In this paper the use of Doehlert's designs for the optimization of analytical methods is proposed. The use of Doehlert's designs makes it possible to create sequential designs, and enables the use of blocks and the detection of lack of fit in the calculated model. Similarly, it permits the estimation of the terms of the quadratic models with a small number of experiments.The application of the Lagrange criterion to the estimated response function leads to specific conclusion: either the existence of a maximum that permits the calculation of the optimum values of the variables, or the necessity to make a new design covering the zone previously indicated by the intersection of the straight lines traced in the direction of the maximum variation of the analytical signal.
