The first order calibration methods cannot correctly predict the concentration of analytes in unknown samples with unmodeled interferents[1]. So, developing the methods that can eliminate this limitation, without the need for repeated calibration stages, is extraordinary important. Micelles usually affect on spectral parameters; the intensity of absorption bands can be increased or decreased and shift in absorption maxima of different chemical species are observed [2]. This phenomenon depends on solute and surfactant type. If the analytes in the aqueous and micellar environment have different spectral shapes but the interferents have the spectral shapes which are independent on two phases, the differential spectra (between these two phases) eliminates troublesome unmodeled interferents in the unknown samples. Fortunately, differential spectrum of the analyte is directly proportion to its concentration .So making a first order calibration model using differential spectra, rather than direct spectral data, eliminates the unmodeled interferents in unknown samples. This method is demonstrated for simultaneous determination of new fuchsine, crystal violet and malachite green in the presence of unmodeled interferences found in the unknown samples. The obtained satisfactory results indicate the successful applicability of the proposed method in complex samples.

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