2.2.1 Spin exchange optical pumping
There are three major approaches currently used for preparing an artificially high spin population difference: dynamic nuclear polarization (DNP), para-hydrogen induced polarization (PHIP) and spin exchange optical pumping (SEOP). The function and operation of SEOP7 setup has been optimized for production of hp 129Xe, and will be described here. SEOP functions by creating a non-equilibrium population of electronic spins and transferring this polarization to the nuclei of interest such as xenon-129 or helium-3. It was originally introduced in the 1960s64. The theory and applications of SEOP in the context NMR and MRI experiments have been reviewed by many authors.7,65 Figure 2.2 describes the SEOP process. In the first step, circularly polarized light is used to pump a certain transition in an effective one-electron system such as an alkali metal. Most
polarizer setups, including ours, work with rubidium whose vapor pressure is high and the D1 transition can be driven by a 795 nm diode laser. Rb is heated in a glass cell to increase vapor atmosphere, and is exposed to a magnetic field of 1-10 mT to lift the degeneracy of the ms = ±1/2 states. The σ- light tuned at 795 nm traveling along the field selectively pumps the transition 52S1/2 52P1/2 with a spin flip from ms = +1/2 to mJ = -1/2, due to the conservation of angular momentum. When in the first excited state, frequent collisions between the electron and other gas molecules such as helium allow transitions between mJ = +1/2 and -1/2 states, resulting in an incoherent superposition, or J-randomization, of the two states. This J-randomization leads to an effective mixing and equal populations of the two excited states. Subsequently, the electron decays back to the ground state via quenching collisions with N2 molecules. This non-radiative quenching mediated by N2 (1 ns) happens faster than the natural decay of the excited states (~30 ns), which helps to conserve the non-equilibrium population of electron spins in the ground state. The energy of excited states was dissipated among the many vibrational levels in the N2 molecule, instead of being released via radiative decay. As a result, the ms = -1/2 state was significantly overpopulated while ms = +1/2 state was continuously depleted by the laser.
Polarization of the valence electron of rubidium is transferred to the xenon nucleus through collisional hyperfine interactions. When the gas pressure is low, long-lived van der Waals complexes are major contributors to the polarization transfer.66 However, under high pressure (multi-atmosphere) conditions, the dominating process was thought to be binary collisions between two spins that were close enough, which leads to spin transfer from Rb electron to 129Xe nucleus.7
2.2.2 Hyperpolarizer setup
To prepare for SEOP, an optical cell is coated with octadecyltrichlorosilane (OTS) which consists of long hydrocarbons, to reduce relaxation through the interaction of hp 129Xe with the cell wall. A more detailed procedure was described by Woolley.67 Briefly, after being thoroughly cleaned and dried over flow of nitrogen, the cell was filled with a solution of OTS, chloroform and hexane with volume ratio of 0.1:25:100. The solution was left to bind the surface of the optical cell for 5 minutes. Unbound OTS was removed by three rinses with chloroform. Introducing Rb to the optical cell is performed in a glove box. The coated optical cell, an ampule of rubidium, tools to break the ampule, a Pasteur pipet and a heat gun were transferred into a glove box for the procedure. Rb inside the ampule is heated up to liquid phase, and transferred to the optical cell with a Pasteur pipet.
In our home-built hyperpolarizer (Figure 2.3), diode laser (OptiGrate) is tuned to highest power output (~65W) and passed through a beam expander and a λ/4 wave plate. The alkali-metal Rb is embedded in a high pressure gas mixture (50 psi, 3.4 atm) composed of 1% natural abundance xenon, 10% nitrogen and 89% balancing helium.
2.2.3 Delivery of hp 129Xe
There are two ways to deliver hp 129Xe, serving different experimental purposes. The first is using a cryogenic separation technique68 to obtain a batch of hp xenon. In this method, a cold trap immersed in liquid nitrogen separates xenon (melting point 161.25 K) from the rest of the gas mixture as the gas stream flows through the optical pumping cell. The flow was retained for ~20 min at a rate of 0.5 SLM to allow for hp 129Xe ice accumulation, while
the rest of gas mixture flows out of the outlet. A permanent magnet around the cold trap is used to conserve the 129Xe ice polarization. The relaxation time of solid 129Xe is about 3 h at 77 K.69 Subsequent sublimation of the xenon ice by replacing liquid nitrogen around the trap with hot water generates hp 129Xe gas that can be either sealed in a plastic bag for 129Xe gas imaging or directed to a NMR tube with a J-Young valve for solution NMR experiments.
Alternatively, the gas mixture can be directly delivered into an NMR tube within a magnet, without condensation of xenon. This continuous flow setup allows hp xenon to be repeatedly bubbled into sample solutions, and therefore is particularly useful when acquisitions of multiple spectra are required, such as in Hyper-CEST.