[wsytnik]

The question reads: The base peak in the mass spectrum of propanone (acetone) occurs at m/z  43. What cation does this peak represent? Draw the two contributing resonance structures of this cation in the window below. Draw only the two resonance structures. Do not separate them with '+' or any other symbol.

So I believe the 43 is an isopropyl with a charge on the secondary carbon.  I dont understand how to draw the two resonance structures though.  Wouldn't it be a double bond between either of the carbons?  thanks for any help.