Calibration linear versus nonlinear methods

Option (Valid) presents a graph of relative standard deviation (c.o.v.) versus concentration, with the relative residuals superimposed. This gives a clear overview of the performance to be expected from a linear calibration Signal = A + B Concentration, both in terms of (relative) precision and of accuracy, because only a well-behaved analytical method will show most of the residuals to be inside a narrow trumpet -like curve this trumpet is wide at low concentrations and should narrow down to c.o.v. = 5% and rel. CL = 10%, or thereabouts, at medium to high concentrations. Residuals that are not randomly distributed about the horizontal axis point either to the presence of outliers, nonlinearity, or errors in the preparation of standards. 

The external standard method is the technique used most frequently to gather quantitative information from a chromatogram. In this case, a pure reference substance (ideally the same compound as the one to be determined in the sample) is injected in increasing concentrations and the peak areas or peak heights obtained are plotted versus the concentration (calibration curve ->Chemo-metrics). These calibration curves should show a constant slope (linear curves), and the intercept should be as close to zero as possible. Since the calibration curves usually show nonlinear behavior and flatten off at higher concentrations (.see Section 12.2.6), the quantification should be carried out only within the linear part of the curve. 

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