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

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

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Gas Chromatography
Sample injection is considered successful if

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
the sample is homogenously spread along the column

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Gas Chromatography
Column bleeding occurs when

the column is cracked and stationary phase leaks out
traces of the stationary phase are eluted
elution of the analyte is extended over time
the column breaks during installation and causes personal injury

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