A MULTIWAVELENGTH APPROACH TO THE SELECTION OF ABSORBENCY RATIOS FOR THE ASSESSMENT OF CHROMATOGRAPHIC PEAK PURITY

The routine use of diode-array detectors (DAD), based on the linear photodiode array device, has transformed the practice of UV-vis detection in liquid chromatography (LC). Multiwavelength detection is widely employed to generate absorbance ratios as a relatively non-specific method for characterizing peak purity in LC. If several wavelength pairs are selected the selectivity of the absorbance ratio method and its sensitivity to an interfering impurity can be increased, however, these attributes still depend on the selection of suitable pairs of wavelengths. This paper presents a novel approach to the selection of absorbance ratios for the assessment of peak purity in LC, utilizing a matrix derived from all the spectral data collected. As with single absorbance ratios, the absorbance ratio matrix (ARM) generated (containing all possible finite absorbance ratios) is characteristic for the analyte and independent of the analyte concentration. Moreover, the ARM technique eliminates the need to select "appropriate wavelength pairs", for sensitive discrimination of small spectral differences, when used for peak purity assessment. The ARM is found to give comparably high sensitivity to the presence of co-eluting species, as compared with the use of the wavelength pair selected on the basis of the conventional optimization criteria.