One of the most sensitive gas chromatographic detector for halogen-containing compounds like chlorofluorocarbons. Here's an animation. James Lovelock's early work with this detector led to the discovery that anthropogenic chlorofluorocarbons used most heavily in the Northern hemisphere were very long-lived and therefore well-mixed throughout the troposphere. Later work by Rowland and Molina suggested that CFCs also diffused into the stratosphere where their chlorine atoms could by freed by photolysis-CFC bond breaking by UV light. The result of this is the destruction of ozone molecules in the important ozone layer of the stratosphere.