Orthogonal Projection (OPA) and Needle Search (NS) have been applied to assess the purity of peaks in a spectrochromatogram recorded with a high-performance liquid chromatograph coupled with a diode array detector. The introduction of an anomalous number of compounds in the peak purity assays points out to the existance of an non-linearity artefact in the DAD data set, which leaves unexplained contributions greater than the contributions associated to the known impurities. Decision criteria and certain modifications were introduced in OPA and NS in order to distinguish the artefact peaks from the peaks corresponding to minor components.