This invention generally relates to a method and apparatus for direct detection of one or more markers in pressurized hydrocarbon fluids. The apparatus includes a vessel (130) and one or more valves (150, 160, 170). The hydrocarbon fluid (240) may be a liquid or gas. Markers may include a variety of optical markers, such as fluorescent markers. The apparatus may be coupled to a hydrocarbon fluid source (210), and the vessel (130) may be at least partially filled with hydrocarbon fluid from the hydrocarbon fluid source (210). A> detector (140) coupled to the vessel (130) may be used to detect at least one of the markers in the hydrocarbon fluid (240) while the apparatus is coupled to the hydrocarbon fluid source (210) and without the addition of reagents. Detection of markers may include, for example, fluorescence detection.