Facile oxygen-18 labelling of phosphate and its delabelling under various conditions

Oxygen-18 labelled phosphoric acid can be prepared without any significant loss of enrichment by hydrolysing PCl5 dropwise with H2 [18O]O in an atmosphere of chlorine, subsequently removing the HCl. If the sacrifice of up to one-half of the enrichment is tolerable, then the procedure can be facilitated by simply hydrolysing POCl3 in air. Oxygen-18 enriched phosphate dissolved in sterilized aqueous solution, or kept dry, loses its label very slowly by exchange with oxygen-16. In the presence of living organisms the delabelling process is relatively rapid (Larsen et al., 1989). The effect is applicable to measurement of biological activity, for example in soil. (author)