Almost all elements in the periodic table are found in petroleum products, varying from percent levels for C-H-S to parts per million and parts per billion levels for transition metals. The presence of elements in crude oil is ascribed to its marine animal and vegetative origin. Compared with two other fossil fuels — coal and oil shale — petroleum is a lesser source of environmental pollution. Extensive data on crude oils composition are available. Determination of S, N, V, Ni, and Fe form an integral part of crude assay analysis of oils carried out by all oil companies. Generally it is agreed that V, Ni, and Fe occur as metallo-porphyrins; Hg, Sb, and As as organometallic compounds; Mo and Ge as carboxylic acid salts; and silica and NaCl as colloidal minerals.
The significance of metals either naturally present in crude and gasoline or purposely added to petroleum products is described. Trace metals in used lubricating oils indicate the wear performance of an engine, and can be used as a preventive maintenance check. A number of sample preparation schemes are necessary depending on the analytical technique to be used. This can vary from procedures as simple as dilution for XRF or AAS, to more involved procedures such as oxygen bomb combustion for ion chromatography, or wet ashing for AAS or ICPAES. Frequently, robots are used to speed sample dilution for XRF or ICPAES.
For quality assurance of these analyses, standard reference materials (SRMs) are used. At least three sources of these materials are Alpha Resources, Leco Corporation, and the National Institute of Standards and Technology. SRMs for crude oil, diesel oil, fuel oil, kerosene, lube oil, paraffin oil, and petroleum coke are currently available.