Application of derivative spectroscopy to the determination of chromatographic peak purity

Derivative spectroscopy was used in the interpretation and exploitation of data obtained from rapid-scan UV—VIS absorbance detectors for ascertaining the purity of chromatographic peaks. Two types of derivatives were examined. The derivative of the elution curve with respect to the specific wavelength at which the major compound has a zero derivative (referred to as the spectral derivative null technique) proved to be rapid and useful for the determination of co-eluting impurity peaks which might form in solution or during chromatography from compounds otherwise known to be pure. The derivatives of the spectral curves obtained during chromatography (referred to as the derivative spectral mapping technique) were also examined both by computer simulation and experimentally and found to have the potential for universal applications in screening compounds for possible overlapping impurities in high-performance liquid chromatographic scans. A novel approach using the derivative spectral mapping technique is described to relate impurity detection limits to chromatographic and spectral resolutions of closely absorbing species which are incompletely resolved chromatographically (Rc < 0.5). Using the above techniques, it was possible to detect, under suitable conditions, as little as 0.1% impurity which co-eluted chromatographically with the major compound.