Particle geometry and time of elution

[Venusroulette]

Hello,
I'm new with GC technique. Can you tell me why is there a difference between the retency time with n-butane and isobutane?I can't find anything I have two peaks and I'm not sure which one is n-butane and which one is isobutane. What determine the difference?
Thank u very much!

[Venusroulette]

In addition, we use DC200 in that measurement

[tom jupille]

Other things being equal, on a non-polar stationary phase, the order of elution is dominated by volatility of the analyte, so I would expect isobutane (BP = -12 deg C) to elute before n-butane (BP = -1 deg C).

[Venusroulette]

Thank u very much!