Thin layer chromatography of commercial Sudan Black B on silica gel with chloroform-benzene (1∶1) as the developing solvent reveals two blue main fractions with Rf values of 0.49 and 0.19, SBB-I and SBB-II respectively. Furthermore at least eighteen secondary fractions or impurities have been found. SBB-I and SBB-II were isolated and purified by preparative thin layer chromatography. Commercial Sudan Black B consists of about 20 p.c. SBB-I, 60 p.c. SBB-II and 20 p.c. secondary fractions.
From spectrophotometrical and histochemical investigations it appeared that SBB-I stains lipids more pronounced than SBB-II; moreover SBB-I is more specific for neutral lipids than SBB-II, which fraction may also stain some proteins and acid mucopolysaccharides. Contrary to SBB-II the staining with SBB-I is fairly independent of pH. Finally, the colour of SBB-II changes under the influence of light and air, while SBB-I is much more stable.
A physico-chemical study of the nature of SBB-I and SBB-II, including spectrophotometry, chromatography, infrared spectroscopy and chemical analysis revealed, that SBB-II is a basic dye, while SBB-I in spite of the structural resemblance behaves as a neutral one, dissolving therefore better in neutral lipids.
As yet the chemical composition of SBB-I and SBB-II, and the relation to the scheme of synthesis of Sudan Black B has not been solved. The unspecificity of lipid staining by Sudan Black B is due to the basicity of SBB-II, and to the instability of this dye toward light and air. Moreover the some eighteen impurities may have some influence on the staining properties. The question of solubility or adsorption processes in the case of lipid staining by Sudan dyes is at least partially answered by the proposition of a dissolving fraction SBB-I and an adsorbed fraction SBB-II. The changing absorption spectra by the corresponding solvatochromic and metachromatic effects may give information about the nature of the lipids stained.