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Examination of used motor oils by flame AAS for criminalistic purposes: a diagnostic study
Forensic Science International 91 (1998) 171–179
Examination of used motor oils by flame AAS for criminalistic purposes: a diagnostic study J. Zie¸ba–Palus Institute of Forensic Research, Westerplatte 9, 31 -033 Cracow, Poland Received 27 May 1997; accepted 17 November 1997
Abstract Two kinds of motor oil were examined with respect to changes in their elemental composition during car driving. The concentrations of Ba, Ca, Mg, Zn, Cu, Fe, Pb and Cd in oil samples were determined by flame atomic absorption spectrometry. Significant differences in the metal concentrations of oil samples used during normal car service were observed. Quantitative elemental composition of oils can be recommended as a good feature for differentiation between various oil samples for criminalistic purposes. 1998 Elsevier Science Ireland Ltd. Keywords: Lubricating oils; Degradation; Metals determination; AAS
1. Introduction The wide occurrence of motor oils as stains on clothing makes them important contact materials in forensic science. While the major component of these materials is hydrocarbon, their most important characteristics are the additives. These additives fall into several groups that may be summarised as anti-oxidants, detergents, anti-wear agents, dispersants, corrosion inhibitors and viscosity index improvers. During use some of the additives are broken down and the breakdown products can be characteristic of the particular oil. Degradation of additives and differentiation of oil samples at various degrees of their use can be observed and studied by means of e.g. FTIR spectroscopy [1]. An elemental analysis of oils has special importance. Examination of the concentration of various metals in used lubricating oils is thought to be an effective and practical means of monitoring engine wear, often giving an early indication of a component failure. 0379-0738 / 98 / $19.00 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S0379-0738( 97 )00190-4
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J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
The concentration of certain metals in lubricating oils can be determined in terms of trace element analysis. Atomic absorption spectrometry (AAS) is the most often used analytical method for the determination of metals [2]. The oil sample itself presents handling difficulties, as it may occur in the form of an extremely viscous liquid. In the literature a number of analytical procedures have been described. Most of them are based on ashing or dilution of the oil sample with an organic solvent. Vigler [3] determined 25 elements in petroleum products after sample ashing in the presence of sulphuric acid. Barbooti [4] determined Cr, Fe, Mg and Pb in used lubricating oils using flame AAS after three types of sample preparation: direct dilution with an organic solvent, dry ashing-acid dissolution and dry ashing in the presence of a porous inert material (silica gel). He stated that direct dilution could be applied to highly volatile metals such as Pb which could be lost during heating. Dry ashing was useful for most metals provided that the rate of heating was slow. The presence of silica gel prevented sputtering and volatilisation of the sample. There are some techniques for the determination of Ca, Mg and Zn in oils in which an organic solvent is used as a medium for spraying directly into the flame. Some authors suggest diluting oil with xylene or MIBK [5]. As inorganic standards have obvious advantages over the organometallic ones, being cheaper and readily available, Holding and Matthews [6] developed a mixed-solvent system that permits the use of inorganic standards for the determination of Ca and Zn present as additives at the g / kg level in lubricating oils. However, this method is not suitable for the determination of wear metals present at the parts per million level. Hon and Lau [7] proposed another solvent system that enabled aqueous inorganic standards to be used for the direct determination of both metal additives and wear metals in oils. The samples were diluted with isobutyric acid. Electrothermal AAS is sometimes preferred to flame AAS procedures as it provides better precision for some metals, i.e. Fe and Cu. Saba [8] determined the ten wear metals in aircraft lubricating oils by AAS using a graphite furnace atomizer. The oil samples were diluted with kerosene (1:4) and 0.5–20 ml of a sample were injected to the furnace. The alkylo-arylosulphonate of metals were used as standards. He stated that the precision was worse than this for conventional methods because of the large volume of the sample and the presence of undissolved metal particles. Although AAS is still widely used, other newer techniques such as inductively coupled plasma atomic emission (ICP) [9], neutron activation [10] and energy dispersive X-ray fluorescence spectrometry also play an important role in solving analytical problems of oils. King [9] compared results of wear metal determination in used oils obtained by flame AAS and ICP. He stated that ICP is more sensitive for most elements, especially for P and Ba. The determination of metal contents in oil seems to be useful in criminalistic examination, especially in differentiation between oil samples. However, very little information concerning this problem is available in the literature. The only publication we found was that by Espinoza [11], who applied XRF techniques to the determination of Zn in automotive and locomotive lubricant oils for toxicological purposes. He stated that the presence of Zn is a good feature for differentiation between samples of this type of material.
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
173
The aim of this work was to observe, by means of the widely used flame AAS, how great are the differences between the elemental contents of different oils and how great are the changes in metal concentration of oil samples used during normal car service. The results obtained were expected to be useful for criminalistic purposes, giving information on whether the metal concentration could be an appropriate feature for differentiating one particular oil sample from the others.
2. Experimental
2.1. Apparatus A Pye–Unicam SP-9800 atomic absorption spectrometer equipped with a background correction unit and an automatic gas control unit was used with an air–acetylene flame and other conditions described in the operations manual. Hollow-cathode lamps were used as radiation sources.
2.2. Sample preparation Lotos 15w / 40 and GTX Castrol motor oils were placed in two cars, respectively. Samples of the oils were withdrawn from the sumps for analysis at intervals, usually after the cars had travelled a distance of 500 or 1000 km. The sumps were not filled up with oil during the whole process. The experiment was terminated after the cars had travelled a distance of 5000.
2.3. Analytical procedure The ashing technique was chosen to destroy the organic matrix because aqueous standard solutions of the metals were more available. Although some of the volatile elements could be lost during ashing of the oil samples and the determined concentrations of these metals may differ from the actual values, constant experimental conditions were kept; hence, the comparative examinations, common in criminalistics, were expected to provide satisfactory results. The following procedure was followed to prepare samples for examination. Approximately 3 g of the oil sample were weighed accurately and placed in a silica crucible on the hotplate at approximately 150–2008C in order to allow slow charring of the sample to occur. After charring was completed the crucible was heated in a muffle furnace set at about 5008C until all carbon had been removed. To the cooled ash, 5 ml aliquot of HNO 3 (111) was added to effect dissolution (warming was necessary). The sample solution was transferred quantitatively into a 10 ml volumetric flask and made up to the mark with deionised water. The samples for AAS measurements were prepared by dilution of these solutions 20, 50 or 100 times, according to the kind of metal being determined, using deionised water. All reagents used were of analytical-reagent grade. Barium, calcium, zinc and magnesium originating from additives and iron, copper,
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lead and cadmium from motor wear were chosen for determination because their concentrations were expected to be the most indicative for degradation processes of oils. Aqueous standard stock solutions were used for calibration.
3. Results and discussion
3.1. Verification of the analytical method The analytical method was initially tested using three different kinds of fresh oils. Three samples of each oil were taken for the examination. Each sample was individually treated according to the procedure described above, and then adequately diluted and measured. The mean values of the metal concentrations found and the relative standard deviations (RSD) of the results obtained for each kind of oil are given in Table 1. The method proved to be quite satisfactory in terms of precision. Relatively worse reproducibility was revealed for Fe and Cd but such results could be explained by the low concentration of these metals in the samples. In general, the analytical method could be considered as fully suitable for the purposes desired.
3.2. Examination of the used oils The mean values of the metal contents found in the Lotos and Castrol oil samples examined are listed in Tables 2 and 3. The results obtained, similarly to those presented in Table 1, justify the statement that the metal contents of various fresh oils differ significantly from each other. During consumption of the oils, their elemental compositions changed further in regard to concentration of some metals only. However, the observed differences were small in some cases. In both oils examined, the group of metals originating from additives remained at approximately the same concentrations as in the fresh oils, but the group of wear metals changed in concentration progressively.
Table 1 Element concentration in fresh motor oil of Lotos family Sample
x RSD [%] x RSD [%] x RSD [%]
Metal concentration Zn
Mg
Ca
Ba
Cu
Cd
Fe
Pb
640.42 0.08 2204.51 0.03 1579.91 0.71
489.88 0.07 12.60 0.20 9.15 0.96
395.62 0.18 6704.25 0.01 3564.99 0.18
8.85 0.26 29.62 0.74 30.54 1.17
0.18 4.50 0.26 2.25 0.21 2.71
0.15 1.17 0.13 4.56 0.18 3.15
2.50 1.66 3.58 7.13 2.58 6.10
2.50 0.81 2.83 2.83 1.67 0.91
x: the mean of three results. RSD: relative standard deviation.
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
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Table 2 Element concentration in LOTOS 15 W/ 40 motor oil Distance [km]
0 5 600 1000 1550 2000 3000 5000
Concentration [mg / g] Zn
Mg
Ca
Ba
Cu
Cd
Fe
Pb
2116.50 1939.14 1689.50 1648.03 1626.50 1944.91 1970.66 2011.32
16.01 179.47 195.89 198.83 239.80 307.36 319.95 325.12
5083.86 3908.20 3441.66 3409.21 3399.16 4064.87 4119.99 4131.51
27.31 30.57 34.51 35.66 36.87 37.13 37.57 37.88
0.46 5.66 21.39 27.01 33.69 34.65 34.67 34.72
0.15 0.76 1.13 1.17 1.20 1.27 1.28 1.29
1.29 18.24 36.74 37.85 47.70 87.97 97.74 102.85
2.80 424.12 792.99 905.24 1076.51 1521.96 1774.83 1798.87
3.2.1. Metals originated from additives ( Zn, Ca, Mg and Ba) Contents of zinc, calcium and barium in oil samples of different degrees of decomposition remained almost at the same level. This is clearly seen in Fig. 1 where the current concentrations of these metals are related to those amounts reached after 5000 km. In contrast, the magnesium content rises regularly due to driving, revealing the greatest increase during the first tens of kilometres. This effect could be explained by the wear of bearings and transition of metal particles to oil during normal car service.
3.2.2. Metals originated from motor elements (wear metals) Cu, Cd, Fe and Pb The concentration of copper, cadmium, lead and iron increased regularly due to driving. It is seen in Fig. 2 that the greatest changes occur during the first 2000 kms and thereafter they tend to be smaller. This effect concerns primarily the concentration of Fe, Cu and Pb, and is less visible in the case of Cd, which was determined at levels close to the detection limit. Nevertheless, cadmium should be taken into consideration as anticorrosion coatings on metal parts of engine contain this metal, hence it may appear in oil owing to the wear process of motor. The changes of Fe and Cu contents can be caused by wear of the metal motor elements during car use, including cylinder surfaces (Fe), engine (Fe) and pans (Cu). The increase of Pb concentration may originate from fuel containing compounds of Pb mixed with oil during driving as well as from the wear of bearings or from lubricants in the motor. Table 3 Element concentration in Castrol GTX motor oil Distance [km]
0 1000 4000 6000
Concentration [mg / g] Zn
Ca
Ba
Cu
Fe
Pb
1320 1200 1150 1020
1100 1200 1280 1400
90 80 95 98
0.5 1.0 1.0 2.5
1 10 31 48
3.2 10.8 10.7 15.9
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Fig. 1. The changes in Ba, Ca, Zn and Mg contents in samples of Lotos oil as a function of distance covered.
4. Conclusions From the criminalistic point of view the most valuable information is provided by the presence of such metals in the motor oils as zinc, calcium and barium, since their concentrations are believed to be different in various kinds of oils but very similar in various samples of the same kind of oil. The contents of such metals as lead, iron and copper are even more informative as they change when an oil is used. But the changes observed occur to great extent within a relatively short time of driving. Besides, the differences in mechanical conditions of different motors may have an influence on the degradation rate of the same kind of oil being used by them. Nevertheless, the following general conclusions can be formulated on the basis of the investigations performed: • Quantitative element composition may be accepted as a good feature for differentiation between various oil samples.
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177
Fig. 2. The changes in Cu, Cd, Fe and Pb contents in samples of Lotos oil as a function of distance covered.
• The contents of Zn, Ca and Ba in samples of used oil may be the basis for establishing the kind of oil used in a particular car. • Observation of different amounts of Zn, Ca and Ba in two samples may indicate that they come from two different oils. • The same amount of Zn, Ca and Ba in two samples does not denote that they come from the same oil. Such a result could be considered as quite sure only if Pb, Fe and Cu are also determined at the same levels in both samples. • Different amounts of Zn, Ca and Ba as well as of Pb, Fe and Cu in two samples may indicate that they come from two different oils. However, this conclusion should be confirmed by some additional examination, as many factors can change the composition of the oil in different conditions. Taking all the above into consideration a schema of criminalistic inference can be suggested as presented in Fig. 3. It should be emphasized that these investigations were preliminary and have diagnostic character only. Further examinations including a larger number of various
178
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
Fig. 3. Schema of criminalistic inference on the basis of comparison of Zn, Ca, Ba, Fe, Cu and Pb contents in evidential and control oil samples, respectively.
kinds of oils as well as different car types are in progress and the results are expected to be useful and reliable for criminalistic practice.
References [1] J. Zie¸ba, Examination of lubricating oils by IR spectroscopy, Forensic Science International 27 (1985) 30–33. [2] S.J. Haswell, Atomic Absorption Spectrometry. Theory, Design and Applications. Elsevier, 1991. [3] M.S. Vigler, V.F. Gaylor, Trace metals analysis in petroleum products by AAS, Applied Spectroscopy 28 (1974) 342–344. [4] M.M. Barbooti, N.S. Zaki, S.S. Baha–Udolin, Use of silica gel in the preparation of used lubricating oil samples for the determination of wear metals by flame AAS, Analyst 115 (1990) 1059–1061. [5] M. Kashiki, S. Oshima, Determination of Ca, Zn, Ba in lubricating oils by AAS, Anal. Chim. Acta 55 (1971) 436–438. [6] S.T. Holding, P.H.D. Matthew, The use of the mixed-solvent system for the determination of Ca and Zn in petroleum products by AAS, Analyst 97 (1972) 189–194. [7] P.K. Hon, O.W. Lau, C.S. Mok, Direct determination of metals in lubricating oils using AAS and aqueous inorganic standards, Analyst 105 (1980) 919–921. [8] C.S. Saba, W.E. Rhine, K.J. Eisentraut, Determination of wear metals in aircraft lubricating oils by AAS using a graphite furnace atomizer, Applied Spectroscopy 39 (1985) 689–693. [9] A.D. King, D.R. Hilligass, G.F. Wallace, Comparison of results for the determination of wear metals in used lubricating oils by flame AAS and ICP, Atomic Spectroscopy 5 (1984) 189–191.
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[10] P.G. Shaw, D. McKnown, Trace element determination in shale oil products by neutron activation, Anal. Chim. Acta 123 (1981) 65–74. [11] E.O. Espinoza, M.J. Mann, L. Leonardo, A. Copeland, Differentiation of automotive and locomotive lubricant oil, J. Forens. Sci 39 (1994) 839–843.
Examination of used motor oils by flame AAS for criminalistic purposes: a diagnostic study J. Zie¸ba–Palus Institute of Forensic Research, Westerplatte 9, 31 -033 Cracow, Poland Received 27 May 1997; accepted 17 November 1997
Abstract Two kinds of motor oil were examined with respect to changes in their elemental composition during car driving. The concentrations of Ba, Ca, Mg, Zn, Cu, Fe, Pb and Cd in oil samples were determined by flame atomic absorption spectrometry. Significant differences in the metal concentrations of oil samples used during normal car service were observed. Quantitative elemental composition of oils can be recommended as a good feature for differentiation between various oil samples for criminalistic purposes. 1998 Elsevier Science Ireland Ltd. Keywords: Lubricating oils; Degradation; Metals determination; AAS
1. Introduction The wide occurrence of motor oils as stains on clothing makes them important contact materials in forensic science. While the major component of these materials is hydrocarbon, their most important characteristics are the additives. These additives fall into several groups that may be summarised as anti-oxidants, detergents, anti-wear agents, dispersants, corrosion inhibitors and viscosity index improvers. During use some of the additives are broken down and the breakdown products can be characteristic of the particular oil. Degradation of additives and differentiation of oil samples at various degrees of their use can be observed and studied by means of e.g. FTIR spectroscopy [1]. An elemental analysis of oils has special importance. Examination of the concentration of various metals in used lubricating oils is thought to be an effective and practical means of monitoring engine wear, often giving an early indication of a component failure. 0379-0738 / 98 / $19.00 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S0379-0738( 97 )00190-4
172
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
The concentration of certain metals in lubricating oils can be determined in terms of trace element analysis. Atomic absorption spectrometry (AAS) is the most often used analytical method for the determination of metals [2]. The oil sample itself presents handling difficulties, as it may occur in the form of an extremely viscous liquid. In the literature a number of analytical procedures have been described. Most of them are based on ashing or dilution of the oil sample with an organic solvent. Vigler [3] determined 25 elements in petroleum products after sample ashing in the presence of sulphuric acid. Barbooti [4] determined Cr, Fe, Mg and Pb in used lubricating oils using flame AAS after three types of sample preparation: direct dilution with an organic solvent, dry ashing-acid dissolution and dry ashing in the presence of a porous inert material (silica gel). He stated that direct dilution could be applied to highly volatile metals such as Pb which could be lost during heating. Dry ashing was useful for most metals provided that the rate of heating was slow. The presence of silica gel prevented sputtering and volatilisation of the sample. There are some techniques for the determination of Ca, Mg and Zn in oils in which an organic solvent is used as a medium for spraying directly into the flame. Some authors suggest diluting oil with xylene or MIBK [5]. As inorganic standards have obvious advantages over the organometallic ones, being cheaper and readily available, Holding and Matthews [6] developed a mixed-solvent system that permits the use of inorganic standards for the determination of Ca and Zn present as additives at the g / kg level in lubricating oils. However, this method is not suitable for the determination of wear metals present at the parts per million level. Hon and Lau [7] proposed another solvent system that enabled aqueous inorganic standards to be used for the direct determination of both metal additives and wear metals in oils. The samples were diluted with isobutyric acid. Electrothermal AAS is sometimes preferred to flame AAS procedures as it provides better precision for some metals, i.e. Fe and Cu. Saba [8] determined the ten wear metals in aircraft lubricating oils by AAS using a graphite furnace atomizer. The oil samples were diluted with kerosene (1:4) and 0.5–20 ml of a sample were injected to the furnace. The alkylo-arylosulphonate of metals were used as standards. He stated that the precision was worse than this for conventional methods because of the large volume of the sample and the presence of undissolved metal particles. Although AAS is still widely used, other newer techniques such as inductively coupled plasma atomic emission (ICP) [9], neutron activation [10] and energy dispersive X-ray fluorescence spectrometry also play an important role in solving analytical problems of oils. King [9] compared results of wear metal determination in used oils obtained by flame AAS and ICP. He stated that ICP is more sensitive for most elements, especially for P and Ba. The determination of metal contents in oil seems to be useful in criminalistic examination, especially in differentiation between oil samples. However, very little information concerning this problem is available in the literature. The only publication we found was that by Espinoza [11], who applied XRF techniques to the determination of Zn in automotive and locomotive lubricant oils for toxicological purposes. He stated that the presence of Zn is a good feature for differentiation between samples of this type of material.
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
173
The aim of this work was to observe, by means of the widely used flame AAS, how great are the differences between the elemental contents of different oils and how great are the changes in metal concentration of oil samples used during normal car service. The results obtained were expected to be useful for criminalistic purposes, giving information on whether the metal concentration could be an appropriate feature for differentiating one particular oil sample from the others.
2. Experimental
2.1. Apparatus A Pye–Unicam SP-9800 atomic absorption spectrometer equipped with a background correction unit and an automatic gas control unit was used with an air–acetylene flame and other conditions described in the operations manual. Hollow-cathode lamps were used as radiation sources.
2.2. Sample preparation Lotos 15w / 40 and GTX Castrol motor oils were placed in two cars, respectively. Samples of the oils were withdrawn from the sumps for analysis at intervals, usually after the cars had travelled a distance of 500 or 1000 km. The sumps were not filled up with oil during the whole process. The experiment was terminated after the cars had travelled a distance of 5000.
2.3. Analytical procedure The ashing technique was chosen to destroy the organic matrix because aqueous standard solutions of the metals were more available. Although some of the volatile elements could be lost during ashing of the oil samples and the determined concentrations of these metals may differ from the actual values, constant experimental conditions were kept; hence, the comparative examinations, common in criminalistics, were expected to provide satisfactory results. The following procedure was followed to prepare samples for examination. Approximately 3 g of the oil sample were weighed accurately and placed in a silica crucible on the hotplate at approximately 150–2008C in order to allow slow charring of the sample to occur. After charring was completed the crucible was heated in a muffle furnace set at about 5008C until all carbon had been removed. To the cooled ash, 5 ml aliquot of HNO 3 (111) was added to effect dissolution (warming was necessary). The sample solution was transferred quantitatively into a 10 ml volumetric flask and made up to the mark with deionised water. The samples for AAS measurements were prepared by dilution of these solutions 20, 50 or 100 times, according to the kind of metal being determined, using deionised water. All reagents used were of analytical-reagent grade. Barium, calcium, zinc and magnesium originating from additives and iron, copper,
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
174
lead and cadmium from motor wear were chosen for determination because their concentrations were expected to be the most indicative for degradation processes of oils. Aqueous standard stock solutions were used for calibration.
3. Results and discussion
3.1. Verification of the analytical method The analytical method was initially tested using three different kinds of fresh oils. Three samples of each oil were taken for the examination. Each sample was individually treated according to the procedure described above, and then adequately diluted and measured. The mean values of the metal concentrations found and the relative standard deviations (RSD) of the results obtained for each kind of oil are given in Table 1. The method proved to be quite satisfactory in terms of precision. Relatively worse reproducibility was revealed for Fe and Cd but such results could be explained by the low concentration of these metals in the samples. In general, the analytical method could be considered as fully suitable for the purposes desired.
3.2. Examination of the used oils The mean values of the metal contents found in the Lotos and Castrol oil samples examined are listed in Tables 2 and 3. The results obtained, similarly to those presented in Table 1, justify the statement that the metal contents of various fresh oils differ significantly from each other. During consumption of the oils, their elemental compositions changed further in regard to concentration of some metals only. However, the observed differences were small in some cases. In both oils examined, the group of metals originating from additives remained at approximately the same concentrations as in the fresh oils, but the group of wear metals changed in concentration progressively.
Table 1 Element concentration in fresh motor oil of Lotos family Sample
x RSD [%] x RSD [%] x RSD [%]
Metal concentration Zn
Mg
Ca
Ba
Cu
Cd
Fe
Pb
640.42 0.08 2204.51 0.03 1579.91 0.71
489.88 0.07 12.60 0.20 9.15 0.96
395.62 0.18 6704.25 0.01 3564.99 0.18
8.85 0.26 29.62 0.74 30.54 1.17
0.18 4.50 0.26 2.25 0.21 2.71
0.15 1.17 0.13 4.56 0.18 3.15
2.50 1.66 3.58 7.13 2.58 6.10
2.50 0.81 2.83 2.83 1.67 0.91
x: the mean of three results. RSD: relative standard deviation.
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Table 2 Element concentration in LOTOS 15 W/ 40 motor oil Distance [km]
0 5 600 1000 1550 2000 3000 5000
Concentration [mg / g] Zn
Mg
Ca
Ba
Cu
Cd
Fe
Pb
2116.50 1939.14 1689.50 1648.03 1626.50 1944.91 1970.66 2011.32
16.01 179.47 195.89 198.83 239.80 307.36 319.95 325.12
5083.86 3908.20 3441.66 3409.21 3399.16 4064.87 4119.99 4131.51
27.31 30.57 34.51 35.66 36.87 37.13 37.57 37.88
0.46 5.66 21.39 27.01 33.69 34.65 34.67 34.72
0.15 0.76 1.13 1.17 1.20 1.27 1.28 1.29
1.29 18.24 36.74 37.85 47.70 87.97 97.74 102.85
2.80 424.12 792.99 905.24 1076.51 1521.96 1774.83 1798.87
3.2.1. Metals originated from additives ( Zn, Ca, Mg and Ba) Contents of zinc, calcium and barium in oil samples of different degrees of decomposition remained almost at the same level. This is clearly seen in Fig. 1 where the current concentrations of these metals are related to those amounts reached after 5000 km. In contrast, the magnesium content rises regularly due to driving, revealing the greatest increase during the first tens of kilometres. This effect could be explained by the wear of bearings and transition of metal particles to oil during normal car service.
3.2.2. Metals originated from motor elements (wear metals) Cu, Cd, Fe and Pb The concentration of copper, cadmium, lead and iron increased regularly due to driving. It is seen in Fig. 2 that the greatest changes occur during the first 2000 kms and thereafter they tend to be smaller. This effect concerns primarily the concentration of Fe, Cu and Pb, and is less visible in the case of Cd, which was determined at levels close to the detection limit. Nevertheless, cadmium should be taken into consideration as anticorrosion coatings on metal parts of engine contain this metal, hence it may appear in oil owing to the wear process of motor. The changes of Fe and Cu contents can be caused by wear of the metal motor elements during car use, including cylinder surfaces (Fe), engine (Fe) and pans (Cu). The increase of Pb concentration may originate from fuel containing compounds of Pb mixed with oil during driving as well as from the wear of bearings or from lubricants in the motor. Table 3 Element concentration in Castrol GTX motor oil Distance [km]
0 1000 4000 6000
Concentration [mg / g] Zn
Ca
Ba
Cu
Fe
Pb
1320 1200 1150 1020
1100 1200 1280 1400
90 80 95 98
0.5 1.0 1.0 2.5
1 10 31 48
3.2 10.8 10.7 15.9
176
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
Fig. 1. The changes in Ba, Ca, Zn and Mg contents in samples of Lotos oil as a function of distance covered.
4. Conclusions From the criminalistic point of view the most valuable information is provided by the presence of such metals in the motor oils as zinc, calcium and barium, since their concentrations are believed to be different in various kinds of oils but very similar in various samples of the same kind of oil. The contents of such metals as lead, iron and copper are even more informative as they change when an oil is used. But the changes observed occur to great extent within a relatively short time of driving. Besides, the differences in mechanical conditions of different motors may have an influence on the degradation rate of the same kind of oil being used by them. Nevertheless, the following general conclusions can be formulated on the basis of the investigations performed: • Quantitative element composition may be accepted as a good feature for differentiation between various oil samples.
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
177
Fig. 2. The changes in Cu, Cd, Fe and Pb contents in samples of Lotos oil as a function of distance covered.
• The contents of Zn, Ca and Ba in samples of used oil may be the basis for establishing the kind of oil used in a particular car. • Observation of different amounts of Zn, Ca and Ba in two samples may indicate that they come from two different oils. • The same amount of Zn, Ca and Ba in two samples does not denote that they come from the same oil. Such a result could be considered as quite sure only if Pb, Fe and Cu are also determined at the same levels in both samples. • Different amounts of Zn, Ca and Ba as well as of Pb, Fe and Cu in two samples may indicate that they come from two different oils. However, this conclusion should be confirmed by some additional examination, as many factors can change the composition of the oil in different conditions. Taking all the above into consideration a schema of criminalistic inference can be suggested as presented in Fig. 3. It should be emphasized that these investigations were preliminary and have diagnostic character only. Further examinations including a larger number of various
178
J. Zie¸ba–Palus / Forensic Science International 91 (1998) 171 – 179
Fig. 3. Schema of criminalistic inference on the basis of comparison of Zn, Ca, Ba, Fe, Cu and Pb contents in evidential and control oil samples, respectively.
kinds of oils as well as different car types are in progress and the results are expected to be useful and reliable for criminalistic practice.
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