A $5\mathrm{.}00-$mL sample of blood was treated with trichloroacetic acid to precipitate proteins. After centrifugation, the resulting solution was brought to $pH3$ 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 $283\mathrm{.}3nm.$ Five$-$milliliter aliquots of standard solutions containing $0\mathrm{.}400$ and $0\mathrm{.}600ppm$ 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 assuming that Beer's law is followed.