C.L. Bray 1, S. Iannopollo 1, G. Ferrante 3, N.C. Schaller 2, D.Y. Lee 1, J.P. Hornak 1 1 Magnetic Resonance Laboratory and 2 Computer Science Department, RIT, Rochester, NY USA 3 Stelar s.r.l, Mede (PV) – 27035, Italy Motivation The relationship between pore size and the NMR spin-lattice relaxation rate (R 1 ) has been the focus of many studies. 1,2 We are interested in the relationship between the particle size distribution in hydrated soils and the proton R 1 of the water within voids between the particles. An earlier study displayed biexponential R 1 behavior in some natural soil samples. 3 This NMR study examines the R 1 distribution of water in hydrated randomly close packed 4 synthetic soils as a function of particle diameter (d) and magnetic field strength. Conclusions The results of this study suggest that prediction of particle size in simple monodispersed systems can be made based on R 1 and perhaps field strength. Bi-modal R 1 recoveries seen in prior work with real soil samples 3 may be attributed to sample packing inhomogeneities and not because of slow exchange between the structured and bulk water environments. Future studies are planned to examine the relationship between the packing geometry, 4 void size, and R 1. Results & Discussion A mono-modal R 1 distribution was obtained for all particle diameters and field strengths. At 300 MHz, where the largest range of d values were studied and the most repetitions performed, R 1 values increased with decreasing particle diameter from the bulk water value at large particle diameters to a value of 1.36 s -1 at the smallest diameter. (See Fig. 1.) A similar trend was seen at 10, 1, and 0.01 MHz. These results are consistent with a fast exchange between structured and bulk water environments where the structured to bulk water fraction increases as particle diameter decreases. R 1 values increase with decreasing field strength as expected. References 1.G. Liu, et al., Chem. Phys. 149:165 (1990). 2.E.W. Hansen, et al. J. Phys. Chem. B 101: 9206 (1997). 3.C.L. Bray, et al., A Fast Field Cycling Study of Soil, 6th International Conference on Magn. Reson. in Porous Media, Ulm, Germany, A.R. Kansal, et al., Phys. Rev. E., 55: (2002). 5.E. Anoardo, G. Galli, G. Ferrante, Appl. Magn. Reson. 20:365 (2001) 6.G.C. Borgia, R.J.S. Brown, et al.,Mag. Res. Imaging 16:549 (1998). 7.S.W. Provencher, Phys. Comm. 27:229 (1982). 8.J. Gong, J.P. Hornak, J. Magn. Reson. Imag. 10:623 (1992). Methods Sample Preparation Mono-dispersed glass bead samples (Quackenbush, Crystal Lake, IL, and Whitehouse Scientific, Ltd., Chester, UK) ranging from < d < 2 mm were cleaned with 2 molar KOH followed by multiple rinses with 18 M  water. Beads were placed in either 5 mm or 1 cm diameter NMR tubes, and hydrated with the 18 M  water. Samples were centered in the RF coil and occupied less than 90% of the RF coil length to minimize variation in the rotation angle across the sample. 300 MHz Acquisitions Proton NMR spectra were recorded at 23 ºC using DRX-300 (Bruker, Billerica, MA) NMR spectrometer. An inversion recovery sequence was used at 300 MHz. A logarithmic distribution of 127 inversion time values between 25  s < TI < 15s, with the denser sampling at low TI values, 6 a 15s repetition time, and eight phase cycling averages per TI value were used. Magnetization recovery curves were produced from the area of the water peak. The TI=15 s point was repeated twice, once at the beginning and end of the acquisition, to detect drift during the 5 hr acquisition. Relaxation curves with drift were discarded. Field Cycling Acquisitions Field cycling studies were performed at 0.01, 1.0, and 10.0 MHz using a Spinmaster FFC-2000 (Stelar, Mede, Italy) NMR spectrometer using the standard field stepping procedure. 5 The same logarithmic distribution of 127 stepping time values between 25  s < TI < 15s, a 15s repetition time, and eight phase cycling averages per TI value were used. Analysis All magnetization recovery curves were converted to exponential decays with an intentional 5% DC offset. The distribution of R 1 between 1x10 -4 and 1x10 +4 s -1 was estimated using CONTIN. 7 The 5% offset converted the any uncertainty in the equilibrium (long TI) signal 8 into an R 1 peak at ~5x10 -4 s -1 in the CONTIN output. (See Appendix.) An R 1 Distribution Study of Hydrated Randomly Close Packed Synthetic Soils Appendix Uncertainty in the equilibrium magnetization is a major cause of error in fitting exponential recovery data. 8 The uncertainty can be removed by adding an offset, , to all curves before CONTIN analysis. As a validation of this procedure, synthetic mono exponential decay curves with R 1 =0.333 s -1 and 0 <  < 0.3 as defined by Eqn. 1 were analyzed with CONTIN. [1] The offset did not affect the calculation of the actual R 1 of the decay. (See Table 1.) The offset was represented by an additional peak in the CONTIN output located at R 1  5x10 -4 s -1. The location of this additional peak is much less than the smallest possible R 1 value for water. Table 1. Comparison of actual R 1 values from synthetic data with various offsets. Offset (  ) Actual R 1 (s -1 ) Offset Induced R 1 (s -1 ) x x x10 -4 Fig. 1. Proton R 1 of hydrated glass bead samples at 300, 10, 1, and 0.01 MHz compared to bulk water. Dashed lines drawn to guide the eye. For an electronic copy of this poster, please see: ENC-2004