The three-dimensional response surface plot, which is a 3D graphic representation of the regression equations, shows the
individual and cumulative effects of the variables and the mutual interactions between the variables (Fig. 6). This figure indicates the quadratic nature of the surface with the maxima and minima of the response and the significance of the coefficient of the canonical equation. Whereas an isoresponse contour plot is a projection of the response surface on a 2D plane, a contour plot gives a better understanding than a 3D surface plot of the influence of variables and their interaction on the response [29]. Fig. 7 is a contour plot representing the combined effect of pH and temperature on the NDMA FPO3/NDMA FPO3maximum, shown as absorbance. The maximum NDMA FPO3/NDMA FPO3maximum
occurred at pH 8.95 and temperature 20 8C.