A spectrophotometric method for the quantitative analysis of Pb²⁺ in blood yields an absorption reading of 0.193 when a 1.00 mL sample of blood is diluted to 5.00 ml. A second 1.00 ml sample of blood is spiked with 1.00 μL of a 1560-ppb Pb²⁺ external standard and diluted to 5.00 mL, yielding a reading of 0.419. What is the concentration (in ppb) of Pb²⁺ in the original sample of blood?