Mass spectrometry of avermectins: structural determination of two new derivatives of Ivermectin B(1a).

Fragmentation pathways of Avermectins were studied by electron impact (EI), chemical ionisation (CI), electrospray ionisation (ESI) and by collision experiments. Structure characterisation was obtained using ESI combined with multi-stage (MS(n)) tandem mass spectrometry, analysis of homologues, and effects on fragment masses of H/D exchange. By these approaches the structures of two new derivatives of Avermectins were characterised.