Energy Fuels 2010, 24, 1099–1105 Published on Web 11/02/2009
: DOI:10.1021/ef900973s
Detection of Trace Metals in Asphaltenes Using an Advanced Laser-Induced Breakdown Spectroscopy (LIBS) Technique Mohammed A. Gondal,† Mohammad N. Siddiqui,*,‡ and Mohamed M. Nasr§ † Laser Research Laboratory, Physics Department and ‡Department of Chemistry, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia, and §Department of Natural Sciences, College of Dentistry and Pharmacy, Riyadh 11671, Saudi Arabia
Received September 1, 2009. Revised Manuscript Received October 19, 2009
The significance of trace elements in the petroleum industry has increased because of the role of nonhydrocarbon components in the elucidation of the mechanisms of migration and origin of crudes. Determining the amounts of trace elements is also very important in the petroleum industry for refining and processing of crudes and even exploration. In the development of different instrumental analytical techniques for trace elemental analysis of crudes and its products, little attention has been devoted to the broad variation in data because of poor sampling and the proper nature of the matrix. Accurate detection of trace metals in asphaltene samples using conventional methods is not a trivial task. In this work, the determination of 13 trace elements in asphaltenes including Co, Fe, V, Ni, Pb, P, Mo, Ca, Si, Ti, Mn, Cd, and Cu are reported for the first time using an advanced laser-based analytical technique. For this purpose, laser-induced breakdown spectroscopy (LIBS) using a Nd:YAG laser at 1064 nm was employed to determine the concentration of these trace elements present in four asphaltene samples derived from the marketable Saudi Arabian crude oils. The concentration determined with our LIBS setup of different trace metals present in the same samples was also measured using a standard technique, such as inductively coupled plasma (ICP), and results achieved with LIBS are in close agreement with ICP results. The limit of detection for these elements was also determined and reported. This is the first time, to the best of our knowledge, the LIBS technique has been applied for the determination of trace metals in asphaltenes. The unique features of LIBS are no or little sample preparation requirement, fast and rapid analysis, and in situ detection, which are lacking in the conventional analytical techniques.
diminishes the conversion efficiency achievable in the hydrocracking process.3,4 Petroleum asphaltenes represent a solubility class of petroleum liquids and are considered an important factor that causes hindrance in petroleum operations.5 The main problem in studying the composition of asphaltenes is its inherited chemical complexity. It has also been reported that asphaltenes consist of highly condensed polyaromatic rings bearing long aliphatic and alicyclic substituents along with metals and heteroatoms as part of a ring system.6-9 Therefore, the petroleum industry has genuine concern and interest in the determination of trace metals present in heavy crude oils and asphaltenes in particular. Trace metals are of great significance because of their vital role in the genesis of petroleum and its refining. The information concerning the origin, migration, and maturation of petroleum may be obtained from the true nature of these metals and their abundance in petroleum. Trace metals occur in varying quantities depending upon the nature of the crude.10-12
Introduction The consumption of crude oils is on the rise because of the increasing demand for energy and fuels worldwide as a consequence of the growth in world population. The heavy petroleum residue constitutes about 70% of the drilled crude oils, and a fairly low percent is being used without much processing.1 Thus, the increasing demand for transportation fuel (gasoline and diesel fuel) has necessitated the need for processing the heavier residues containing higher contents of asphaltenes. Asphaltenes are considered to be the most troublesome fractions in petroleum cracking and refining processes. The tendency of asphaltenes to precipitate during crude oil recovery can cause severe consequences as a sharp decline in oil flow or even blockage of pipelines and processing equipment.2 Asphaltenes also adversely affect the overall rate of hydrodesulfurization during catalytic hydroprocessing; they act as coke precursors, leading to catalytic deactivation and sludge formation, which *To whom correspondence should be addressed. Telephone: þ966-38602529. Fax: þ966-3-8604277. E-mail:
[email protected]. (1) Speight, J. G. Fuel Science and Technology Handbook; Marcel Dekkar: New York, 1990. (2) Cimino, R; Correra, S.; Del Bianco, A.; Lochhart, T. P. In Asphaltenes Fundamentals and Applications; Sheu, E. Y., Mullins, O., Eds.; Plenum Press: New York, 1995; p 97. (3) Bartholomew, C. H. In Catalytic Hydroprocessing of Petroleum and Distillates; Oballa, M. C., Shih, S. S., Eds.; Marcel Decker: New York, 1994; p 42. (4) Miyauchi, Y.; de Wind, M. Hydroprocessing. Proceedings of the Akzo Nobel Catalysts Symposium, Amsterdam, The Netherlands, 1994; pp 123-140. r 2009 American Chemical Society
(5) Sheu, E. Y. Energy Fuels 2002, 16, 74. (6) Speight, J. G. Fuel 1970, 49, 134. (7) Hasan, M.; Siddiqui, M. N.; Arab, M. Fuel 1988, 67 (8), 1131. (8) Shirokoff, J. W.; Siddiqui, M. N.; Ali, M. F. Energy Fuels 1997, 11, 561. (9) Siddiqui, M. N. Pet. Sci. Technol. 2003, 21 (9 and 10), 1601. (10) Valkovic, V. Trace Elements in Petroleum, Part 2; The Petroleum Publishing Company: Tulsa, OK, 1978; pp 62-83. (11) Yen, T. F. Energy Resour. 1974, 1, 447. (12) Yen, T. F. Chemical aspect of metals in native petroleum. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1975; p 31.
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The nature of each element is also important, regardless of their abundance, as being the potential source of environmental pollution, the cause of corrosion to equipment, and the poisoning of catalysts during refining. Metals are present in very complex form in crudes, and their removal is not an easy task.13,14 Demetalation of petroleum becomes difficult because of the association of metals to heteroatoms present in asphaltene fractions. The heavy metals, such as vanadium and nickel, which are primarily associated with asphaltenes, occupy either heteroatom (N, S, and O) bounded sites or are strongly associated with the aromatic sheets of asphaltenes via π-π bonding. Different analytical techniques have been applied in the determination and identification of trace metals in the crude oils and asphaltenes. A literature review carried out by Duyck et al.15 on the determination of trace elements in crude oil and heavy molecular mass fractions, including saturates, aromatics, resins, and asphaltenes (SARA), by inductively coupled plasma-mass spectrometry (ICP-MS), ICP-optical emission spectroscopy (OES), atomic absorption spectroscopy (AAS), and spectrochemical analytical techniques was discussed. Escobar et al.16 used SARA, gas chromatography (GC)-MS, ICP-OES, and UV-vis techniques to study the diverse trace metals and biomarker-derived parameters for a suite of 30 crude oil samples. Dreyfus et al.17 developed a method to analyze direct trace and ultra-trace metal elements in crude oil and its fractions (maltenes-asphaltenes) by ICP-MS. A technique for introducing crude oils directly into an ICP mass spectrometer based on the formation of oil-inwater microemulsions and greatly simplifying the determination of trace metals in oil is reported.18,19 Proton-induced X-ray emission (PIXE) analysis was also used for the direct determination of the distributions and abundances of trace metals in crude oils.20 Buenafama et al.21 applied neutronactivation analysis for the determinations of 17 trace elements in heavy crude oils. Ali et al.22 used atomic absorption in a study of the trace metals of crude oils from the different oilproducing fields of Saudi Arabia. It is worth mentioning that all of these methods are cumbersome and highly expensive and require sophisticated equipment and high-level technical skills. In addition, these methods are highly time-consuming and require special chemicals and sample preparation prior to analysis. To minimize coke formation and develop advanced and more efficient technologies for heavy oil upgrading, a better
and comprehensive understanding of metal contents in asphaltene is highly essential. To have a rapid and on-line analysis of asphaltene, advance techniques are required. For this purpose, laser-induced breakdown spectroscopy (LIBS) was developed locally and applied for the first time to determine the maximum number of trace metals in asphaltenes, which are not easily detectable with other conventional techniques, as mentioned above, because of different reasons on rapid-time scale. In LIBS, a plasma spark is created by focusing the high-energy laser beam at the sample of asphaltene and spectrally resolved emissions are recorded with a spectrometer having reasonable resolution (0.1 nm) to identify the elements present in the asphaltene sample. This analytical technique enables the determination of elemental compositions23-25 of different trace elements present in the asphaltene samples. The unique features of LIBS are no or little sample preparation requirement, fast and rapid analysis, and in situ detection, which are lacking in the conventional analytical technique, such as atomic absorption and ICP.23-25 Yaroshchyk et al. have used LIBS for the quantitative analysis of wear metals in engine oil. A limit of detection (LOD) was determined for several trace elements. They compared the LIBS results to the ICP-AES analysis of the same samples, and a good relation was reported.26,27 The work on LIBS for environmental and other analytical applications is a continuity of laser-based research activities being developed at the Physics Department, KFUPM, including laser intensity direction and ranging (LIDAR) and photoacoustic spectroscopy.28-31 Experimental Section Sample Collection. The following four varieties of marketable crude oils produced by the Saudi Arabian Oil Company (Saudi Aramco) were procured from the Ras Tanura refinery, Saudi Arabia. The asphaltenes used in this study were isolated from the following four Saudi Arabian crude oils: (1) Arab Berri (AB) is a relatively high American Petroleum Institute (API) gravity (38.50° API), low-sulfur, and paraffinic-type crude oil and has the lowest asphaltene contents (2.93%). Arab Extra Light, which comes from the Berri, Saudi Arabia field, is produced from the upper Jurassic age Arab zone reservoirs, generally oolitic and dolomitic limestones. (2) Arab Light (AL) is a moderately high-gravity (33.80° API), medium-sulfur, and medium-paraffinic crude oil and has moderate asphaltene contents (6.83%). Arab Light is produced from the Ghawar field, which is the largest onshore oil field in the world, and is also derived from the upper Jurassic age Arab zone reservoirs. (3) Arab Medium (AM) is a medium-gravity (30.40° API) and paraffinicwax-containing crude oil and has higher asphaltene contents (9.18%). Arab Medium is produced from the Jurassic age Arab zone reservoirs as multi-stage separated oil from a blend of the following fields: 65% Khursaniya, 25% Qatif, and 10%
(13) Filby, R. H.; Branthaver, J. F. Metal Complexes in Fossil Fuels; American Chemical Society: Washington, D.C., 1987; ACS Symp. Ser. 344. (14) Filby, R. H.; Shah, K. R. Neutron activation methods for trace elements in petroleum. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1975. (15) Escobar, M.; Da Silva, A.; Azuaje, V.; Esteves, I. Rev. Tec. Fac. Ing., Univ. Zulia 2007, 30, 391–400. (16) Duyck, C.; Miekeley, N.; Porto da Silveira, C. L.; Aucelio, R. Q.; Campos, R. C.; Grinberg, P.; Brandao, G. P. Spectrochim. Acta, Part B 2007, 62B (9), 939–951. (17) Dreyfus, S.; Pecheyran, C.; Magnier, C.; Prinzhofer, A.; Lienemann, C. P.; Donard, O. F. X. Elemental analysis of fuels and lubricants. American Society for Testing and Materials (ASTM) Special Technical Publication, 2005; STP 1468, pp 51-58. (18) Lord, C. J., III. Anal. Chem. 1991, 63 (15), 1594–1599. (19) Zaki, N. S.; Barbooti, M. M.; Baha-Uddin, S. S.; Hassan, E. B. Appl. Spectrosc. 1989, 43 (7), 1257–1259. (20) Fischbeck, H. J.; Engel, M. H.; Ruffel, A. V.; Weaver, B. L. Nucl. Instrum. Methods Phys. Res., Sect. B 1987, 24-25 (part 2), 655–657. (21) Buenafama, H. D.; Lubkowitz, J. A. J. Radioanal. Nucl. Chem. 1977, 39 (1), 293–300. (22) Ali, M. F.; Bukhari, A.; Saleem, M. Ind. Eng. Chem. Res. Dev. 1983, 22, 691.
(23) Gondal, M. A; Hussain, T.; Yamani, Z. H.; Baig, M. A. Talanta 2007, 72, 642–649. (24) Gondal, M. A.; Hussain, T. Talanta 2007, 71, 73–80. (25) Gondal, M. A.; Hussain, T.; Ahmad, Z.; Bakry, A. J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 2007, 42, 879– 887. (26) Yaroshchyk, P.; Morrison, R. J. S.; Body, D.; Chadwick, B. L. Spectrochim. Acta, Part B 2005, 60, 986–992. (27) Yaroshchyk, P.; Morrison, R. J. S.; Body, D.; Chadwick, B. L. Spectrochim. Acta, Part B 2005, 60, 1482–1485. (28) Gondal, M. A. Appl. Opt. 1997, 36, 3195–3201. (29) Gondal, M. A.; Mastromarino, J. Appl. Opt. 2001, 40, 2010– 2017. (30) Gondal, M. A.; Mastromarino, J. Talanta 2000, 53, 147–154. (31) Striganove, A.; Sventitski, N. Table of Spectral Lines of Neutral and Ionized Atoms; Plenum: New York, 1968.
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Figure 1. Schematic diagram of the LIBS system applied for the analysis of asphaltene samples.
atmosphere oven for about 2 h at 105 °C to obtain a constant weight. The filtrate (maltenes) was recovered by evaporating the n-heptane on the steam bath using a rotavapor with continuous blowing of dry nitrogen until a constant weight of maltenes was obtained. For LIBS measurement, the pellets of asphaltene samples were prepared. A 4.0 g asphaltene sample was poured in a stainless-steel dye having a cylindrical shape. The pellets of this sample were made in a hydraulic press machine by applying a load of 12 000 psi for a 0.5 h duration. These pellets have a diameter of 20 mm and thickness of 10 mm. To test the homogeneity of our samples, several LIBS measurements were performed at different locations of the surface of the pellet samples. Laser-Induced Breakdown Spectrometer Details. A schematic diagram of the laser-induced breakdown spectrometer applied in this study is depicted in Figure 1 and is discussed in detail in earlier publications.23-27 The LIBS system applied in this study consists of an Ocean Optics LIBS 2000þ spectrometer, a special home-built chamber, OOILIBS software, and a Nd:YAG laser (Spectra Physics, model GCR100). The Nd:YAG laser can deliver a maximum pulse energy of 1 J with a pulse width of 8 ns and operate at a 10 Hz pulse repetition rate, operating in Q-switched mode. Here, 1064 nm radiations emitted at a fundamental frequency from the Nd:YAG laser were applied for the production of the plasma spark at the asphaltene test sample. The laser energy was measured with a calibrated energy meter (Ophir model 300) for the study of the dependence of the LIBS signal upon incident laser energy. The pulse energy used in this experiment was in the range of 80-120 mJ. The light from the plasma spark is collected by a collimating lens using a UV-grade fused silica 1 m, multimode sampling fiber with a SMA connector and is transferred to the LIBS 2000þ spectrometer (Ocean Optics). Our LIBS 2000þ has four spectrometer modules to provide high resolution [full width at half maximum (fwhm) of 0.1 nm] in the 200-620 nm wavelength region. The detector has a gated charge-coupled device (CCD) camera having 14 336 pixels. This makes it possible to measure a LIBS spectrum over a broad spectral range (200-620 nm) simultaneously with spectral resolution (0.1 nm). The plasma emission was recorded at a 90° angle to the laser pulse. Software built in the spectrometer read the data from the chip and reconstructed the spectrum. The concentrations of different trace metals present in asphaltene
Abu-Safah. (4) Arab Heavy (AH) is a relatively low-gravity (28.03° API), high-sulfur, and paraffinic-wax-containing crude oil and has the highest asphaltene contents (13.5%). Arab Heavy is comprised of crude oil from an offshore field, Safaniya, located about 125 miles northwest of the exporting terminal Ras Tanura and known to be the world’s largest offshore oil field. Safaniya oil is produced from the lower Cretaceous age Arab zone reservoirs. All crude oil leaving the Saudi Aramco production fields, with the exception of those at Safaniya, Marjan, and Zuluf, are sour and contain toxic hydrogen sulfide. Sour crude is sweetened, stabilized, and pumped to the storage facilities before shipment. All crude oil samples studied in this work are stabilized crude from the storage facilities in the Ras Tanura refinery. Chemicals and Materials. High-performance liquid chromatography (HPLC)-grade normal heptane, which has 99.99% purity, was procured from Fluka and used for the precipitation of asphaltenes. Separation of Asphaltenes. First, 7.0 g of heavy residue and 5 mL of HPLC-grade n-heptane were transferred into the 200 mL Pyrex beaker. The beaker was heated on the hot plate at around 70-80 °C temperature for 15 min with constant swirling to homogenize the solution. This residue solution, when well-mixed, was carefully transferred to a 2 L Pyrex flask, and 700 mL of HPLC-grade n-heptane was added to the same flask. The flask containing the residue solution was fitted with a mechanical stirrer and placed on the water bath. The residue solution was heated at 90 °C on the steam bath with continuous stirring for about 2 h to maximize the solubility of the residue in n-heptane. After 2 h of mixing, the residue solution covered with aluminum foil was left on the working bench to cool at room temperature for about 24 h. The long cooling time produces more efficient precipitation of asphaltenes. The residue solution was filtered using a Millipore filtration apparatus with 0.8 μm (37 mm) pore size filter paper. All insoluble material was Soxhlet-extracted with 25 mL of toluene for 2 h at 110 °C temperature and filtered again using the same filtering apparatus. The insoluble material was removed as sludge (coke), and soluble material (asphaltenes) was recovered after evaporating toluene completely. The asphaltenes were collected in a 250 mL beaker and washed several times with small portions of n-heptane, to remove any traces of maltenes, until washings became colorless. The recovered asphaltenes were dried in an inert 1101
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Figure 2. Typical LIBS spectra of the Arab Light asphaltene sample recorded in the region of 200-550 nm. The change in LIBS signal intensity depending upon the concentration of trace metals present in the sample can be noticed from the y-axis scale.
Figure 3. Typical LIBS spectra of the Arab Medium asphaltene sample recorded in the region of 200-570 nm.
sample of asphaltene was digested with nitric acid (99.99%, Fisher Scientific, Pittsburgh, PA) and left overnight. The resulting residue was then ashed at 500 °C. The ashed sample was further diluted with nitric acid (99.99%, Fisher Scientific, Pittsburgh, PA), and the resulting solution was analyzed for trace metals using an ICP spectrometer calibrated using reference standards of three levels of accuracy.
samples were also measured with a calibrated ICP spectrometer to verify the results achieved with our calibrated LIBS method. For each LIBS analysis, a fresh asphaltene test sample was kept in the LIBS chamber. To test the homogeneity of our test samples, several LIBS measurements were performed at the surface of asphaltene test samples. For calibration purposes, iron, cobalt, and lead metals were used. All of these metals in powder forms were of high purity (99.99%) and procured from Fisher Scientific of Pittsburgh, PA. For the construction of the calibration curves, pure metals in powder form were mixed with the Ras Tanura asphalt sample, which contains an asphaltene matrix. The metals were thoroughly mixed with a warm asphalt sample. ICP Analysis. To verify our results for the determination of trace metals present in asphaltene samples, ICP analysis was also performed on an ARL model 3580 OES ICP atomic emission spectrometer. For the ICP analysis, each individual
Results and Discussion In LIBS, there are many processes that occur when a pulsed laser beam interacts with any solid material, resulting in the production of intense plasma, thermionic emission, sample heating, melting, atomization, excitation, and ionization. The trace elements present in the samples can be identified by LIBS spectral analysis. There are many variables that influence the LIBS signal intensity for the investigation of solid samples. 1102
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Figure 4. Typical LIBS spectra of the Arab Heavy asphaltene sample recorded in the region of 200-570 nm.
Figure 5. Typical LIBS spectra of the Arab Berri asphaltene sample recorded in the region of 200-570 nm.
These are the laser pulse width, shape, spatial and temporal fluctuation of the pulse and power fluctuation, laser wavelength, laser energy, and the physical and chemical properties of the target material. To enhance the sensitivity of the LIBS system, for analysis of asphaltene samples, the optimal experimental conditions mentioned above, which can affect the LOD in LIBS, were explored. Hence, prior to the analysis of test samples, different parameters, such as laser energy, delay time, focusing lens for incident laser radiation, and collecting lens for laser-produced plasma emission, were optimized. The reproducibility for each data point was calculated with a confidence level of 95%. Spectral Analysis and Trace Element Identification Using the LIBS Technique. Figures 2-5 depict the emission spectra of the asphaltene samples for the spectral region of 200592 nm. The laser pulse energy was 100 mJ. The distance between the optical fiber and the plasma was 10 mm. The LIBS spectra of the element under investigation were recorded in the above-mentioned spectral region to find the most sensitive lines
for each element. The major elements detected in the sample are Co, Fe, V, Ni, Pb, P, Mo, Ca, Si, Ti, Mn, Cd, and Cu. The trace elements present in different asphaltene samples detected by our LIBS system and counter verified by ICP are listed in Table 1. The fingerprint wavelength of each element is also listed in Table 1. Table 2 shows the trace metals and their spectral assignments determined in different asphaltene samples using LIBS. These emission lines have minimal interference from other emission lines, do not involve the ground state, so that self-absorption is almost absent, and are intense enough. Because of these reasons, these lines are useful for quantitative analysis. All of the spectral lines for the above-mentioned elements recorded with our LIBS setup were identified using the work performed by Striganove et al.31 and also using the National Institute of Standards and Technology (NIST) atomic spectral database.32 (32) http://physics.nist.gov/PhysRefData/ASD/index.html?nist_atomic_spectra.html.
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Table 1. Concentration (ppm) of Different Elements Detected with LIBS at 400 mJ and ICP Methods Arab Light sample 1 trace elements
Arab Medium sample 2
Arab Heavy sample 3
Arab Berri sample 4
wavelength (nm)
LIBS
ICP
LIBS
ICP
LIBS
ICP
LIBS
ICP
422.7 341.5 467.8 498.2 412.1 324.3 358.6 293.9 553.3 253.6 405.8 570.8 292.4
179.65 16.1 115.2 45.2 88.1 48.5 85.7 35.5 475.5 92.1 87.11 46.2 97.3
195.3 14.95 114.65 44.8 86.6 49.4 86.6 36.18 481.25 90.62 88.45 45.33 99.53
7.27 11.8 97.8 80.5 85.3 227.1 54.6 107.7 113 298.21 63.03 55 26.77
6.79 10.9 98.9 79.68 84.6 223.42 53.49 105.6 111.91 295.88 67.06 53.93 23.91
865.2 153.2 70.9 140.9 162.8 327.1 186.9 62.8 116.8 127.2 81.29 204 8.21
937 150.21 75.37 139.59 173.21 323.82 183.3 62.14 114.56 135.31 80.49 200 7.33
178 9.6 18.7 326.8 117.7 80.2 85.7 16.7 148.4 59.2 83.6 153 7.22
193.5 8.88 19.89 323.62 114.56 79.41 86.6 18.09 150.21 62.98 84.05 150 6.45
Ca Ni Cd Ti Co Cu Fe Mn Mo P Pb Si V
Table 2. Assignment of the Transitions Applied for the Fingerprint Wavelengths and the LOD trace element Ca Ni Cd Ti Co Cu Fe Mn Mo P Pb Si V
fingerprint wavelength (nm)
LOD (ppm)
422.6 341.5 467.8 498.2 412.1 324.3 358.6 293.9 553.3 253.6 405.7 569.6 292.4
9 10 7 7 3 4 5 5 2 4 4 8 8
assignment of observed transition
references
S0-1P1 3 D3-3F4 3 P0-3S1 4 F5-4G6 2 F7/2-2G9/2 2 S1/2-2P3/2 5 F5-5G6 4 S0-4P1 5 S2-P2 2 P3/2-2P3/2 2 P3/2-2S1/2 3 P1-3P1 4 F7/2-4F7/2
32 32 32 32 32 32 32 31 32 32 31 31 31
1
Calibration Curves for Trace Elements in Asphaltenes. In this work, dependent upon the element under investigation, known concentrations of Fe, Pb, and Co metals, as indicated in Figures 6-8, were prepared in the asphalt sample containing an asphaltene matrix and LIBS spectra were recorded for these different concentrations of each series of element. For example, the standard samples prepared for the calibration of Co were 10, 50, 95, 190, 285, 375, and 475 ppm in the asphalt matrix. Similarly, for the calibration curve of Fe, the concentration of Fe was 18, 30, 45, 60, 77, 90, 120, 200, 300, 400, and 500 ppm, and for the calibration curve of Pb, the concentration of Pb was 250, 400, 1250, 1500, and 2250 ppm. All of these spectra were recorded with an average of 20 laser shots, at three different locations on the sample surface. Typical calibration curves for LIBS measurements of trace metals (Fe, Pb, and Co) are depicted in Figures 6-8. It is worth mentioning here that the Ras Tanura asphalt used as base material for the calibration curves of Fe, Co, and Pb trace metals was analyzed by Siddiqui et al.33 and no traces of Fe, Co, and Pb metals were detected. These curves were plotted by recording the LIBS signal intensity of the emission lines at fingerprint wavelengths of each trace metal at abovementioned concentrations from the standard pellet samples. Average spectra were recorded for each data point. This averaging of the spectra reduces the background noise to a great extent when compared to the single shot spectrum of the sample. Concentration and Role of Trace Elements. The concentration of each trace element present in the four asphaltene
Figure 6. Calibration curves obtained using LIBS measurements of the different concentrations (ppm) of Fe metal mixed with asphalt samples.
Figure 7. Calibration curves obtained using LIBS measurements of the different concentrations (ppm) of Pb metal mixed with asphalt samples.
samples as determined with LIBS and ICP analyses is presented in Table 1. The vanadium and nickel concentrations are the geochemical indicators commonly used in the characterization of oil seeps, crude oils, and bitumen, but the concentrations of other trace elements are also important for many purposes. It is also noteworthy that the concentration of these metals can be influenced by processes of thermal alteration, deasphalting, biodegradation, and washing water,
(33) Siddiqui, M. N.; Ali, M. F. Studies on the aging behavior of Arabian asphalts. Fuel 1999, 78 (9), 1005.
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serve as accumulators and, therefore, found in enhanced concentrations in crude oils. Table 1 shows the total number and amount of trace metals present in all four asphaltenes. There were 13 metals detected in asphaltenes, but V, Ni, Pb, P, Fe, Mo, Ca, and Cd metals are more prominent than the rest. V, Ni, and Fe are the most important metals present in asphaltenes. Even though present in small amounts (ppm), these metals are very detrimental in the processing of whole oils and responsible for rapid catalyst poisoning. Sensitivity of Our LIBS Spectrometer. The LOD or sensitivity of any instrument is very important for samples under investigation. Sensitivity or LOD here means the lowest concentration that can be detected in samples under investigation with our LIBS system.24 The LOD of the LIBS system can be estimated using the following eq 1: LOD ¼
Figure 8. Calibration curves obtained using LIBS measurements of the different concentrations (ppm) of Co metal mixed with asphalt samples.
2σb S
ð1Þ
where σb is the standard deviation of the background and S is the calibration sensitivity (=slope of the calibration curve), which is the ratio of the LIBS signal intensity to the known concentration of each trace metal in a standard sample. The LOD for LIBS analysis of asphaltene samples under investigation was calculated using the above-mentioned 1equation. The relative standard deviation (RSD) is given by the following eq 2:
or during migration, the V/Ni ratio could change trends to be constant because of the structural similarities among organometallic compounds that contain vanadium and nickel. This trait means that the ratio is one of the most useful parameters for determining both the origin of precursor organic materials and oil-oil or oil-source rock correlations. As clear from Table 1, the vanadium concentration is 97, 27, 8, and 7 ppm for Arab Light, Medium, Heavy, and Berri asphaltenes, respectively. Similarly, the concentration of Ni is 16, 18, 153, and 92 ppm for Arab Light, Medium, Heavy, and Berri asphaltenes, respectively. The non-porphyrin vanadium and nickel occupy either heteroatom (N, S, and O) bonded sites or are strongly associated with the aromatic sheets of asphaltenes via π-π bonding in metalloporphyrin. In general, the concentration of V and Ni in asphaltenes increases linearly with the increasing API gravity of the crude oils. Many correlations of V/Ni ratios have been made in attempts to obtain information on the geological origin of petroleum or the age of the reservoir rock.22 V and Ni metals are most concentrated in crude oils because of their well-known organic complexing mechanisms. These have been mostly concentrated in crude oils as compared to their presence in the earth’s crust and the organic matter from where petroleum is originated. The V content is largely but not exclusively associated with the asphaltic content and also with medium to high sulfur levels. Ni, the prevalent metal in crude oils after vanadium, is abundant in the various sectors of the living environment. Plankton accumulate Ni from seawater and marine animals. Valkovic has extensively discussed the origin and role of most of the trace elements present in the petroleum matrix.10 Mn, Fe, and Cu is present in the earth’s crust at an average concentration and is prevalent in significant quantities in the organic matter in nature from which crude oils is derived. Ca and Mg are widely distributed with substantial concentrations found in seawater and brine deposits and associated with crude oils largely as extrinsic matter. A reasonable concentration of Pb metal is detected in crude oils. P is widespread in the natural organic world; thus, the presence of P in fossil fuels is expected but in low quantity. Mo is another essential element for which many plants and animals
RSD ð%Þ ¼
standard deviation 100 mean
ð2Þ
The RSD decreased with the number of shots, but no improvement in RSD was noticed after 20 laser shots. The RSD value in our case was 3.4%. The asphaltene sample results with the LIBS method were comparable to the results obtained by ICP emission spectroscopy. The LOD for some elements under investigation is given in Table 2. Conclusions Trace metals present in four asphaltene samples were detected using LIBS. The concentrations of 13 trace metals (Co, Fe, V, Ni, Pb, P, Mo, Ca, Si, Ti, Mn, Cd, and Cu) present in asphaltene were estimated, and results achieved are in good agreement with the conventional techniques, such as ICP spectrometry. The maximum concentrations measured with our setup and verified by the ICP method for V (∼97 ppm), Ni (∼153 ppm), Pb (87 ppm), Mn (107 ppm), Cd (115 ppm), and Mo (475 ppm) are higher than the permissible safe limits. The parametric dependence of the signal intensity for improvement of the sensitivity of the LIBS spectrometer was carried out. The LOD for these elements was also determined and reported. This work demonstrates that the LIBS technique could be applied for rapid on-line analysis of asphaltene samples and simulation studies for remediation of oil well production, petroleum pipe lines, and catalyst poisoning could be undertaken. Acknowledgment. The authors acknowledge the facility and financial support provided by the King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia, through Project Grant SAB 2007/020 (to M.N.S.) and IN 080401 (to M.A.G.) for carrying out this work that has resulted in the preparation of this paper. One of the authors (M.M.N.) is thankful to KFUPM for its hospitality and permission to work at its research facilties. 1105
