Answer: La+3 binds to PO4-3 in the urine sample which prevents the formation of Ca3(PO4)2 and frees the Ca+2 in the sample to be atomized.
Explanation:
In the presence of phosphates, the calcium ions form a refractory compound (calcium phosphate) which is not atomized in the plasma/flame in atomic absorption spectroscopy.
The addition of a releasing agent like lanthanum to the sample minimizes the formation of refractory compounds as the lanthanum binds with the phosphate ions to form more volatile compounds.
