Characterization of water distribution in cut tobacco using a low-field 1H NMR technique
Cut tobacco was studied by low-field 1H NMR at various water contents and temperatures. The spin-spin relaxation times (T2) were measured with Carr-Purcell-Meiboom-Gill (CPMG) sequences. Three populations were observed in cut tobacco, namely, population T21, with relaxation times in the range of 0.1-1ms; population T22, with relaxation times in the range of 1-10ms; and population T23, with relaxation times in the range of 10-100ms. The population T21 was referred to free water which was significantly affected by the moisture content in cut tobacco. An increase in the relaxation time of this population was observed when water content was increased. The intensity of T21 evidently increased with the increase of water content from 8% to 18% w/w. The population T22 was referred to bound water. The intensity of T22 was relatively stable, however, its ratio to total water decreased with the increase of water content in tobacco. The population T23 was an intermediate state of water which was less sensitive to the moisture content in tobacco. In addition, the spin-spin relaxation times (T2) were also measured at -15.8 °C, -6.8 °C, 0 °C and 32 °C. Results showed that the intensities of T22 and T23 were relatively stable with temperature, as opposed to the population T21 which intensity sharply decreased with the temperature decreased from 32 °C to -15.8 °C.