The samples were prepared in a glove bag (AtmosBag, Sigma-Aldrich, Dorset, UK) which was purged with dry nitrogen gas (relative humidity < 1% ) to avoid moisture sorption from atmospheric water vapour. The lyophilised powder samples were pressed into 13 mm diameter flatfaced pellets, using a load of 1.5 metric tons. The resulting pellets were between 300 700 µm in thickness and 70 mg in weight each, and were placed between two-quartz windows. This sandwich structure was sealed in the sample holder, as described previously.