the present invention relates to methods and means including a process and system for producing an elemental - mercury - free reference flue gas which is used to obtain background photometric measurements thereby eliminating the photometric interferences of flue gas components to the photometric measurement of mercury . in the process , elemental mercury is selectively removed from a portion of a gaseous stream and the intensity of radiation absorbed is measured by a uv spectrometer on the resulting mercury - free reference gas , and thereafter the concentration of mercury in the remainder of the gaseous stream is determined through utilization of the ratio of the transmitted radiation of the mercury - containing flue gas with respect to that of the mercury - free reference portion of the gaseous stream . the mercury concentration can then be determined without interference by other absorbing moieties at 254 nm in the flue gas . the gist underlying the inventive concept of the instant invention relates to use of a porous bed of gold - coated material to effectively remove mercury from the reference portion of the gaseous stream without altering the composition of other radiation - absorbing components therein at the wavelengths utilized for measuring mercury . an important feature of this process is that it can be used to measure elemental mercury in a flue gas upon extractive sampling from a flue stack or after the extractive sample undergoes a process to convert the mercury species in the stream to elemental mercury prior to a total mercury measurement . the ability of the instant invention to monitor mercury in the presence of interfering agents is an essential feature for mercury emission control and for compliance by fossil - fueled industries and waste combustors .