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

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

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Gas Chromatography
Sample injection is considered successful if

the sample is homogenously spread along the column
all of the sample in the injector has been added to the column
the sample is concentrated at the start of the column
the sample is spread evenly along the column

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