Solid-liquid nuclear magnetic resonance relaxation and signal amplitude relationships with ranking of seasoned softwoods and hardwoods

Abstract

In H1 NMR (nuclear magnetic resonance) relaxation measurements for a set of eight hardwood and softwood samples, each free induction decay (FID) is fitted by the sum of a “solid” signal of the form Aexp[−c(t∕TS)2][1−g(t∕TS)2+h(t∕TS)4] plus a “liquid” signal Bexp(−t∕T2-FID). Distributions of longitudinal (T1) relaxation times were computed separately for the solid and liquid components, giving also the solid/liquid H1 ratio α. From measurements on the samples dried, seasoned, and hydrated, the moisture content (liquid/solid weight ratio) was found to be approximately 0.50∕α. For each of the “seasoned” samples (10%–13% moisture content) a single T1 peak was found for the solid and two for the liquid, with the longer liquid T1 close to that of the solid, but with some differences exceeding perceived experimental uncertainties. None of the solid or liquid-long T1’s is much less than 20ms, even though liquid-short times go as low as 0.35ms, appearing to negate simple solid-to-liquid exchange on a millisecond time scale. Data for six of the samples (all except for two resin-containing pine species) can be formally fitted by a two-site exchange model, in which cases the solid-to-liquid exchange times are a few tens of milliseconds. For our set of wood samples, each of the above three T1 values, and also the overall liquid geometric-mean and rate-average T1’s, as well as the liquid long-T1 fraction, for a seasoned hardwood is longer than the corresponding value for any softwood, suggesting that relaxation parameters may provide a useful ranking of seasoned woods.