Osmometric molecular mass determinations of neutral medium-polar model compounds in the range of 1000-1500 could be carried out with an error of less than 8%. Electron impact (El) mass spectrometry failed. The fast atom bombardment (FAB) method provided MHQ or (M -H)Q ions but with low abundance.
Molmassenbestimmung einiger Organischer
Verbindungen im Bereich von 1500 An Hand neutraler, maBig polarer Modellverbindungen im Bereich von 1000 -1500 gelingt die osmometrische Molmassenbestimmung mit einem Fehler von weniger als 8%. Die Elek-tronenstoB-Massenspektrometrie (EI) versagte; die Fast-Atom-Bombardment-(FAB-)Methode lieferte MHO-oder (M -H)O-Tonen mit z. T. geringer Intensitat.
During structure elucidation of an amorphous green fungus pigment from Roesleria hypogea ') (an ascomyces in the family Caliciaceae), molecular mass determinations of solutions in chloroform were made by osmometry2). Values varying from 1358 to 1472 were obtained from different samples; in acetone solution a value of 1628 was obtained. Therefore, a proof of accuracy for the method in this range was necessary. For compounds with molecular masses up to 300-400 an error of + _ 2 % is the average in our laboratory but the potential error clearly increases with increasing molecular mass.
Such an effect has been observed applying cryoscopic methods to large organic molecules3). The potential errors can of course be minimized by extrapolation from different concentrations, as described in the literature4).
In this communication, molecular mass determinations of some model compounds 1-4 (see Table ) by osmometric measurements have been compared with data obtained from EIand FAB-mass spectrometry.
From the Table it is evident that conventional osmometry provides correct molecular masses with an error of less than 8% for crystalline well-dried compounds. The data for amorphous materials are too small on account of occluded solvent molecules.