Analytical Chem istry - DePauw University

158 Analytical Chemistry 2.0assumed relationshipactual relationshipS sampFigure 5.2 Example showing how a single-pointstandardization leads to a determinate error in ananalyte’s reported concentration if we incorrectlyassume that the value of k A is constant.S stdC std(C A ) reported(C A ) actualC std is known as a calibration curve. The exact standardization, or calibrationrelationship is determined by an appropriate curve-fitting algorithm.There are at least two advantages to a multiple-point standardization.First, although a determinate error in one standard introduces a determinateerror into the analysis, its effect is minimized by the remaining standards.Second, by measuring the signal for several concentrations of analyte weno longer must assume that the value of k A is independent of the analyte’sconcentration. Constructing a calibration curve similar to the “actual relationship”in Figure 5.2, is possible.Appending the adjective “external” to thenoun “standard” might strike you as oddat this point, as it seems reasonable to assumethat standards and samples mustbe analyzed separately. As you will soonlearn, however, we can add standards toour samples and analyze them simultaneously.5C.2 External StandardsThe most common method of standardization uses one or more externalstandards, each containing a known concentration of analyte. We callthem “external” because we prepare and analyze the standards separate fromthe samples.Si n g l e Ex t e r n a l St a n d a r dA quantitative determination using a single external standard was describedat the beginning of this section, with k A given by equation 5.5. After determiningthe value of k A , the concentration of analyte, C A , is calculatedusing equation 5.6.Example 5.1A spectrophotometric method for the quantitative analysis of Pb 2+ inblood yields an S std of 0.474 for a single standard whose concentration oflead is 1.75 ppb What is the concentration of Pb 2+ in a sample of bloodfor which S samp is 0.361?