Instrument: Micromass chemical ionization (methane)*
Mass spectrum:163(M+1, 43%,161 (M-1, 100%), 145 (M-17, 35%
129 (M-33, 30%), 75 (C3H7S, 75%), 73(C3H5S, 60%)

Surprisingly, the base peak obtained was not M+1, but M-1. Also appeared relatively large peaks at 145 and 235

The peak at 145 admits two interpretations
I have imagined a mechanism in which the protonated allicin undergo an internal hydrogen abstraction followed by water elimination to form an allyl cation particulary stable.
The peak at 161 could be produced by hydrogen abstraction by an unknown species.

A scheme of the proposed mechanism is shown below.

Another possibility is the decomposition of allicin during the storage period previous to the mass spectrum with formation of thioacrolein which condense in a DIELS-ALDER mode to yield a dithiin (MH+145)

The peak at 235 could correspond to (E,Z) ajoene. This condensation product could have been previously formed during the the allicin storage in dichloromethane used for extraction.

*Mass spectrum made at the Faculty of Pharmacy of Sevilla

Cruz Villalon, G. 2010 Ph.D.mass spectrum (thesis)

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