Abstract: 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 (the reference portion) 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.