the study was performed using a portable low-cost NMR instrument with which it is possible to investigate wood samples of any size. The main result of this study was that at RH = 94% the relaxation time T2,2, associated with the cell wall bound water, can be used as a marker to discriminate among softwoods and hardwoods. Graphical abstract

Wood is a hygroscopic material that can reach an equilibrium moisture content when ambient temperature and relative humidity are constant. Moisture affects all properties of wood, as well as its preservative treatment. The hygroscopic behavior of wood can be attributed to the hydroxyl groups of its constituents. Since hemicellulose shows the greatest water affinity, it can be considered the main responsible for water entering into the wood mass. Below the fiber saturation point (FSP), wood moisture is only stored in the cell walls. Proton Nuclear Magnetic Resonance (NMR) is a relative method used for the evaluation of moisture content distribution in wood and NMR relaxation is an excellent tool to study the hygroscopic behavior of different woods below the fiber saturation point. This work aimed to test the hypothesis of discriminating among softwoods and hardwoods of different botanical species and identifying further sub-clusters of woods on the basis of the NMR proton spin-spin (T2) and spin-lattice (T1) relaxation times of their cell wall water in the hygroscopic moisture range. Towards this goal, a portable low-field NMR instrument was used to develop a specific non-invasive NMR protocol for in situ investigation useful in the wood industry and/or cultural heritage applications.