The states of water in Norway spruce (Picea abies (L.) Karst.) studied by low-field nuclear magnetic resonance (LFNMR) relaxometry: assignment of free-water populations based on quantitative wood anatomy

Abstract

The states of water in Norway spruce (Picea abies (L.) Karst.) studied by low-field nuclear magnetic resonance (LFNMR) relaxometry: assignment of free-water populations based on quantitative wood anatomy DOI 10.1515/hf-2016-0044 Received February 23, 2016 accepted August 15, 2016; previously published online September 22, 2016 Abstract: Low-field nuclear magnetic resonance (LFNMR) relaxometry was applied to determine the spin-spin relaxation time (T 2 ) of water-saturated Norway spruce (Picea abies (L.) Karst.) specimens cut from mature sapwood (sW) and mature and juvenile heartwood (hW), where earlywood (EW) and latewood (LW) were separated. In combination with quantitative wood anatomy data focusing on the void volumes in various morphological regions, the NMR data served for a more reliable assignment of free-water populations found in water-saturated solid wood. Two free-water populations were identified within most sample types. One was assigned to water in the tracheid lumen and the other to water inside bordered pits. Whether water in the ray cell lumina was included in one or the other of these two populations depends on the curve-fit method applied (continuous or discrete). In addition, T 2 differences between the different tissue types were studied and, for comparison, sorption isotherms were measured by means of a sorption balance. There was a significant difference between EW and LW as well as between juvenile wood and mature wood in terms of T 2 related to the cell wall water. However, no differences were seen between the sorption isotherms, which indicates that the observed T 2 differences were not due to differences in cell wall moisture content (MC).

Experimental techniques for characterising water in wood covering the range from dry to fully water-saturated

“…The decreasing sorption hysteresis at high RH might, however, be an artefact due to desorption being initiated from a non-saturated state (often by conditioning to 95% RH). Thus, in recent results by Fredriksson and co-workers (Fredriksson and Johansson 2016;Fredriksson and Thygesen 2017), sorption hysteresis in untreated Norway spruce gradually increases up to at least 95% RH (see Fig. 2).…”

“…As automated sorption balances are becoming a common research tool in wood research laboratories, the literature is abundant with sorption isotherms determined with such equipment, also for various types of modified wood Jalaludin et al 2010a, b;Popescu and Hill 2013;Xie et al 2010Xie et al , 2011. While the low sample mass makes possible detailed studies of differences in water sorption between earlywood and latewood (Fredriksson and Thygesen 2017), even at the growth ring level (Hill et al 2015;Song et al 2014), it also requires the user to consider how to collect representative samples for the study in question. For generating representative samples of larger wood volumes, several studies have used milled wood Himmel and Mai 2015;Xie et al 2010;Zaihan et al 2009Zaihan et al , 2011, but it could be speculated if this gives a representative material or might produce unwanted side effects, for example a change in sample chemistry.…”

“…lignin-rich middle lamella fragments) are over-represented in smaller size fractions that are more easily lost upon handling the milled material. Instead of milling, several studies have used wood slices cut with a microtome or razor blade to produce a large sample surface area while preserving the microscopic wood structure (Fredriksson and Thygesen 2017;Fredriksson et al 2010;Glass et al 2017;Hill et al 2015;Hosseinpourpia et al 2016;Song et al 2014), while a few use small wood blocks (Fredriksson and Johansson 2016). All of these studies use the automated humidity control by which RH is automatically changed upon reaching a specified mass stability (dm/dt) criterion.…”

“…Note that the desorption and absorption isotherms do not coalesce at 97% RH; b absolute sorption hysteresis of untreated wood from moisture content data acquired with automated sorption balances. Based on data for Norway spruce (Fredriksson and Johansson 2016;Fredriksson and Thygesen 2017;Källbom 2015;Sedighi-Gilani and Schwarze 2015) and Sitka spruce ). Note that in all studies except (Fredriksson and Johansson 2016;Fredriksson and Thygesen 2017) Besides deriving MC in equilibrium with various climates, automated sorption balances have been used for studying the kinetics of sorption.…”

“…Based on data for Norway spruce (Fredriksson and Johansson 2016;Fredriksson and Thygesen 2017;Källbom 2015;Sedighi-Gilani and Schwarze 2015) and Sitka spruce ). Note that in all studies except (Fredriksson and Johansson 2016;Fredriksson and Thygesen 2017) Besides deriving MC in equilibrium with various climates, automated sorption balances have been used for studying the kinetics of sorption. In many polymeric materials, the sorption process exhibits two-stage behaviour (Christensen and Kelsey 1959;Crank 1953;Kelly and Hart 1970;Newns 1956) and cannot be explained by Fickian diffusion into the material.…”