In all experiments, the sample solution consisted of 5 mM Ir-IMes catalyst and 125 mM ben- Hyperpolarization. The sample was initially bubbled with nitrogen gas at a pressure of around 5 bar for five minutes to remove residual O 2 . Then the sample was pressurised under pH 2 to approx. 6 bar. Catalyst activation was achieved by bubbling pH 2 through the solution for 15 seconds every 3 minutes, repeated 12 times. After catalyst activation, hyperpolarization was accomplished by pulsed bubbling of pH 2 through the sample in short, controlled cycles. Each cycle involved 2.6 seconds of bubbling followed by a 0.5-second pause, repeated five times in total. Throughout the experiment, a flow rate of 20 scc/min was applied (mass flow controller Sierra Instruments).