Near infrared spectroscopy has proved highly suitable for the analysis of pharmaceutical formulations. However, its limited sensitivity can severely restrict its scope of application. In this work, we determine the active principle and two preservatives in a pharmaceutical preparation available as a hydrogel. The matrix of the pharmaceutical preparation exhibits strong absorption in the NIR spectral region; also, the two preservatives (parabens) are very similar in chemical and spectral terms, and present at low concentrations in the pharmaceutical. These complications make it rather difficult to accurately quantify the active principle and the preservatives, which can only be accomplished by using an effective design in order to prepare the samples to be included in the calibration set and select the optimum spectral range for measuring each analyte. The evaporation of solvents during the measurement process produces increasing errors related with sample's air exposition; the introductions of new samples with a wider range of the volatile components correct this effect. An ANOVA of the predictions obtained with the new models shows that correct the error due to evaporation. The proposed method was validated for the analytical control of the studied preparation.