Formation mechanism of solvent induced artifact arising from chromotographic purity testing of gamma-irradiated chloramphenicol

Different gamma -irradiated chloramphenicol (CAP) samples were screened for impurities by recording melting point, solubility, pH and occurrence of ad ditional 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(j) 0.7) which had an intensity > 5% of the educt spot, depend ing on the irradiation dose, was eluted and concentrated from preparative T LC plates. The structure was confirmed by IR and H-1 or C-13-NMR and determ ined as the cyclic ketale condensation product of acetone and chloramphenic ol. Formation of this by-product, which is acid-catalyzed, was determined a s an artifact arising from purity testing. Not the gamma -irradiation proce dure itself but traces of acid in combination with acetone as the TLC solve nt led to this condensation product which is especially detectable for smal l 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 a rtifact from chromatographic purity investigations.