Analytical Chem istry - DePauw University

46 Analytical Chemistry 2.0(a)Figure 3.4 Two determinations ofthe concentration of K + in serum,showing the effect of precision onthe distribution of individual results.The data in (a) are less scatteredand, therefore, more precisethan the data in (b).5.8 5.9 6.0 6.1 6.2ppm K(b)5.8 5.9 6.0 6.1 6.2ppm KConfusing accuracy and precision is acommon mistake. See Ryder, J.; Clark, A.U. Chem. Ed. 2002, 6, 1–3, and Tomlinson,J.; Dyson, P. J.; Garratt, J. U. Chem.Ed. 2001, 5, 16–23 for discussions of thisand other common misconceptions aboutthe meaning of error.You will find a more detailed treatment ofprecision in Chapter 4, including a discussionof sources of errors.Confidence, as we will see in Chapter 4,is a statistical concept that builds on theidea of a population of results. For thisreason, we will postpone our discussion ofdetection limits to Chapter 4. For now,the definition of a detection limit givenhere is sufficient.3D.2 PrecisionWhen a sample is analyzed several times, the individual results are rarely thesame. Instead, the results are randomly scattered. Precision is a measure ofthis variability. The closer the agreement between individual analyses, themore precise the results. For example, in determining the concentrationof K + in serum the results shown in Figure 3.4(a) are more precise thanthose in Figure 3.4(b). It is important to understand that precision doesnot imply accuracy. That the data in Figure 3.4(a) are more precise doesnot mean that the first set of results is more accurate. In fact, neither set ofresults may be accurate.A method’s precision depends on several factors, including the uncertaintyin measuring the signal and the ease of handling samples reproducibly.In most cases we can measure the signal for a total analysis methodwith a higher precision than the corresponding signal for a concentrationmethod. Precision is covered in more detail in Chapter 4.3D.3 SensitivityThe ability to demonstrate that two samples have different amounts of analyteis an essential part of many analyses. A method’s sensitivity is a measureof its ability to establish that such differences are significant. Sensitivityis often confused with a method’s detection limit, which is the smallestamount of analyte that we can determine with confidence.Sensitivity is equivalent to the proportionality constant, k A , in equation3.1 and equation 3.2. 3 If DS A is the smallest difference that we can measurebetween two signals, then the smallest detectable difference in the absoluteamount or relative amount of analyte is3 IUPAC Compendium of Chemical Terminology, Electronic version, http://goldbook.iupac.org/S05606.html.