A technique, including associated method and system, for on-line measurement of a trace element in a crude or heavy fuel stream for a refinery, including in one embodiment: providing at least one x-ray fluorescence (“XRF”) analyzer at a point for the refinery; analyzing the petroleum stream for chlorine using the analyzer; and providing results from the analyzer to refinery operators, to improve refinery operations. The analyzer may be a monochromatic wavelength XRF analyzer, wherein the analyzer focuses energy to/from the stream using an x-ray engine having at least one focusing, monochromating x-ray optic. The analyzer may be an MWDXRF or ME-EDXRF analyzer; and the trace element may be one or more of the following elements: S, Cl, P, K, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, Hg, As, Pb, and Se; and in one embodiment the stream is crude, and the trace element is chlorine.