Fuel marking is a process of adding a unique identifier (marker) to petroleum products at the point of production or importation. The marker is a harmless, chemical compound that is difficult to remove and can be detected in very small quantities. Fuel markers are typically added to fuel at the point of production or importation. They are then dispersed throughout the fuel using special mixing equipment. Fuel markers can be detected using a variety of methods, including gas chromatography-mass spectrometry (GC-MS), X-ray fluorescence (XRF), and portable field analyzers. Fuel marking is a widely used and effective way to combat fuel adulteration and smuggling, ensure fuel quality, and collect excise taxes. It is used by governments and oil companies in many countries around the world.