Cairns et al., 2007, prepared rare earth element (REE) standards, according to the method described in . "The finished glass contains SiO2, Rare Earth (RE) oxide [REO] (generally RE2O3), CaO and Al2O3. The gels were made in a similar manner to that described in . The gel starting materials were prepared by mixing the required weights of standard solutions of aluminium, calcium and rare earth nitrates to produce a final weight of either 5g or 10g. Ethanol was then added to the nitrate mixture. This was to ensure the miscibility of tetraethyl orthosilicate …, used for the silica component, which was added next. Concentrated ammonia … was then added to form a gelatinous precipitate of hydroxides. The mixture was then covered and left for at least 16 hours to ensure the complete hydrolysis of the TEOS. The gels were then slowly dried," … etc. .