Analytical Chem istry - DePauw University

Chapter 5 Standardizing Analytical Methods161S sampstandard’smatrixsample’smatrix(C A ) reported(C A ) actualFigure 5.4 Calibration curves for an analyte in thestandard’s matrix and in the sample’s matrix. If thematrix affects the value of k A , as is the case here, thenwe introduce a determinate error into our analysis ifwe use a normal calibration curve.ent in the external standard’s matrix, which usually is a much simpler matrixthan that of our samples. When using an external standardization weassume that the matrix does not affect the value of k A . If this is not true,then we introduce a proportional determinate error into our analysis. Thisis not the case in Figure 5.4, for instance, where we show calibration curvesfor the analyte in the sample’s matrix and in the standard’s matrix. In thisexample, a calibration curve using external standards results in a negativedeterminate error. If we expect that matrix effects are important, then wetry to match the standard’s matrix to that of the sample. This is known asmatrix matching. If we are unsure of the sample’s matrix, then we mustshow that matrix effects are negligible, or use an alternative method of standardization.Both approaches are discussed in the following section.The matrix for the external standards inFigure 5.3, for example, is dilute ammonia,which is added because the Cu(NH 3 ) 42+complex absorbs more strongly thanCu 2+ . If we fail to add the same amountof ammonia to our samples, then we willintroduce a proportional determinate errorinto our analysis.5C.3 Standard AdditionsWe can avoid the complication of matching the matrix of the standards tothe matrix of the sample by conducting the standardization in the sample.This is known as the method of standard additions.Si n g l e St a n d a r d Ad d i t i o nThe simplest version of a standard addition is shown in Figure 5.5. First weadd a portion of the sample, V o , to a volumetric flask, dilute it to volume,V f , and measure its signal, S samp . Next, we add a second identical portionof sample to an equivalent volumetric flask along with a spike, V std , of anexternal standard whose concentration is C std . After diluting the spikedsample to the same final volume, we measure its signal, S spike . The followingtwo equations relate S samp and S spike to the concentration of analyte, C A , inthe original sample.