This work reports the first systematic study of relaxation experienced by the hyperpolarized noble gas isotope 83Kr (I = 9/2) in the presence of surfaces with small overall areas. The spin-lattice relaxation of 83Kr is found to depend strongly on the chemical composition of the surfaces in the vicinity of the gas. This effect is caused by quadrupolar interactions during surface adsorption of the atoms that is the dominating source of 83Kr longitudinal spin relaxation in the studied materials. Simple model systems of closest packed glass beads with uniform but variable bead sizes are used for the relaxation measurements. The observed relaxation rates depend strongly on the treatment of the glass surface and on the surface to volume ratio. Hp 83Kr NMR relaxation measurements are also applied to probe a number of porous polymers with void space diameters of 70 – 250 m.

Figure 1: Signal intensity decay resulting from T1 relaxation and the application of a series of medium flip angle r.f. pulses. Using stopped flow optical pumping, hyperpolarized 83K is transferred into a sample of porous polyethylene (mean pore size 70 m). Signal is acquired by applying a series of sixteen 12.3o r.f. pulses spaced evenly every 0.4 s. Similarly robust decay patterns are observed in all of the porous samples studied.

Figure 2: Relaxation rates versus inverse bead radii from the experimental results summarized in Table 1. The inverse radius is directly proportional to the surface to volume ratio in the porous samples. Relaxation rates for the various bead surfaces are represented by the following symbols: siloxane treated – open circles, pretreated – closed diamonds, and untreated – closed circles. The single data point for a flourosilane treated surface (1.0 mm beads) is represented by an open square. Error bars represent standard deviations resulting from at least four replicate measurements.

Figure 3: 83Kr NMR spectra of 70 m hydrophobic porous polymer (-1.8 ppm) and of (bulk) gas phase at 100 kPa (0 ppm). The bulk gas phase is contained inside a 3 mm PFA tube located with axial symmetry in a cylindrical polymer sample. The negative (upfield) shift of the krypton inside the material is caused by shielding through the macroscopic magnetic susceptibility of the polymer. Fig. 3 A-C shows 83Kr NMR spectra at different delay times, , after the transfer of the hyperpolarized gas into the sample. The gas phase peak is clearly identified by its slower relaxation compared to the krypton inside the porous polymer. Note that sample top and bottom also allow for some diffusion between the two regions, leading to somewhat faster relaxation in the gas phase compared to gas phase without contact with the polymer. For the same reason the relaxation rate in the material is reduced compared to a sample without bulk gas phase (see Table 1).

Figure 4: Micrograph of porous polyethylene obtained from an inverted microscope. The sample has a mean pore size of 70 m as characterized by the supplier. For 83Kr NMR spectra of this sample see Fig. 1 and 3.

It is shown, that hp 83Kr NMR relaxation measurements are highly sensitive to the chemical composition of surfaces and to the surface to volume ratios in porous materials. In contrast to direct observation of surfaces by traditional solid state NMR techniques, hp 83Kr NMR does not require any line narrowing. Sensitivity is often a limiting factor in NMR of surfaces but is not an obstacle for hyperpolarized 83Kr. Furthermore, there is no need to discriminate between signals arising from the sample surface and from the bulk gas phase, since the technique is entirely surface selective. An advantage of hp 83Kr NMR over optical and other surface techniques is that it can be easily applied to opaque and amorphous materials even under atmospheric conditions. Like the well established 129Xe NMR spectroscopy, the new technique is only an indirect probe for surface structure. However, for the first time a hyperpolarized quadrupolar noble gas is available for materials science studies that provides information highly complementary to that obtained from 129Xe NMR. The closest packed glass beads are an ideal test system for the exploration of the surface sensitivity of new technique. Hp 83Kr NMR might be helpful for instance, in characterizing the homogeneity of grafting, wetting and other surface treatments in amorphous materials.