Anal. Chem. 1994,66, 730-734
Glow Discharge Mass Spectrometric Analysis of Trace Metals in Petroleum Christopher M. Barshlck,’ David H. Smith, Juiian H. Hackney, Beth A. Cole, and Jeffery W. Wade Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1-6375
A previously reported method of analyzing solutionresidues by glow discharge mass spectrometry (CDMS) has beenextended to the analysis of petroleum. Results showed that cathode formation could not be achieved simply by drying and pressing with a conductive host matrix (as in the case of the aqueous solutions), and an additional low-temperatureashing step was incorporated to reduce the organic material to a dried residue. The residue was pressed with a conducting host into a pin, which was subjected to conventional CDMS analysis. The results from two NIST aqueous reference solutions were compared to those from two SPEX standard reference oils; only small differences in relative ion yields (less than 10% average) were observed between the two types of sample. This is well within experimental error. By applying relative sensitivity factors, good accuracy was obtained (better than 5%averageerror); this indicates minimal matrix effects. Good agreementwas also observed betweenresults obtainedby using the GDMS methodology and those obtained using EPAapproved inductively coupled plasma optical emission spectroscopy calibration curve procedures. One limitation noted in the CDMS analysis was the increased number and type of polyatomic interferences, presumably caused by the residual organic constituents of the oil. The trace elemental composition of petroleum is of interest not only to those involved in exploration and refining but also to those concerned with the impact that a release would have on the environment. Methodology has long been in place to measure such elements as copper, arsenic, and sulfur that cause corrosion of refinery vessels,’ nickel and vanadium that adversely affect catalysis,2and a variety of other trace metals that can serve as “fingerprints” to identify the sources of contamination during refining. The need for analysis of oils also arises because some trace elements are used as additives to stabilize fuels and lubricants and to improve thecombustion characteristics of heating the presence of other elements serves as an indication of wear, providing important clues to how well the oil is f~nctioning.~ The determination of small amounts of metals in crude oil was reported as far back as 1910,j and several groups have (1) Oluwole, A. F.; Asubiojo, 0. I.; NWachukwu, J. I.; Ojo, J. 0.;Ogunsola, 0. J.;Adejumo, J . A.; Filby, R. H.; Fitzgerald, S.;Grimm, C.J. Radioanal. Nucl. Chem. 1993, 168, 145. ODonnell, J. In Criteria for Quality of Petroleum Products; Allinson, J. P., Ed.; Halsted Press: New York, 1973. (3) VlakoviC, V. Trace Elements in Petroleum; The Petroleum Publishing Co.: Tulsa, OK, 1978; p 39. (4) Hitchon, B.; Filby, R. H.; Shah, R. R. In The Role of Trace Elements in Petroleum; Yen, T. F., Ed.; Ann Arbor Science Publisher: Alberta, Canada, 1975. ( 5 ) Hewett, D. F. Am. Insr. Mining Eng. Tram. 1919, 40, 274.
730 Analytical Chemistry, Vol. 66, No. 5, March 1, 1994
detected upwards of 30 elements in various crude oil samples from around the world.6 Several books have been written on the subject of trace elements in p e t r ~ l e u m , ~and , ~ *a ~greater number of review articles have been written on various aspects of their migration, collection, and elemental concentration.*-1° The determinationof tracequantities of metals in petroleum has also been important to those concerned with the effect that an accidental spill would have on the en~ironment.~ Perhaps the most abused victim of environmental pollution has been the sea, but from the standpoint of metals being the primary concern in affecting living tissue, the burning of fossil fuels is of more concern and chiefly responsible for changing the concentrations of trace elements in the atmosphere.” Whatever the source and wherever the problem, determining the trace composition of waste oil is regarded as being of utmost importance as part of an overall environmental protection and restoration program. Several techniques have been developed to prepare petroleum samples for analysis, and although examples can be cited where the “direct” analysis of oil has proven valuable (e.g., neutron activation analysis1J2),for most analyses of metals in crude oils, some combination of preconcentration and dilution is required. Traditionally, crude oil samples have been prepared by reducing the hydrocarbon component of oil by either dry ashing or by a combination of ashing and wet oxidation. One technique frequently used is to char the sample by heating it with sulfuric acid. This is followed by the addition of nitric acid in small increments and oxidation with hydrogen peroxide. Variations of this technique have been applied for different types of petroleum and for the analysis of specific elements; these include the addition of benzenesulfonicacid13J4 or p-xylenesulfonic acid,15 the purpose of which is to render the metals less volatile. Most methods have shown generally close agreement in the determination of concentrations of metals contained in oil; some, however, are superior to others in terms of recoveries, limits of detection, and sample preparation time.16 (6) AI-Shahristani, H.; AI-Alyia, M. J. Radioanal. Chem. 1973, 14, 401. (7) Erickson, R. L.; Myers, A. T.; Horr, C. A. Am. Assoc. Pet. Geol. Bull. 1954, 38, 2200. ( 8 ) Yang, J.; Xianjin, Z . Fenxi Huaxue (Anal. Chem.) 1991, 19, 362. (9) Eisentraut, K. J.; Newman, R. W.; Saba. C. S.;Kauffman, R. E.; Rhine, W. E. Anal. Chem. 1984, 56, 1086A. (10) Terrell, R. E. A w l . Chem. 1983.55, 245R. (1 1) Salooja, K. C. Chart. Mech. Engr. 1968, 15, 301. (12) Zaghloul, R.; Obeid, M.; Staerk, H. Radiochem. Radioanal. Lett. 1973,15, 363. (13) Rowe, W. A.; Yates, K. P. Anal. Chem. 1963, 35, 368. (14) Shott, J. E.; Garland, T. J.; Clark, R. 0. Anal. Chem. 1961, 33, 507. (15) Udoh, A. P.; Thomas, S.A.; Ekanem, E. J. Talanta 1992, 39, 1591.
0003-2700/94/03660730$04.50/0
0 1994 American Chemical Soclety
Numerous measurement techniques have been applied to nitric acid/50% water at 95 OC for 10-15 min. To ensure this problem. In addition to neutron activation a n a l y ~ i s , ~ J ~ J ~completeoxidation,an additional 5 mL of concentrated HNO3 X-ray fluorescence’* and various optical methods have all was added and the solution allowed to reflux for 30 min. This been employed. Examples can be found where flame,19 arc was followed by oxidation with 3-10 mL of 30% H202 until and spark,20 and inductively coupled p l a ~ m a ~excitation l-~~ the solution no longer effervesced. The organic phase was were used to stimulate emission for subsequent optical decanted, and the aqueous phase containing the dissolved detection. Mass spectrometers with several different types of elements was filtered, diluted to 40 mL in approximately 5.0% ionization sources have also been employed, and similarly to HCl and 1.0% HNO3, and centrifuged to clear the solution. optical spectrometry,spark source and thermal ionization mass This solution is referred to as the aqueous “leachate”. The analysis25have given way to inductively coupled plasma mass total time for its preparation was about 8 h. analy~is.263~~ For analysis by ICP-OES, a 1:l dilution of the leachate with water was aspirated directly into an argon ICP powered We report here on methodology that falls in complexity by a Plasma-Therm rf generator (Jobin Yvon JY48 ICPsomewhere between simply pipetting the sample into a vial OESinstrument). The generator provided 1-kW power at 27 prior to neutron activation analysis and the extensive ashing MHz. If the elemental emission signal was beyond the linear procedures used prior to some ICP analyses. The method portion of the calibration curve (>50 ppm), a serial dilution requires no dissolution, uses G O O pg of total sample, and relies on atomization and ionization driven by a glow discharge. was made prior to reanalysis. In earlier work, we showed that dried residues from small Glow discharge cathodes were prepared by pipetting 5 0 volumes (
