The primary factor that determines the elution order of compounds during gas chromatography, is the boiling point of the compounds. The lower the boiling point of a substance, the shorter retention time the substance will have. The retention time is the time it takes for the substance to be injected into the GC and reach the detector.
A lower boiling compound will elute faster (shorter retention time) than a higher boiling compound. The reason for this is the mobile phase of GC is a gas while the stationary phase is a liquid. Therefore, the more time a compound spends in the mobile phase, the faster it will elute.
The boiling point of anisole and d-limonene are 153.8 °C and 176 °C, respectively. Therefore, anisole will have a shorter retention time since it has the lower boiling point.
