On the stability of sesquiterpene lactones in the officinal arnica tincture of the German pharmacopoeia

An investigation of sesquiterpene lactone content in Arnica tincture (German Pharmacopoeia) after storage for three years at different temperatures (+4, +25 and +30 degrees C) was carried out by GC and GC-MS analysis. A decrease (13, 32 and 37%, respectively) in the content of the main active compounds in this preparation, 11 alpha,13-dihydrohelenalin esters, correlating with storage temperature was found. This change was shown to be caused by addition of ethanol to the cyclopentenone structure of these molecules leading to 2-ethoxy-2,3,11,13-tetrahydrohelenalin derivatives.