The linear relationship between the centrate absorbance and polymer concentration was observed at three selected wavelengths (191.5 nm, 200 nm and 210 nm) and at both dilution ratios (1:30 and 1:50) (Fig. 5a and b). The dotted lines show the 95% C.I. for the regression lines. Since the absorbance of the samples were highest at 191.5 nm, the slopes of the regression lines were also highest at 191.5 nm, suggesting that this would be the wavelength that is most sensitive to changes in the polymer concentration. In addition to Hydrex 3572, FloPolymer CB 4350, was also tested under the same conditions, and similar results were obtained (data not shown).