Analytical Chem istry - DePauw University

164 Analytical Chemistry 2.0S= k Csamp A A⎛S = k C⎝⎜spike A AVoVo+ Vstd+ CstdVoVstd+ Vstd⎞⎠⎟5.10SsampCA=CAVoVo+ VstdSspike+ CstdVoVstd+ Vstd5.11Example 5.4A fourth spectrophotometric method for the quantitative analysis of Pb 2+in blood yields an S samp of 0.712 for a 5.00 mL sample of blood. After spikingthe blood sample with 5.00 mL of a 1560-ppb Pb 2+ external standard,an S spike of 1.546 is measured. What is the concentration of Pb 2+ in theoriginal sample of blood.V o + V std = 5.00 mL + 5.00×10 –3 mL= 5.005 mLSo l u t i o nTo determine the concentration of Pb 2+ in the original sample of blood, wemake appropriate substitutions into equation 5.11 and solve for C A .0. 712 1.546=CA ⎛ 500 . mL ⎞C1A⎝⎜5.005 mL⎠⎟ + 560⎛−3500 . × 10 mL⎞ppb⎝⎜5.005 mL ⎠⎟0. 712 1.546=C 0. 9990C+ 1.558 ppbA A0.7113C A + 1.109 ppb = 1.546C AC A = 1.33 ppbThe concentration of Pb 2+ in the original sample of blood is 1.33 ppb.Mu l t i p l e St a n d a r d Ad d i t i o n sWe can adapt the single-point standard addition into a multiple-pointstandard addition by preparing a series of samples containing increasingamounts of the external standard. Figure 5.7 shows two ways to plot astandard addition calibration curve based on equation 5.8. In Figure 5.7awe plot S spike against the volume of the spikes, V std . If k A is constant, thenthe calibration curve is a straight-line. It is easy to show that the x-interceptis equivalent to –C A V o /C std .