Formation mechanism of solvent induced artifact arising from chromatographic purity testing of γ-irradiated chloramphenicol

Different γ-irradiated chloramphenicol (CAP) samples were screened for impurities by recording melting point, solubility, pH and occurrence of additional spots on thin-layer plates according to the European Pharmacopoeia. Significant decomposition was detected as an intense spot in the TLC-test. The spot (r f 0.7) which had an intensity >5% of the educt spot, depending on the irradiation dose, was eluted and concentrated from preparative TLC plates. The structure was confirmed by IR and1H or13C-NMR and determined as the cyclic ketale condensation product of acetone and chloramphenicol. Formation of this by-product, which is acid-catalyzed, was determined as an artifact arising from purity testing. Not the τ-irradiation procedure itself but traces of acid in combination with acetone as the TLC solvent led to this condensation product which is especially detectable for small levels of protons (ca. 10−4 mol L−1) stemming from hydrolysis of CAP or decomposition of residual solvents (CHCl3) at low irradiation doses (8 kGy). Substitution of acetone by methanol solvent avoids this and misleading artifact from chromatographic purity investigations.