NOVELTY - A composition comprises a solution of at least one organosiloxane (0.5-3 wt.%) free of elements other than silicon, oxygen, carbon and hydrogen, having a boiling point of 35-375 degrees C, in an organic solvent having a boiling point of 64-125 degrees C. The solvent is free of elements other than oxygen, carbon and hydrogen.    USE - For calibrating an X-ray silicon analyzer; measuring silicon in a fuel mixture (claimed); and analyzing silicon levels in hydrocarbons or biofuels, e.g. refinery laboratories, pipeline terminals, additive plants and inspection laboratories.    ADVANTAGE - The existence of silicon in gasoline, for instance, can be detrimental to certain parts of an automobile engine, including oxygen sensors, catalytic converters and spark plugs. This leads to the unwanted cost of repairing and/or replacing these parts. The composition mitigates this damage by analyzing the amount of silicon present in petroleum products and other fuel mixtures.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) calibrating (M1) an X-ray silicon analyzer involving: providing the solution; and measuring the response to known concentration;    (2) measuring (M2) silicon in a fuel mixture involving: providing the solution, where the organosiloxane has a boiling point matched to within the boiling point range of the fuel mixture; calibrating an X-ray silicon analyzer with the solution; providing the fuel mixture to the X-ray silicon analyzer; and measuring the silicon concentration in the fuel mixture;    (3) system for practicing the method (M1); and    (4) system for measuring silicon in a fuel mixture comprising: an X-ray silicon analyzer, calibrated with the solution, where organosiloxane has a boiling point matched to within the boiling point range of the fuel mixture; and the fuel mixture, where the X-ray silicon analyzer measures silicon in the fuel mixture responsive to providing the fuel mixture to the X-ray silicon analyzer.