Analytical Chem istry - DePauw University

Chapter 5 Standardizing Analytical Methods169Example 5.7A sixth spectrophotometric method for the quantitative analysis of Pb 2+ inblood uses Cu 2+ as an internal standard. A standard containing 1.75 ppbPb 2+ and 2.25 ppb Cu 2+ yields a ratio of (S A /S IS ) std of 2.37. A sample ofblood is spiked with the same concentration of Cu 2+ , giving a signal ratio,(S A /S IS ) samp , of 1.80. Determine the concentration of Pb 2+ in the sampleof blood.So l u t i o nEquation 5.13 allows us to calculate the value of K using the data for thestandardKC= ⎛ ⎝⎜CISA⎞⎠⎟stdS× ⎛ ⎝⎜SThe concentration of Pb 2+ , therefore, isC SIS Appb CuC = × ⎛ ⎞= 225AK ⎝⎜SIS⎠⎟ppb Cusamp 305 .ppb PbAIS⎞⎠⎟std225 . ppb Cuppb Cu= × 237 . = 3.052+175 . ppb Pbppb Pb. 2+2+2+× 180 . = 1.33 ppb Cu2+2+2+Mu l t i p l e In t e r n a l St a n d a r d sA single-point internal standardization has the same limitations as a singlepointnormal calibration. To construct an internal standard calibrationcurve we prepare a series of standards, each containing the same concentrationof internal standard and a different concentrations of analyte. Underthese conditions a calibration curve of (S A /S IS ) std versus C A is linear witha slope of K/C IS .Example 5.8A seventh spectrophotometric method for the quantitative analysis of Pb 2+in blood gives a linear internal standards calibration curve for which⎛S⎝⎜SAIS⎞⎠⎟std-1= ( 211 . ppb ) × C −0.006AAlthough the usual practice is to preparethe standards so that each contains anidentical amount of the internal standard,this is not a requirement.What is the ppb Pb 2+ in a sample of blood if (S A /S IS ) samp is 2.80?So l u t i o nTo determine the concentration of Pb 2+ in the sample of blood we replace(S A /S IS ) std in the calibration equation with (S A /S IS ) samp and solve for C A .