Gas Chromatography
Column bleeding occurs when

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

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Gas Chromatography
In column switching chromatography

the flow to the column is switched on and off repeatedly
one column is removed and replaced by another
any of the above
compounds trapped on one column are eluted to another column

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