Gas chromatography, specifically gas-liquid chromatography, involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of an inert, gaseous mobile phase. The column itself contains a liquid stationary phase that is absorbed onto the surface of an inert solid. The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. Gas-liquid chromatography is the most widely used chromatographic technique for environmental analyses. Analysis of organic compounds is possible for a variety of matrices such as water, soil, soil gas, and ambient air. It is often used to test hazardous waste sites for determining personal protective equipment (PPE) levels and emergency response testing.