Direct CIMS for the determination of mixtures of fatty alcohols from octanol to octadecanol

Abstract

The possibility of using direct chemical-ionization mass spectrometry to determine fatty alcohols in technical mixtures was examined. CH 4, i-C 4H 10, H 2S, NH 3 and (CH 3) 3− n NH n were examined as reactant gases. Methane, because its proton affinity is significantly lower than those of the alcohols, and isobutane lead to an overlapping pattern of fragment ions, which prevents evaluation of the spectra. H 2S does not form such interfering fragment ions; it forms only the quasi-molecular ions [ M − 17] +, which serve for the quantitative detection, and [ M − 1] + at low intensity. The detection limits of the alcohols in methanol are between 0.007 and 0.025%. NH 3 and the amines (CH 3) 3− n NH n also generate no fragment ions. Instead, they form adduct ions of the form [ROH···H n+1 N(CH 3) 3− n ] +, where n = 0–3. The catalytic action of the fatty alcohols in the formation of the dimers ((CH 3) 3− n NH n ) 2H + does interfere here, however. The detection limits by NH 3-CIMS of the alcohols in a mixture are between 0.01 and 0.02%.