Energy & Fuels 1987, 1 , 257-262
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Elemental Profiles of Hydrocarbon Materials by Size-Exclusion Chromatography/Inductively Coupled Plasma Atomic Emission Spectrometry W. R. Biggs,* R. J. Brown, and J. C . Fetzer Chevron Research Company, Richmond, California 94802 Received September 17, 1986. Revised Manuscript Received January 9, 1987 The ability of size-exclusion chromatography combined with inductively coupled plasma atomic emission spectrometry (SEC-ICP) to generate selected element profiles of hydrocarbon-based materials is explored. Iron, nickel, vanadium, and sulfur profiles of crude oils, residua, and derived subfractions are reported. Accuracy, precision, and detection limits of the technique are detailed. Use of the technique for the study of the refining process is demonstrated. Introduction The removal of metals, sulfur, and nitrogen from crudes, particularly heavy crudes, is an important step in the upgrading of these materials to usable products. In order to understand processing differences between crudes, it may be useful to know the nature of and differences in the chemical environments of these elements within a crude. Traditionally, the bulk analysis value for the elements of interest, rather than information related to speciation, has occupied the interest of researchers. However, the development of specific element detection coupled with a size-exclusion separation approach [size-exclusion chromatography/inductively coupled plasma (SEC-ICP)] can, in principle, provide speciation information. With correlations between these data and processing characteristics, SEC-ICP becomes both a useful tool for probing the refining process and a relatively simple test providing speciation information. Previous work has established the qualitative potential of SEC/specific element detection in applications to coal liquids'" and shale oil!,' Also, three recent papersg10 have qualitatively explored the distribution of selected elements in crude oils. This work was undertaken with four goals in mind: (1) to evaluate experimental design from a chromatographic standpoint, (2) to correlate features of element profiles with identifiable subfractions, (3) to establish quantitative guidelines for the accuracy, precision, and limits of the detection technique, and (4) to compare elemental profiles among crude oils. Experimental Section Chromatographic Apparatus and Reagents. The mobile phase of o-xylenelpyridinelo-cresol (79.5/20.0/0.5 vol %) was pumped at 1.0 mL/min through lo4- and 500-A Ultragel sizeexclusion columns (Analytical Sciences, Inc.) by a Beckman Model 100 A pump. Samples were injected from a 1OO-wL loop fitted (1) Hausler, D.; Hellgeth, J.; Taylor, L.; Borst, J.; Cooley, W. Fuel 1981,60,40-46. (2)Hausler, D.;Taylor, L. Anal. Chem. 1981,53, 1223-1227. (3)Hausler, D.;Taylor, L. Anal. Chem. 1981,53,1227-1231. (4)Brown, R.;Hausler, D.; Hellgeth, J.; Taylor, L. ACS Symp. Ser. 1982,NO.205,163-183. ( 5 ) Taylor, L.; Hausler, D.; Squires, A. Science (Washington, D.C.) 1981,213,644-646. (6)Brinckman, F.; Fish, R. N B S Spec. Publ. US. 1981, No. 618. (7)Fish, R.;Jewett, K.; Brinckman, F. Enuiron. Sci. Technol. 1982, 16,174-179. (8) Fish, R. H.; Komlenic, J. J. Anal. Chem. 1984,56, 510-517. (9)Hausler, D.Spectrochim. Acta, Part B 1985,40B389-396. (10)Biggs, W.R.; Fetzer, J. C.; Brown, R. J.; Reynolds,J. G. Liq. Fuels Technol. 1985,3,397-421.
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to a Rheodyne Model 7125 injection valve. All solvents used in the study were obtained from Burdick and Jackson except for o-cresol (Eastman Kodak). Detection System. The effluent from the size-exclusion columns was carried by 0.009-in. i.d. Teflon tubing to a Meinhard concentric glass nebulizer fitted to the spray chamber of an ARL Model 3580 emission spectrometer operating at 1600-W incident power. The detection lines used were as follows: V(II), 292.40 nm; Ni(II), 231.60 nm; Fe(II), 259.94 nm; S(I),182.04 nm. The plasma was viewed 15 mm above the load coil. The argon flow rates were as follows: 13 L/min coolant; 2.2 L/min plasma support; 0.7 L/min carrier. Data System/Electronics. The phototube outputs of the elements of interest were routed to Keithley (Keithley Instruments Inc., Cleveland, OH) Model 427 current-to-voltage amplifiers. The amplified signals were then sent to Nelson Analytical (Cupertino, CA) Series 760 interface boxes previously downloaded with the analysis protocol. The analysis protocol was developed from options provided by Nelson Analytical Chromatography software (Revision 6.22). At the completion of a run, the time vs. element response chromatograms were uploaded to a Hewlett-Packard (Fort Collins, Colorado) Model 9816 computer fitted with an additional 0.75-Mbyte RAM board. The chromatograms were processed by using the Nelson software run by a modified Basic 3.0 operating system. After processing, the raw data were stored on 31/2-in.microfloppy disks by using a Hewlett-Packard Model 9122 disk drive. Plots of raw data were obtained by using a modified version of XYPLOT (Nelson Analytical) and a HewlettPackard Model 7470 plotter. Preparation of Standards. Dibenzothiophene and iron, nickel, and vanadium meso-tetraphenylporphines were used as external standards. Sufficient amounts of the metal complexes were dissolved in o-xylene to produce a 7 pg of element/mL solution. The exact concentrations were determined by a wet-ash bulk analysis. Dibenzothiophene was dissolved in o-xylene, producing a 100 pg of S/mL solution whose exact concentration was established by oxidative Dohrmann analysis. Very small amounts of S were detectable in the o-xylene but were ignored as it contributed only a constant background signal. Wet-Ash Analysis. Confirmatory metals analysis was performed by a wet-ash procedure. Light components were first removed by gentle (
