Analytical Chem istry - DePauw University

Chapter 5 Standardizing Analytical Methods163S spike of 0.419. What is the concentration of Pb 2+ in the original sampleof blood?So l u t i o nWe begin by making appropriate substitutions into equation 5.9 and solvingfor C A . Note that all volumes must be in the same units; thus, we firstcovert V std from 1.00 mL to 1.00 × 10 –3 mL.CA0.193=⎛100. mL ⎞C⎝⎜500 . mL⎠⎟A0.419⎛10. 0 mL ⎞100 . 10 3 m1560 ppb⎝⎜500 . mL⎠⎟ + ⎛ × − L⎞⎝⎜500 . mL ⎠⎟0.1930.419=0.200C0. 200C+ 0.3120 ppbA A0.0386C A + 0.0602 ppb = 0.0838C A0.0452C A = 0.0602 ppbC A = 1.33 ppbThe concentration of Pb 2+ in the original sample of blood is 1.33 ppb.It also is possible to make a standard addition directly to the sample,measuring the signal both before and after the spike (Figure 5.6). In thiscase the final volume after the standard addition is V o + V std and equation5.7, equation 5.8, and equation 5.9 becomeadd V std of C stdV oV oConcentrationof AnalyteC ACAVoVo+ Vstd+ CstdVoVstd+ VstdFigure 5.6 Illustration showing an alternative form of the method of standard additions.In this case we add a spike of the external standard directly to the samplewithout any further adjust in the volume.