Sample purification for spectroscopic high-pressure investigations by dynamic supercritical fluid extraction

A method for the high resolution purification of 1,4-bis-(n-alkylamino)-9,10-anthraquinone dyestuff samples based on a modified supercritical fluid chromatography (SFC) equipment was developed to minimize errors in high-pressure solubility measurements. Impurities and by-products were eluted in a continuous CO2 flow, the purification process could be observed by a diode array spectrophotometer. The fact that both static and dynamic solubility experiments of a selected dyestuff gave consistent results turned out to be a credit point for this new method, which can be universally used for other purifications, too. Besides, the necessity of calibrating directly in the solvent gas is demonstrated, since some organic solvents lead to characteristic spectral effects which can strongly disagree with those in CO2. Our experiments showed a weakening of these effects towards longer alkyl chains of this substance class. In a separate section, some remarks are given concerning the use of wavenumbers instead of wavelengths in such experiments.