The following wet-ashing procedure was used to measure arsenic in organic soil samples byatomic absorption spectroscopy: A 0.1- to 0.5-g sample was heated in a 150-mL Teflon bomb in a microwave oven for 2.5 min with 3.5 mL of 70% HNO3. After the sample cooled, a mixture containing 3.5 mL of 70% HNO3, 1.5 mL of 70% HClO4, and 1.0 mL of H2SO4 was added and the sample was reheated for three 2.5-min intervals with 2-min unheated periods in between. The final solution was diluted with 0.2 M HCl for analysis. Explain why HClO4 was not introduced until the second heating.