Chemistry 155 Introduction to Instrumental Analytical Chemistry

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Chem 155 Unit 1 Page 30 of 3131.8.5 Direct InterferenceIn order to measure a concentration directly, withoutcorrections, k B,A , k C,A must be approximately:Zero!If there is interference, it is necessary to know both:k B,A , k C,AandC B , C Cbefore one can determine the desired quantity C A !Page 30 of 313
Chem 155 Unit 1 Page 31 of 3131.9 Linear RegressionLeast-SquaresRegression orLinear RegressionAssuming that a set of x,y data pairs are welldescribed by the linear function y = a+bx, andassuming that most error is in y (x is moreprecisely known) and the errors in y are not afunction of x, then the coefficients that minimizethe residual function:χ 2 = Σ(y I – (a + bx I )) 2can be found from the following equations:Non-linearRegressionStandard AdditionsPlotSample Matrix /Standards MatrixMatrix EffectInternal Standardsfrom Data Reduction and Error Analysis for thePhysical Sciences, Philip R. Bevington, cw. 1969Mc Graw Hill.Methods for fitting arbitrary curves to data sets.A nearly matrix-effect free form of analysis. Astandard additions plot is a linear plot ofinstrument signal versus quantity of a standardanalyte solution ‘spiked’ or added to the unknownanalyte sample. The unknown analyteconcentration is derived from the concentrationaxis-intercept of this plot.The matrix is the solution, including solvent(s)and all other solutes in which an analyte isdissolved or mixedA matrix effect refers to the case where theinstrumental sensitivity is different for the sampleand standards because of differences in thematrix.A calibration method in which fluctuation in theinstrument signals due to matrix effects are,ideally, cancelled out by monitoring thefluctuations in the instrument sensitivity tochemicals, internal standards, that are chemicallysimilar to the analyte.Page 31 of 313
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Chemistry 155 Introduction to Instrumental Analytical Chemistry

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