FIG. 4.

Characterization of fatty acids from a crude preparation of N33 Nod factors by negative-ion GC-MS of their perfluorobenzyl derivatives. (a) Total ion current (GC-MS profile). Peaks labeled by asterisks corresponded to fatty acids not characterized as Nod factor substituents by examination of the LSIMS mass spectra. This profile was recorded in the negative-ion electron capture mode. i, iso. (b) MS-MS profile (CID monitored ion kinetic energy spectrometry) of the carboxylate ion at m/z 267 from 15-methyl-2-hexadecenoic acid (iso-C_17:1 Δ²). The low abundance of the ion at m/z 237 (loss of ethane) demonstrated the presence of a methyl group on C₁₅. The higher abundance of the ion at m/z 83 (allylic cleavage), associated with a lack of ions at lower m/z values, was characteristic of Δ² unsaturations.