The Florida Phosphate Rock (120c) certified reference material
(CRM) from the National Institute of Standards and Technology was
employed in the accuracy studies. A mass of 0.25 g of the CRM was accurately
weighed out and transferred to a digestion tube, followed by
addition of 5.0 mL of hydrochloric acid and 2.0 mL of water. The tube
was placed in a block digester and heated at 150 °C for 3 h. After digestion
and cooling, the resulting digest was transferred to a 25 mL volumetric
flask, followed by addition of 1.25 mL of hydrogen peroxide
and 9 mL of Ti stock solution, and the volume was made up with deionized
water. The addition of Ti to the samples and the CRMavoided interference
in the PO measurements due to the formation of stable calcium
phosphate molecules [14]. The accuracy of the proposed HR-CS FAAS
method was assessed using addition/recovery tests. Aliquots of 5.0 mL
of the CRM digests were spiked with 250 μL of 20000 mg L-N,400 μLof5000mgL−1
P, and 20 μL of 25000 mg L−1 K stock standard solutions, in order to obtain solutions containing 1000 mg L N,400 mg L−1 P, and 100 mg L−1 K. These studies were also carried out using 20000 mg L
−1 stock standard solutions of NH4+−1NO3−−1-N and urea-N.