Question

Which of the following influences the rate at which a molecule elutes from a GC column?

a)		The size of the molecule.
b)		The polarity of the molecule.
c)		The vapour pressure of the molecule.
d)		All of the above.
c)		None of the above.

Answer 1

d) All the above

Polarity : One of the key factors when setting up a GC method is to choose the polarity of the stationary phase. The polarity is chosen using knowledge of the sample matrix and what separation is required. If the polarity of the target compound and the stationary phase are similar, then there is likely to be a greater interaction between the two. Consequently, the retention time will be longer for polar compounds on polar stationary phases and shorter on non-polar stationary phases.

Vapor pressure & size of molecule: If a component has a low boiling point, then it is likely to spend more time in the gas phase. Therefore its retention time will be lower than a compound with a higher boiling point. A compound’s boiling point can be related to its polarity. Boiling point as we know is affected by vapor pressure and size of molecule