Question 5.8: A sixth spectrophotometric method for the quantitative deter...
A sixth spectrophotometric method for the quantitative determination of Pb^{2+} levels in blood uses Cu^2+ as an internal standard. A standard containing 1.75 ppb Pb^{2+} and 2.25 ppb Cu^{2+} yields a ratio of SA/SIS of 2.37. A sample of blood is spiked with the same concentration of Cu^{2+}, giving a signal ratio of 1.80. Determine the concentration of Pb^{2+} in the sample of blood.
Equation 5.11 allows us to calculate the value of K using the data for the standard
K=\left\lgroup\frac{C_{IS}}{C_A} \right\rgroup \left\lgroup\frac{S_A}{S_{IS}} \right\rgroup_{stand} =\frac{2.25}{1.75} \times 2.37=3.05
The concentration of Pb^{2+}, therefore, is
C_A=\left\lgroup\frac{C_{IS}}{K} \right\rgroup \left\lgroup\frac{S_A}{S_{IS}} \right\rgroup_{stand} =\frac{2.25}{3.05} \times 180=1.33 ppb Pb^{2+}