Gas chromatography works on the principle that different compounds will be more or less attracted to other materials. Gasses are pumped through a column and are attracted for a characteristic time period to materials in the column. As a result, different compounds exit the column at different times. A detector at the end of the column, often a flame- or photo-ionization detector, determines the amount of the compound exiting the column at its characteristic time, giving the concentration of that compound in the gas being analyzed.