[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The concentration of these metals increases while the

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The concentration of these metals increases while the engine is working. Thus, analysis of used oil in order to determine the concentration of metals has been successfully applied for many years in the monitoring of degree of use of the engine, often making possible the detection of damage or faulty working of the engine [686]. In criminalistic practice, a sample for identification and comparative exam- ination is obtained by extraction of a lubricant from blots visible on clothing, obtaining, in effect, one to several droplets of the sample. The high content of organic compounds and the considerable viscosity of the oil have always caused significant difficulties in the preparation of a sample for elemental analysis [683, 690, 691]. The main difficulty in obtaining repeatable results of analysis stems from the fact that the sample is a suspension of metal particles that in time undergo sedimentation. Therefore, analytical results are burdened with some errors. Better results can be achieved by preparing oil samples for examination by means of the incineration technique and then dissolving the obtained residue in a mineral acid. Formerly, atomic absorption spectrometry (AAS) or atomic emission spec- trometry (AES) were commonly used. Nowadays, a convenient method of elemental analysis of oil is X-ray fluorescence (XRF). Less sensitive in com- parison to AAS, it does, however, have one big advantage. This method makes it possible to analyse an oil sample directly in the unchanged state or possibly after dilution with an organic solvent [688]. However, the XRF method for the quantitative determination of elements requires complicated calibration with standard samples. So, comparison of the composition of oil samples is often performed by a nonquantitative method such as peak rationing. A model experiment was performed in order to establish the possibility of discriminating between used oil samples on the basis of their elemental composition [691]. Oil samples were withdrawn from the sumps of two cars after various time of exploitation. Elements originating both from the addi- tives and from the wearing away parts of the engine were determined in an oil sample. The semiquantitative method was applied, comparing character- istic signals for particular elements with the signal of a chosen element in the sample. It was observed that as the oil in the car was used up, the concentra- tions of, among other things, iron, cadmium, lead and copper increased. The concentration of these metals in oil is related to its degree of use, whereas the concentration of calcium, barium, magnesium, zinc and phosphorus is stable. Providing information on the kind and quantities of refining additives can be helpful in differentiating between types of oil. A schema of criminalistic inference on the basis of comparison of metal contents was proposed as well.
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