fragment ions for branched hydrocarbons

[Nomvalt]

6) Consider the structures of butane and methylpropane. Both of these molecules have the molecular formula C₄H₁₀.
b) Suggest the possible fragment ions that would remain if only one of the bonds is broken at a time in each molecule.

methylpropane fragment ions: CH₃C⁺H₂CH₂, CH₃CH₂⁺, CH₃⁺

I know what the fragment ions for butane would be and other straight chain hydrocarbons, however I often have trouble with working out the fragment ions for branched hydrocarbons. What is the reasoning behind this? Why are the fragment ions for methylpropane what they are?

[kyzoo]

Methylpropane = CH3.CH.(CH3).CH3

...I don't see where the CH3CH2+ fragment comes from

[Nomvalt]

Methylpropane = CH3.CH.(CH3).CH3

...I don't see where the CH3CH2+ fragment comes from

What about the other one? Why is CH₃C⁺H₂CH₂ a fragment ion for methylpropane? Why is it positive on the second carbon? How do you know which carbons/hydrogens to eliminate from the original formula for methylpropane?

[kyzoo]

Again, I don't see how CH3.CH2.CH3+ is a fragment for methylpropane, it's a fragment for butane though.

[Mao]

Again, I don't see how CH3.CH2.CH3+ is a fragment for methylpropane, it's a fragment for butane though.

CH3-C⁺H-CH3 is a fragment of methylpropane

CH3-C⁺H2-CH3 is not a fragment of anything
CH3-CH2-CH3+ is not a fragment of anything
CH3-CH2-CH2+ is a fragment of butane
CH3-CH2+ is a fragment of butane

To find what fragments you have, draw out the structure and cut any C-C bond, the two parts are the fragments. Repeat this process to find all the fragments (only cut one C-C at a time)