analyses was 16%. The 27Alline was used as the standard, and transmittance areas were measured. In another application, the overall precision with which spectra of microsamples in solution could be replicated was determined with image width measurements. This test included the sample loading, electrode sparking, plate development, photometry, and image measurement steps. Gold support electrodes were loaded with 3-pg aliquots of gadolinium from solution and sparked to produce singleexposure spectra. Image widths were then measured to determine the absolute variation of the spectra. The averages
of the image widths and their standard deviations are plotted in Figure 9. The pooled relative standard deviation of 12 % corresponds to a variation in gadolinium concentration of a factor of 4. The precision of the gadolinium determination is increased by (to a factor of 1.5) when data for all seven of the isotopes are utilized.
d?
RECEIVED for review April 12, 1968. Accepted February 18, 1969. The information in this article was developed during the course of work under Contract AT(07-2)-1 with the U.S. Atomic Energy Commission.
Separation of Crude Oil Fractions by Gel Permeation Chromatography H. J. Coleman, D. E. Hirsch, and J. E. Dooley Bartlesville Petroleum Research Cenfer, Bureau of Mines, U.S. Department o f the Interior, Bartlesville, Okla.
This study was undertaken by the Bureau of Mines to investigate, extend, and apply gel permeation chromatographic (GPC) techniques to the fractionation of highboiling petroleum samples. The use of GPC with specially built columns and appropriate size polystyrene gel successfully effected the separation of several diver,se petroleum samples. In general, these separations were made by molecular size. Analytical data such as molecular weight, nuclear magnetic resonance (NMR) spectra, mass spectra, and per cent sulfur on the GPC subfractions provided a measure of the separations attainable. These data establish GPC as a useful supplementary method for separating high-boiling petroleum fractions without exposing the samples to the thermal hazards of distillation.
CHARACTERIZATION of the heavy ends of petroleum involves many separation and identification techniques. In the past, distillation and liquid-solid chromatography were the major tools of separation but newer methods such as gel permeation chromatography (GPC) are presently under investigation. Characterization studies on high-boiling fractions are progressing with the application of such analytical techniques as mass spectrometric analysis, high temperature programed gasliquid chromatography (GLC), nuclear magnetic resonance (NMR), infrared and ultraviolet spectroscopy, and various chemical techniques. Although development of GPC techniques began some years ago, early applications dealt mostly with materials soluble in an aqueous media. However, the development of special gels for operation with organic solvent systems ( I , 2) provided a basis for extensive work with polymers and other high molecular-weight materials. Such use of GPC with organic solvents for polymer analysis is described by Altgelt and Moore (3). Altgelt, in an earlier paper (4), (1) J. C. Moore, J . Polymer Sci., A2,835 (1964). (2) M. Joustra, B. Soderqvist, and L. Fischer, J. Chromafogr., 28, 21 (1967). (3) K. H. Altgelt and J. C . Moore in “Polymer Fractionation,” Manfred J. K. Cantow, Ed., Academic Press Inc., New York, N. Y., 1967. (4) K . H. Altgelt, Makromol. Chem., 88,75 (1965). 800
ANALYTICAL CHEMISTRY
discussed GPC as applied to various high molecular-weight petroleum asphaltenes, maltenes, and asphalts while Rosscup and Pohlmann (5)emphasized GPC separations obtained for metalloporphyrins in crude oil residue samples. Other papers (6-10) describe GPC studies of individual lower molecular-weight organic compounds in determining structure effects on gel fractionating capability. This paper describes the GPC procedures used in fractionating some Wasson, Tex., crude oil fractions boiling above 400 “C. Physical property data, such as molecular weight, NMR spectra, mass spectra, and sulfur percentages on the fractions, are summarized. These data and related information show the usefulness of GPC for fractionating highboiling petroleum cuts without exposing them to the thermal hazards of distillation. EXPERIMENTAL PROCEDURES Preparation of Starting Samples. Samples were prepared from Wasson, Tex., crude oil. The origin of the pentane deasphaltened Wasson residue has been given (11). Molecular distillation of this residue in a Jena, all-glass still produced the distillate and residue fractions listed in Figure 1. The residue shown on the right of this figure and the aromatic concentrates from three distillate fractions were fractionated by GPC. Preparation of GPC Columns. Commercially available, rigid, crosslinked polystyrene gel (Waters Associates, Inc., “Styragel”) of uniform bead size and having a permeability range to match the molecular size of the sample to be separated was used. This polystyrene gel comes in its expanded
(5) R. J. Rosscup and H. P. Pohlman, American Chemical Society, Division of Petroleum Preprints, 12, No. 2, A-103 (1967). (6) Jack Cazes and David R. Gaskill, Sep. Sci., 2,421 (1967). (7) Beveridge J. Mair, Philip T. R. Hwang, and Raffaele G. Ruberto, ANAL.CHEM., 39,838 (1967). (8) T. Edstrom and B. A. Petro, J . Polymer Sci., C21, 171 (1968). (9) G. D. Edwards and Q.Y . Ng, ibid.,p 105. (10) J. G. Hendrickson, ANAL.CHEM., 40,49 (1968). (11) C. J. Thompson, N. G. Foster, H. J. Coleman, and H. T. Rall, U. S. Bureau of Mines Rept. of Invest. 6879, Bartlesville, Okla., 1966.
form suspended in diethylbenzene. The columns consisted
of standard 1-inch by 6-foot glass pipe fitted at the bottom with a stainless steel cone and needle valve and at the top with a reservoir flask. The column for the GPC fractionation of the three aromatic-compound concentrates had the bottom half packed with 100- and the top half with 400-angstrom pore size gel; the column for GPC fractionation of the residue contained only 1,000-angstrom gel. A small, filter-paper disk atop each gel column eliminated stirring of the gel surface during sample or solvent introduction, and a small cotton plug at the bottom of each column prevented loss of gel through the open valve. GPC Column Operation. High-purity benzene was chosen as the solvent system because of its ready availability and its use prevented effects of impurity on quantitative and qualitative measurements of the separated sample fractions. The aromatic concentrate fractions were easily soluble in the benzene solvent system. The amount of sample charged was near the maximum permissible without column overloading to provide large enough samples for subsequent analysis, although both higher gel-to-sample ratios and greater column height-to-diameter ratios would have given better resolution. The quantity of sample charged was somewhat dependent on molecular weight ; for molecular distillation cut No. 1, 2.0 grams was dissolved in and diluted to 5 ml with benzene. For cut 2 the charge was 1.8 grams; for cut 3, 1.7 grams; and for the crude oil residue, 1.0 gram. Following sample introduction, sufficient nitrogen pressure (ca. 3 psi) was applied to assure lOO-ml/hr eluate flow from the bottom of the column, and 10-ml fractions were collected. Column operation was at ambient temperature (ca. 25 "C). Repeated sample chargings (eight times for molecular distillation cut No. 1) provided enough of the fraction for further analysis. Analytical measurements followed solvent evaporation in either a rotary film vacuum evaporator or a vacuum oven and the analytical data secured on selected fractions in addition to fraction weight and elution volume were as follows : molecular weight by vapor-pressure osmometry; weight per cent sulfur by the ASTM (D1551) quartz-tube, combustion method (12); GLC for qualitative determination of composition as to type and carbon-number distribution; mass spectrometry for both molecular weight and compound-type analysis; and NMR spectrometry for compound structure analysis. RESULTS AND DISCUSSION
I
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GPC of Aromatic Concentrates from High-Boiling Petroleum Distillates. GPC evaluation, in its simplest form, involves a single compound per test for establishing relative retention volumes on given columns and for determining the effects of molecular weight, molecular volume, and molecular structure. Admittedly, aromatic fractions from limited-boilingrange, molecular distillation cuts (400-470 "C, 470-495 "C, and 495-525 "C) are quite different from individual compounds. Yet the use of such aromatic concentrates takes advantage of another separation step that is valuable in having removed normal paraffins, isoparaffins, cycloparaffins, and heteroatom-containing compounds that would affect both the GPC separation and interpretation of the resulting analytical data. The three GPC chromatograms in Figure 2 are plots of sample weight eluted us. solvent volume eluted. These quantitative elution volume data are valuable but additional data, such as molecular weights and weight per cent sulfur make data interpretation more meaningful. Retention or elution volume data on known reference compounds (toluene (12) American Society for Testing and Materials, 1968 Book of ASTM Standards, Pt. 17, 575, Philadelphia, Pa., 1968. VOL. 41, NO. 6, MAY 1969
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The fact that a minute portion of these high-boiling aromatic fractions elutes slightly later than toluene is indeed proof that these GPC separations are not totally a function of molecular size but must be influenced slightly by compound polarity. The 70 "C boiling range for cut 1 us. 25 "C and 30 "C for cuts 2 and 3, respectively, has an obvious effect on the sharpness of separation. Molecular weights were determined by thermoelectric differential vapor pressure lowering of benzene at 37 "C in a Mechrolab vapor pressure osmometer. For selected GPC fractions from each of the aromatic concentrates, these data are consistently higher (6-230/0) than the mass spectral profiles given in Figure 3, but in the absence of mass data, they are helpful for indicating definite molecular-weight trends and showing the consistency or repeatability of the replicate GPC separations. The higher molecular weights for the early eluted material of aromatic fraction 3 compared to those of fraction 2 and those of fraction 2 compared to those of fraction 1 clearly indicate an increase in molecular weight with increase in fraction number which agrees with the boiling point and average-molecular-weight data given in Figure 1. Although molecular weight data might indicate that these GPC separations occur primarily by molecular weight, the sulfur data given in each panel of Figure 2 clearly show that more is involved than just molecular weight. For most petroleum distillate fractions, sulfur content increases with increasing boiling point, and many high-sulfur, crude oil residues theoretically contain over 100% sulfur compounds calculated on the assumption of only one atom of sulfur per molecule. A calculation of the weight-per cent-sulfur compounds in these GPC fractions, based on one atom of sulfur per molecule, shows that the high-molecular-weight compounds, as separated by GPC, are low in sulfur compounds in comparison to that of high-molecular-weight compounds separated by distillation. For instance, the calculated weight per cent-sulfur compounds for the right-hand panel (mol. dist. cut 3) vary roughly from a low of 2670 for the high molecularweight compounds eluted early (440 ml) to a high of 94% for the low-molecular-weight compounds eluted late (630 ml). Such a large increase of sulfur compounds occurring across the GPC chromatogram is but one measure of struc802
ANALYTICAL CHEMISTRY
tural differences related to molecular volume that certainly do occur across the GPC elution curve. Such data, together with mass and NMR data, strongly substantiate the findings of Henrickson (IO)in his GPC study on effective chain lengths for various groups of atoms. The mass spectral data of Figure 3 are useful in discerning various separations occurring during GPC fractionation of the three aromatic Concentrates of this study. The low-ionizing voltage profiles shown were derived from raw, peak-height data without having made allowances for sensitivity differences of the various compound types illustrated. Thus the data demonstrate qualitative analytical trends in GPC fractionation rather than an absolute quantification of specific compound types. For each of the three aromatic concentrates, mass data shown in Figure 3 indicate that the early eluted GPC fractions have high molecular weights and that these molecular weights decrease as sample elution proceeds. Carbon number ranges also decrease from about 11 in the earlier GPC fractions to about 6 in the later [ignoring slight tailing which may result from adsorption on the polystyrene gel (6)]. In addition, an increase in boiling point of the aromatic concentrates shows a corresponding increase in average molecular weight. Bimodal distribution curves probably result from more than one compound type in the same series. Structural formulas of compound types shown are primarily for illustration. Isomers indicated for each type represent only one of several isomers possible at a given mass. However, there is evidence to support the presence of long chain alkylbenzenes in the earlier GPC fractions as denoted by the dominant, - 6 series profile (A).Smaller quantities of mono-, di-, and trinaphthenobenzenes also occur in the earlier fractions, but more condensed ring structures and increasing sulfur compound concentrations are indicated in the later eluted fractions. High-ionizing voltage fragmentation patterns, high resolution mass spectrometry, and sulfur data support these observations. Collectively, all mass data obtained for these GPC separations of the three aromatic concentrates suggest GPC fractionation is largely by molecular volume, modified slightly by adsorption effects for some of the polycyclic aromatics having highly condensed ring systems. Proton magnetic resonance measurements with a Varian
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Figure 3. Low-voltage mass spectral data on selected gel permeation chromatographic fractions of aromatic portion of Wasson, Texas, petroleum Associates DP-60 NMR spectrometer provided additional data on the GPC separations. GPC fractions 11 and 29 were selected for NMR analysis because by elution volume, they represented widely differing samples (see the chromatogram in the left hand panel of Figure 2 for the aromatic concentrate of molecular distillation cut 1). The proton magnetic resonance spectra of these fractions, shown in Figure 4, and the calculated integration data of Table I, show that GPC effectively fractionated the high-boiling aromatic concentrate. The proton chemical shifts and spectral pattern obtained on the early eluted fraction 11 suggest a high degree of parafFinicity (more than 80%) with respect to paraffinic methyl and methylene hydrogen. The large, resonance-band intensity of the methylene peak at roughly 1.2 parts per million (ppm) chemical shift below tetramethylsilane (TMS) in conjunction with its shape, strongly indicates high paraffinicity in the form of long (>lo carbon atoms) alkyl groups (13). Thus the proton magnetic resonance data for GPC fraction (13) Kenneth W. Bartz and Nugent F. Chamberlain, ANAL. CHEM., 36,2151 (1964).
Table I. Proton Magnetic Resonance Data for Selected GPC Fractions from the Aromatic Concentrate of Molecular Distillation Cut No. 1 GPC GPC fraction 11 fraction 29 Wt. % hydrogen as paraffinic CH, (0.5 to 1.05 ppm) 20.9 5.9 Wt. hydrogen as paraffinic CH and CH2plus CHI beta to aromatic ring (1.05 to 2.0 ppm) 62.2 18.3 Wt. % hydrogen as aromatic CH, CHz, and CH, alpha to aromatic ring (2.0 to 3.5 ppm) 12.1 36.5 Wt. hydrogen as aromatic H directly on ring (ca. 7 to 8 ppm) 4.8 39.3 11, given in Figure 4 and Table I, in conjunction with average molecular-weight data obtained by both vapor pressure osmometry and mass spectrometry, point to a long-chain, alkyl substitution on a monocyclic aromatic plus some lesser VOL. 41, NO, 6,MAY 1969
0
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Figure 4. NMR spectra of selected gel permeation chromatographic fractions
Figure 5. Molecular weight data obtained from GPC analyses on selected Wasson, Texas, crude oil fractions boiling above 400 "C
quantities of long-chain, alkyl-substituted mono-, di-, and trinaphthenobenzenes. The proton chemical shifts and spectral pattern of the latereluted fraction 29 (see bottom panel, Figure 4) suggest a singular lack of paraffinicity. To compare the integral data (Table I), note the major compound type differences that are denoted by the weight per cent hydrogen as aromatic ring hydrogen increasing by a ractor of 8 over that of fraction 11 and by the occurrence of this aromatic ring hydrogen further downfield from 7 ppm. Without discussing these chemical shift data in detail the conclusion may be drawn that compound type structures eluted as GPC fraction 29 must be predominantly highly condensed, polycyclic aromatics with relatively short, alkyl chain-length substitution plus some methyl group substitution. Although both mass and NMR analyses show that significant separations were achieved by GPC, the resulting fractions are still too complex to show individual compound peaks by programed-temperature GLC with either a polar or nonpolar column. The detailed GLC data are not included in this paper, but the analysis with both polar and nonpolar columns for GPC fractions 17 and 41 of the aromatic concentrate from cut 2 substantiates the general paraffinicity or aromaticity determined by both mass and NMR. Such information is even further evidence of the validity of the various analytical data pointing to the compound type separations occurring with molecular weight separations; that is, separation by molecular size. GPC Analysis of Crude Oil Residues. GPC separations of untreated crude oil residues or associated high molecularweight, complex materials are difficult to evaluate fully. Results of our studies with such materials do show that the
804
ANALYTICAL CHEMISTRY
less complex samples, such as the topped (653 "C) and deasphaltened Wasson crude oil residue, give much more meaningful separations. Figure 5 , with its molecular weight us. sample elution plots and other appropriate data for this residue sample and the aromatic concentrate fractions, provides a summary and a basis for fraction selection in carrying out additional physical of chemical separations. For example, should the first 25% of the Wasson residue eluted by GPC (2.55% of original crude) be selected, a cut would be provided containing essentially all the material having molecular weights above 1800; thelast 2501, would contain most of the material with molecular weight less than 1000; and the middle 50% would have a comparatively limited 800 molecular weight range. An extension of this same type of sample selection to the lower boiling fractions of Figure 5 indicates the possibility of obtaining very limited molecular-weight range samples suitable for further studies. ACKNOWLEDGMENTS
The authors are indebted to A. B. King of Gulf Research and Development Co., Pittsburgh, for supplying high resolution mass data and to Daryl Doughty, formerly of the Bureau of Mines Bartlesville Petroleum Research Center and presently a graduate student at University of Oklahoma, for the proton magnetic resonance spectra. RECEIVED for review October 1, 1968. Accepted March 10, 1969. Investigation performed is part of the work of American Petroleum Institute Research Project 60 on "Characterization of Heavy Ends of Petroleum," which the Bureau of Mines is cooperatively conducting at Bartlesville (Okla.) and Laramie (Wyo.) Petroleum Research Centers. Reference to specific brands of equipment is made for identification only and does not imply endorsement by the Bureau of Mines.
