Alkynes typically fragment to form the 3-propynyl cation or propargyl cation, which is resonance-stabilized.

The alkynes can be either terminal or internal.

In terminal alkynes, the 3-propynyl cation signal appears at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted and appears at a higher mass-to-charge ratio.

Another type of fragmentation frequently observed is the loss of the hydrogen atom from the terminal alkyne carbon bearing the triple bond.

So, terminal alkynes exhibit an intense mass signal at a mass-to-charge ratio of 1 u lower than the molecular ion of the analyte molecule. This peak is denoted as the M–1 peak.