A tetracycline hydrochloride sample was analyzed using High Performance Liquid Chromatography with Diode Array Detection (HPLC DAD). The HPLC DAD chromatograms were investigated using chemometric methods such as the Orthogonal Projection Approach (OPA), the Fixed Size Moving Window Evolving Factor Analysis Approach (FSW-EFA), Multivariate Curve Resolution Alternating Least Squares Approach (MCR-ALS) applied on the single HPLC DAD chromatograms (two-way data) and on sets of such chromatograms (three-way data).