A $5\mathrm{.}00-$mL sample of blood was treated with trichloroacetic acid to precipitate proteins. After centrifuga$-$ tion, the resulting solution was brought to pH $3$ and extracted with two $5-$mL portions of methyl isobutyl ketone containing the lead$-$complexing agent APCD. The extract was aspirated directly into an air$/$acetylene flame and yielded an absorbance of $0\mathrm{.}502$ at $283\mathrm{.}3$ nm$.$ Five$-$milliliter aliquots of standard solutions containing $0\mathrm{.}400$ and $0\mathrm{.}600$ ppm of lead were treated in the same way and yielded absorbances of $0\mathrm{.}396$ and $0\mathrm{.}599.$ Find the concentration of lead in the sample in ppm assum$-$ ing that Beers law is followed.