Analytical Chem istry - DePauw University

188 Analytical Chemistry 2.0amount of sample. The resulting y-intercept gives the signal in the absenceof sample, and is known as the total Youden blank. 13 This is the trueblank correction. The regression line for the three samples in Table 5.3 isS samp = 0.009844 × W samp + 0.185giving a true blank correction of 0.185. As shown by the last row of Table5.4, using this value to correct S samp gives identical values for the concentrationof analyte in all three samples.The use of the total Youden blank is not common in analytical work,with most chemists relying on a calibration blank when using a calibrationcurve, and a reagent blank when using a single-point standardization.As long we can ignore any constant bias due to interactions between theanalyte and the sample’s matrix, which is often the case, the accuracy of ananalytical method will not suffer. It is a good idea, however, to check forconstant sources of error before relying on either a calibration blank or areagent blank.5FUsing Excel and R for a Regression AnalysisAlthough the calculations in this chapter are relatively straightforward—consisting, as they do, mostly of summations—it can be quite tedious towork through problems using nothing more than a calculator. Both Exceland R include functions for completing a linear regression analysis and forvisually evaluating the resulting model.A B1 Cstd Sstd2 0.000 0.003 0.100 12.364 0.200 24.835 0.300 35.916 0.400 48.797 0.500 60.42Figure 5.15 Portion of a spreadsheetcontaining data from Example5.9 (Cstd = C std ; Sstd =S std ).5F.1 ExcelLet’s use Excel to fit the following straight-line model to the data in Example5.9.y = β + β x0 1Enter the data into a spreadsheet, as shown in Figure 5.15. Dependingupon your needs, there are many ways that you can use Excel to completea linear regression analysis. We will consider three approaches here.Us e Ex c e l ’s Bu i l t -In Fu n c t i o n sIf all you need are values for the slope, b 1 , and the y-intercept, b 0 , you canuse the following functions:=intercept(known_y’s, known_x’s)=slope(known_y’s, known_x’s)where known_y’s is the range of cells containing the signals (y), and known_x’sis the range of cells containing the concentrations (x). For example, clickingon an empty cell and entering13 Cardone, M. J. Anal. Chem. 1986, 58, 438–445.