How does oxygen actually damage the polysiloxane phase in capillary GC columns?

The presence of oxygen in the carrier gas in combination with elevated temperatures can "unzip" the polysiloxane bonded phase of a GC column, resulting in column bleed.

Sources of oxygen include the following:

• Contamination in carrier gas - solution is to use traps that remove oxygen (water and hydrocarbons are other contaminants that should be removed from carrier gas) and change them as needed
• Loose fittings - oxygen can enter columns and increase bleed; so ensure fittings are tight. With Vespel ferrules, they shrink with increasing temperatures, so always be sure to check them after oven comes up to temperature.