Analytical Chem istry - DePauw University

Chapter 5 Standardizing Analytical Methods187Table 5.4 Equations and Resulting Concentrations of Analyte for DifferentApproaches to Correcting for the BlankConcentration of Analyte in...Approach for Correcting Signal Equation Sample 1 Sample 2 Sample 3ignore calibration and reagent blankCAW SAsamp= = 0.1707 0.1610 0.1552W kWsampAsampuse calibration blank onlyuse reagent blank onlyCCAAW S −CBA samp= =W kWsampAsampW S −RBA samp= =W kWsampAsamp0.1441 0.1409 0.13900.1494 0.1449 0.1422use both calibration and reagent blank CAW SA= =Wsampsamp−CB−RBkWAsamp0.1227 0.1248 0.1261use total Youden blankCAW S − TYBA samp= =W kWsampAsamp0.1313 0.1313 0.1313C A = concentration of analyte; W A = weight of analyte; W samp = weight of sample; k A = slope of calibration curve (0.075–seeTable 5.3); CB = calibration blank (0.125–see Table 5.3); RB = reagent blank (0.100–see Table 5.3); TYB = total Youden blank(0.185–see text)tions for calculating the analyte’s concentration using each approach, alongwith the resulting concentration for the analyte in each sample.That all four methods give a different result for the analyte’s concentrationunderscores the importance of choosing a proper blank, but does nottell us which blank is correct. Because all four methods fail to predict thesame concentration of analyte for each sample, none of these blank correctionsproperly accounts for an underlying constant source of determinateerror.To correct for a constant method error, a blank must account for signalsfrom any reagents and solvents used in the analysis, as well as any bias resultingfrom interactions between the analyte and the sample’s matrix. Boththe calibration blank and the reagent blank compensate for signals fromreagents and solvents. Any difference in their values is due to indeterminateerrors in preparing and analyzing the standards.Unfortunately, neither a calibration blank nor a reagent blank can correctfor a bias resulting from an interaction between the analyte and thesample’s matrix. To be effective, the blank must include both the sample’smatrix and the analyte and, consequently, must be determined using thesample itself. One approach is to measure the signal for samples of differentsize, and to determine the regression line for a plot of S samp versus theBecause we are considering a matrix effectof sorts, you might think that the methodof standard additions is one way to overcomethis problem. Although the methodof standard additions can compensate forproportional determinate errors, it cannotcorrect for a constant determinate error;see Ellison, S. L. R.; Thompson, M. T.“Standard additions: myth and reality,”Analyst, 2008, 133, 992–997.