Gas Chromatography
Helium is generally preferred as carrier gas over nitrogen and hydrogen because

it doubles up as a party gas for balloons and funny voices
it has a lower viscosity
all of above
it is inert

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Gas Chromatography
What does the selectivity factor describe?

The relative separation achieved between two species
The proportional difference in widths of two chromatographic peaks
The maximum number of different species which a column can separate simultaneously
None of these

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