[3,4-D2]Thiophen: synthesis and scrambling on electron impact

[3,4-D,]Thiophen (1) has been prepared by hydrogen-deuterium exchange reactions using trifluoroacetic acid. [2,5-D2]Thiophen (2) and (1) show an unexplained difference in the amount of scrambling in the formation of the CHS+ ion on electron impact. This difference is maintained when the electron beam and ion draw-out voltages are varied.