Four-way multivariate calibration using ultra-fast high-performance liquid chromatography with fluorescence excitation–emission detection. Application to the direct analysis of chlorophylls a and b and pheophytins a and b in olive oils

• A four-way multivariate calibration method, based on LC-fluorescence detection, is described.
• Three-way emission-elution time matrices were obtained at different excitation wavelengths.
• Elution time excitation-emission fluorescence data were processed with multi-way algorithms.
• The methods were directly applied to the determination of pigments in olive oils.

A four-way multivariate calibration approach, based on the combination of ultra-fasthigh-performance liquid chromatographic data and four-way algorithms, is described for the first time. To achieve this goal, several emission wavelength–elution time matrices (ETMs) were recorded as a function of the excitation wavelength. Each sample was injected into the chromatograph eight times, in sequential mode, each time exciting at a different wavelength across the excitation spectra of the compounds of interest, and the emission spectra were recorded along the full chromatogram using a fast scanning fluorescence detector. The data were obtained in a very short time with an ultrafast chromatographic system operating in gradient mode. The three-wayETM data thus obtained for the calibration sample set and for each of the test samples were joined into a single four-way array, which was subsequently analyzed with parallel factor analysis (PARAFAC), unfolded partial least-squares with residual trilinearization (U-PLS/RTL) and multi-way partial least-squares with residual trilinearization (N-PLS/RTL) multivariate calibration algorithms. Best results were found when either U-PLS/RTL or N-PLS/RTL algorithms were used to perform the multivariate calibration. The method was applied to the direct determination of chlorophylls a and b and pheophytins a and b in olive oil samples, in the presence of other interfering fluorescent compounds, and without previous sample treatment.