4T,300K=2.27×105

times the thermal equilibrium signal at 9.4 T and 300 K Vincristine purchase corresponds to 100% polarization. For comparison, the thermal polarization for 83Kr is P83Kr9.4T,300K=4.53×10-6 ( fmax9.4T,300K=2.21×105), and for 129Xe is P129Xe9.4T,300K=8.92×10-6 ( fmax9.4T,300K=1.12×105). Using the stopped-flow optical pumping method, 131Xe signal enhancements on the order of 5000 times greater than thermal signal at B0 = 9.4 T, 150 kPa, and 297 K were achieved (i.e. approximately 2.2% spin polarization) when mixture I was used. The 131Xe polarization build-up reached a steady-state relatively quickly compared to other noble gas isotopes (3He, 129Xe and, 83Kr – at similar SEOP conditions). The time dependence for the hp 131Xe polarization build-up is shown in Fig. 4 for the three different mixtures (5%, 20% and 93% Xe) under C59 wnt 40 W of σ− circularly polarized 794.7 nm laser light.

To monitor the 131Xe polarization build-up, the magnetic field at the SEOP cell was initially switched off, while the cell was maintained under constant laser illumination at a constant temperature (453 K) and pressure (150 kPa) for 5–10 min. This procedure produced a ‘starting point’ at stable SEOP conditions but with no hyperpolarized 131Xe present and allowed for regeneration of the rubidium vapor after the shuttling procedure. The magnetic field of a pair of Helmholtz coils was then turned on for incremented time period, tp, after which the hp 131Xe was transferred to the sample cell where it was detected. The polarization value was obtained from the hp 131Xe signal intensities through comparison to the thermal signal

of 131Xe described in the experimental section. The time dependent build-up of hyperpolarization is described as [72]: equation(3) P131XeSEOP=γseγse+Γ·γopγop+∑iκsdi[Mi](1-e-(γse+Γ)tp),where STK38 γse   is the Rb–Xe spin exchange rate and Γ   = 1/T  1 is the quadrupolar driven fast self-relaxation rate of 131Xe. The destruction of Rb spin polarization by collisions with inert gas atoms is described by the sum of the products of the rate constants, κsdi, with their corresponding gas atom number densities [Mi]. The optical pumping rate per Rb atom, γop, depends on experimental parameters such as laser power, SEOP cell design, and SEOP temperature that were kept constant for all build-up experiments reported here. However only a reduced form of Eq. (3) was used for fitting of the experimental data since γse and Γ were unknown under the SEOP conditions used in this work: equation(4) P131XeSEOP(t)=A(1-e-Btp). The lower the xenon concentration used in the gas mixture, the larger was the resulting pre-exponential parameter A  . The steady-state polarization P131XeSEOP(max) (i.e. at infinite long SEOP times) determined through A   was 2.24 ± 0.03 for mixture I (5% xenon), 0.438 ± 0.007 for mixture II (20% xenon), and 0.0256 ± 0.0005 for mixture III (93% xenon). The ratios between the values obtained for A   were 1:0.20:0.