Analytical Chem istry - DePauw University

160 Analytical Chemistry 2.0method’s sensitivity is a function of the analyte’s concentration. In Figure5.2, for example, the value of k A is greatest when the analyte’s concentrationis small and decreases continuously for higher concentrations of analyte.The value of k A at any point along the calibration curve in Figure 5.2 is givenby the slope at that point. In either case, the calibration curve provides ameans for relating S samp to the analyte’s concentration.Example 5.2A second spectrophotometric method for the quantitative analysis of Pb 2+in blood has a normal calibration curve for whichSstd-1= ( 0. 296 ppb )× C + 0.003What is the concentration of Pb 2+ in a sample of blood if S samp is 0.397?So l u t i o nTo determine the concentration of Pb 2+ in the sample of blood we replaceS std in the calibration equation with S samp and solve for C A .CAS −0.003samp 0. 397 −0.003=== 133 . ppb-1-10.296 ppb 0. 296 ppbIt is worth noting that the calibration equation in this problem includesan extra term that does not appear in equation 5.6. Ideally we expectthe calibration curve to have a signal of zero when C A is zero. This is thepurpose of using a reagent blank to correct the measured signal. The extraterm of +0.003 in our calibration equation results from the uncertainty inmeasuring the signal for the reagent blank and the standards.stdThe one-point standardization in this exerciseuses data from the third volumetricflask in Figure 5.3.Practice Exercise 5.1Figure 5.3 shows a normal calibration curve for the quantitative analysisof Cu 2+ . The equation for the calibration curve isS std = 29.59 M –1 × C std + 0.0015What is the concentration of Cu 2+ in a sample whose absorbance, S samp ,is 0.114? Compare your answer to a one-point standardization where astandard of 3.16 × 10 –3 M Cu 2+ gives a signal of 0.0931.Click here to review your answer to this exercise.An external standardization allows us to analyze a series of samplesusing a single calibration curve. This is an important advantage when wehave many samples to analyze. Not surprisingly, many of the most commonquantitative analytical methods use an external standardization.There is a serious limitation, however, to an external standardization.When we determine the value of k A using equation 5.5, the analyte is pres-