CHAPTER-6 ELEMENTAL ANALYSIS OF LUBRICATING OIL SAMPLES USING EDXRF TECHNIQUE

Transcription

1 CHAPTER-6 ELEMENTAL ANALYSIS OF LUBRICATING OIL SAMPLES USING EDXRF TECHNIQUE 6.1 INTRODUCTION MATRIX EFFECTS SAMPLE PREPARATION EXPERIMENTAL METHODS EVALUTION PROCEDURE RESULTS AND DISCUSSION 150 REFERENCES 152

2 6.1 INTRODUCTION Ever demanding environmental regulations determine the advances required in engine construction technology, emission after-treatment systems, fuels and lubricants to be used in vehicles. Besides providing lubrication, a little bit of lubricant oil always burns in the combustion chamber of the engine used in a vehicle and hence affects the exhaust gas after-treatment systems of vehicles. The economical and environmental requirements demands that the lubricanting oils with longer drain intervals shall be used with the lowest possible content of metals. The metallic content in lubricating oils is due to additives (detergents) or wear and tear of different parts of engine. The knowledge of elemental composition of oil can provide information regarding its degradation (decreased additives) with time as well as wear and tear of the engine. For example, the levels of wear metals such as iron, copper or aluminum can provide information regarding the condition of piston and cylinder. Similarly, the silicon (dirt) levels in lubricant oil can be used to monitor the air intake system of the engine. There are several methods for the quantitative and qualitative analysis of elements in new and used lubricating oils: inductively coupled plasma atomic/optical emission spectroscopy or mass spectroscopy (ICP-AES/OES, ICP-MS), atomic absorption spectroscopy (AAS) or energy dispersive X-ray fluorescence (EDXRF) technique. The energy dispersive X-ray fluorescence (EDXRF) technique [1-8], involving measurement of characteristic X-rays emitted from the target material using semiconductor detectors has been well established to determine the elemental concentrations of different elements simultaneously. The EDXRF is a rapid, sensitive and non-destructive technique for qualitative as well as quantitative information of the trace elements present in the sample. Moreover, this technique does not require much 144

3 of sample preparation as compared to other widely used ICP-AES or ICP-MS techniques. In the present work, elemental composition of the Castrol Activ 4-stoke multigrade lubricating oil (SAE 20W-40) samples collected from a motorbike after traveling specific kilometers have been determined using the EDXRF technique. It may be noted that (SAE 20W-40) represents the viscous behavior of the oil at low and high temperature conditions, respectively, as specified by the Society of Automotive Engineers (SAE). 6.2 MATRIX EFFECTS One of the major difficulties in quantitative analysis using EDXRF technique is the inter-element or matrix effects. These consist of absorption and/or enhancement of the intensity of characteristic X-ray of interest by co-existing elements in the sample matrix. The matrix absorption is the most important factor and it arises from the absorption of both the incident radiation and the emitted characteristic X-rays in the sample. Enhancement is important only if an excited characteristic X-ray of an interfering element (present in large amount) lies above the absorption edge of the element of interest. This effect can be ignored if the specimen is composed of elements of low atomic number. In such cases, only matrix absorption contributes significantly. These matrix effects invalidate the direct relationship between the characteristic X-ray intensity and the elemental concentration as described in section 2.5. Therefore, special attention must be given to methods for overcoming the matrix effects in any specimen for which truly quantitative results are required. There are many methods to take care of the matrix effects [9,10]. One of the methods consists of diluting the sample in known matrices such as cellulose nitrate or 145

4 boron-tetra-fluoride. In this method enhancement is almost absent where as absorption correction is calculated theoretically using the absorption coefficients of the known matrix elements at a given excitation energy. The absorption correction factor can also be determined experimentally [11]. For experimental determination of this factor the sample thickness must be less than the critical thickness (according to Jenkins et al [9], mathematically the critical thickness intensity is adequate). Above this thickness the experimental determination of self-absorption correction factor becomes increasingly difficult. The lower limit for sample thickness is dependent on the capability of making self-supporting pellets, apart from the statistical considerations due to reduced count rates. 6.3 SAMPLE PREPARATION Castrol Activ 4-stoke multi-grade motorbike lubricating oil (SAE 20W-40) samples were collected from the Hero Honda motorbike after traveling specific kilometers. The samples were collected by putting motorbike on main stand and keeping a plastic mug exactly under the drain plug (which is at the bottom of the crankcase). After the collection of required sample the drain plug is closed so that motorbike can travel next specific kilometers. This process was repeated for the collection of all other samples. The five samples of the oil were collected at different distance of 0 km, 750 km, 2000 km, 3000 km and 3685 km. Thin samples of lubricating oil were prepared by sealing about 1ml of oil in form of thin film using polythene sheets with adhesive. 146

5 6.4 EXPERIMENTAL METHODS The oil samples in the form of thin films were analyzed using EDXRF spectrometer available at Panjab University, Chandigarh. In this setup, a 2.4 kw Mo anode X-ray tube (60 kv, water cooled, PW 2274/22, Pananalytic, The Netherlands) in conjunction with Mo absorber/filter was used as a photon source. An LEGe detector in horizontal configuration (100 mm 2 10 mm, 8-μm Be window and FWHM = 150 ev at 5.89 kev, Canberra, US) coupled with PC based multi-channel analyzer was used to collect the fluorescent X-ray spectra from the samples. The X- ray tube, LEGe detector and target holders were arranged in 90 o reflection geometry. The samples were analyzed by operating the Mo X-ray tube at 29 kv and 10mA current. The tube emitted Mo K X-rays along with bremsstrahlung radiation ranging up to maximum applied operational voltage. The Mo absorber was used to improve the detection limit in energy region of interest. The X-ray tube and detector were kept outside the chamber. The alignment of the X-ray tube collimator and chamber collimator was done using laser beam. The samples were placed at 45 o to incident photon beam direction. The X-ray tube generally operated at 29 kv to avoid excitation of the Sn-collimator with K-Shell binding energy, B K = kev and 10mA current. The X-ray spectrum of each sample was collected using PC-based multi-channel analyzer (Multiport II, Canberra, US) for time periods ~1000s. A background spectrum recorded by placing a blank polythene sheet, used to prepare oil films, at the target position is shown in Fig.6.1. The Ar-K peak observed in this spectrum is due to excitation of the air column present between X-ray tube-target and target-detector, where as the Sn-L peak is due to Sn collimator used in front of the detector. The spectra of new lubricating oil sample and that collected after traveling 3000 Km are shown in Fig and Fig.6.3, respectively. 147

6 Counts Mo K X-rays ELEMENTAL ANALYSIS OF LUBRICATING OIL SAMPLES USING EDXRF TECHNIQUE 3.0 x103 Blank polythene sheet Ar Sn-L Channel Number Figure 6.1: A typical spectrum of blank polythene sheet using Mo X-ray tube operated at 29kV. 148

7 Counts Counts ELEMENTAL ANALYSIS OF LUBRICATING OIL SAMPLES USING EDXRF TECHNIQUE x New Lubricating Oil Sample Zn-K 1 Zn-K Ar Sn-L Ca Fe-K Channel Number Figure 6.2: Spectrum of new lubricating oil sample using Mo X-ray tube operated at 29 kv. x Lubricating Oil Sample Zn-K Ar Sn-L Ca Fe-K Cu-K Zn-K Channel Number Figure 6.3: Spectrum of lubricating oil sample collected after traveling 3000 km. 149

8 6.5 EVALUTION PROCEDURE The elemental concentrations (g/cm 2 ) in various samples were determined using the relation x m N /(I Gε σ β ) (6.1) K o where N K is the number of counts per unit time under the K X-ray photo peak, I o G is the intensity of the incident radiation falling on the area of the target visible to the detector, is the detector efficiency, is the self-absorption correction factor which K accounts for the absorption of incident and emitted photons in the target and x σ K represents the theoretical K X-ray production cross sections taken from reference [12]. The self absorption correction factor was negligible because very thin oil samples were used for analysis. Each spectrum was analyzed for photo peak areas (N K ) using software package ORIGIN 6.0 in which a non-linear least squares fitting routine based on chi-square minimization using the Marquardt s algorithm [13] has been implemented. In this code different photo peaks can be fitted using multigaussian function with the option of varying peak centroids, the FWHM as a function of energy and background under the photo peak. 6.6 RESULTS AND DISCUSSION Four elements, namely, Zn, Fe, Cu and Ca could be observed in different oil samples. The concentrations of different elements detected in different samples are given in Table 6.1. The error in measured concentrations is ~8%. The source of Zn and Ca detected in the new oil sample can be the presence of some additives like Zinc-dithiophophate (ZDPP) or Calcium sulfonates which are usually added to the lubricant oil in order to improve its viscosity index. 150

9 Table 6.1: Concentrations (g/cm 2 ) of different elements detected in lubricant oil samples. Elements S1 (New) S2 (750 km) S3 (2000 km) S4 (3000 km) S5 (3685 km) Fe 1.727E E E E E-08 Zn 4.066E E E E E-07 Ca 7.801E E E E E-08 Cu E E E E-09 It is clear from Table 6.1 that the concentrations of Zn and Ca have shown a decreasing trend with increase in distance traveled, thereby indicating the deterioration of quality of lubricating oil. It may be noted that Cu could be observed in oil samples collected after traveling certain distance and its concentration has shown increasing trend with increase in distance traveled, thereby indicating the level of wear and tear of the engine parts (shaft). 151

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