Anal. Chem. 1992, 64, 7337-1344
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Liquid Chromatographic Separation Schemes for Pyrrole and Pyridine Nitrogen Aromatic Heterocycle Fractions from Crude Oils Suitable for Rapid Characterization of Geochemical Samples Maowen Li, Stephen R. Larter,’ and Daniel Stoddart Newcastle Research Group in Fossil Fuels & Environmental Geochemistry, Drummond Building, The Uniuersity, Newcastle upon Tyne NE1 7RU, U.K.
Malvin Bjoray Geolab Nor, Hornebergueien 5, P.O.Box 1581, 7002 Trondheim, Norway
The use of a polar amlnocyano-bonded silica HPLC column (PAC) for the class separation of nitrogencompounds in crude oils is described. Elution of deasphalted oils with solvents of a composltion gradient yielded well-defined pyrrole and pyridine nitrogen fractions which were characterized by gas chromatographyIn conJunctlonwith flame ionizationdetection (FID), nitrogen phosphorus detection (NPD), and mass spectrometry (MS). Comparison Is made wlth two open column chromatographic methods previously reported In the Ilterature. The new separation schemes reported here have made it possible to Identify more easily the isomeric nltrogen compounds, particularly the pyrrole nltrogen specles. On the basis of comparlson of crude oils wlth their potentlal source rocks, p d M e maturationand mlgratlonrelatedfractionations of hindered and less hindered alkylcarbazole and benzocarbazole isomers are observed.
INTRODUCTION Though present in small quantities in most crude oils, typically between 0.1 and 2.0%,organic nitrogen is present predominantly as aromatic heterocycles,’ with a predominance of neutral pyrrole structures over basic pyridine forms. Richter et al.2 report that crude oils have typically one-quarter to one-third of the nitrogen in basic form. Most of the nitrogen in crude oils is associated with high-boiling petroleum fractions, principally asphaltenes, and recent work3 suggests that approximately one-third of asphaltene nitrogen is typically in alkylpyridine and benzo form. While present in trace amounts in most marine petroleums, interactions of nitrogen species in general and basic nitrogen in particular with acidic functionalities elsewhere in the petroleum or with mineral surfaceshave been implicated along with other components in controlling critical Darcy “law” parameters such as petroleum viscosity and interfacial tension and system wettability. Due to the complexityand wide polarity range of petroleum hetero-compoundfractions, isolation and further fractionation accordingto chemical class is a necessary prior step to detailed chemical characterization of nitrogen-rich fractions by gas (1)Snyder, L. R.; Buell, B. E. Anal. Chem. 1968,40,1295-1302. (2)Richter, P. P.;Ceasser, P. D.; Meisel, S. L.; Offenhauser, R. D. Ind. Eng. Chem. 1952,44,2601-2605. (3)Wilhelms, A.; Patience, R. L.; Jorgensen, S.; Larter, S. R. Geochim. Cosmochim. Acta, in press.
chromatography (GC) or gas chromatography-mass spectrometry (GC-MS). Consequently, several separation schemes for nitrogen compound separation have been reported (e.g. see refs 1 and 4-11). Most of these nitrogen compounds schemes developed have been open column liquid chromatography based and, though useful, are often characterized by low selectivity for nitrogen species and incomplete separation of the specific compound classes. Further, large volumes of eluents are necessary and expensive materials and lengthy analysis times also make it difficult and impractical to apply these methods in routine geochemicaloperation (e.g. in the mapping of reservoir petroleum column nitrogen speciation where potentially hundreds of samples might be analyzed; cf. ref 12). Normal-phase high-performance liquid chromatography (HPLC)has been shown to be useful in the rapid separation and relatively detailed characterizationof nitrogen- and other heteroatom-containing compounds in crude oils, with silica gel and basic alumina being commonly used as adsorbents (e.g. see refs 13-16). Although strong retention of nitrogen species on such polar stationary phases is attractive from the viewpoint of separation, irreversible adsorption is a severe problem, especially when recovery of nitrogen compounds for further characterization by GC/GC-MS is required. To overcome retention problems associated with polar adsorbents, various derivatized, stationary phases have been developed, such as amino, alkylamine, alkylnitrile, cyano(4)Schiller, J. E.;Mathiason, D. R. E. Anal. Chem. 1977,49, 12251228. (5)Schiller, J. E. Anal. Chem. 1977,49,2292-2294. (6)Later, D.W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vaeailarm, D. L. Anal. Chem. 1981,53,1612-1620. (7)Schmitter, J. M.; Arpino, P. J. Mass Spectrom. Reu. 1985,4,87121. (8)Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Anal. Chem. 1983,55,225-231. (9)Dorban, M.; Schmitter, J. M.; Garrigues, P.; Ignatiadis, I.; Edward, M.; Arpino, P.; Guiochon, G. Org. Geochem. 1984,7,111-120. (10)Dorban, M.; Ignatiadis, I.; Schmitter, J. M.; Arpino, P.; Guiochon, G.; Toulhoat, H.; Huc, A. Fuel 1984,63, 565-570. (11)Frolov, Y. B.; Smirnov, M. B.; Vanyukova, N. A.; Sanin, P. I. Petrol. Chem. USSR (Engl. Traml.) 1989,29,87-102. (12)Karlsen, D. A.; Larter, S. R. In Correlation in Hydrocarbon Exploration;Graham, S.,Ed.; Norwegian Petroleum Society: Oslo, 1989; pp 77-85. (13)Dong, M.; Locke, D. C. J. Chromatogr. Sci. 1977,15,32-35. (14)Radke, M.; Willsch, H.; Welte, D. H. In Aduances in Organic Geochemistry 1979;Bjoroy, M., Ed.; Wiley: Chichester, U.K., 1980; pp 504-512. (15)Green, J. B.; Grizzle, P. L. In Trace Analyses; Lawrence, J. F., Ed.; Academic Press: New York, 1982;pp 223-265. (16)Matsunaga, A. Anal. Chem. 1983,55,1375-1379.
0003-2700/92/0364-1337$03.00/0 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 84,
NO. 13, JULY 1, 1992
Table I. Sample Descriptions and Basic Geochemical Data sample tYPY origin % nitrogen % extract aliphatic aromatic resins asphaltene
a
A B crude oil heaw crude oil Californian Californian 0.51
C
shale Californian
100
1.30 24.1
11.1
18.9 9.4 39.6 32.1
0.61
0.57
0.41
0.44
0.32
0.08
0.47
0.43
0.07
6.3 38.6 44.0
13.6 52.2 10.1
D shale Californian
F
crude oil North Sea, Norway 0.14
0.69 Group Type Composition 31.9 59.5 10.5 24.2 46.4 15.5 17.8 0.3 Biomarker Parametersa 22S/(22S + 22R)-C32homohopane 0.57 0.60 20S/(20S + 20R)-C29 aaa-sterane 0.10 0.38 a@@/(aaa + a@@)-Czssterane 0.06 0.49
G crude oil North Sea, Norway 0.14
57.4 22.9 17.8 1.9
57.2 24.9
0.60
0.60
0.38
0.34
0.46
0.46
16.9 1.0
Calculated using the appropriate peak heights in the mlz 191 and 217 mass chromatograms, respectively.
and aminocyano-bonded silica. The applications of these stationary phases in the separation of nitrogen heterocyclics from hydrocarbons (particularly polyaromatics) has been reviewed and compared.” We discuss here how a polar aminocyano-bonded silica column (PAC) may be successfully applied in the rapid separation of pyrrole and pyridine nitrogen heterocycles for subsequent GC and GC-MS analysis directly from light whole oils with nitrogen contents as low as 0.1 w t % within 40 min. Comparison is made with two open column liquid chromatographic methods thought to be simple and rapid.6J8 Results on the specific distributions of pyrrole nitrogen compounds obtained by GC-MS and GC-nitrogen phosphorus detection (GC-NPD) from several crude oils and source rocks are also presented as an illustration of the methods’ utility.
EXPERIMENTAL SECTION Sample Preparation. Table I gives a list of some of the crude oils and rock extracts employed in this study. Rock extracts were obtained by Soxhlet extraction of crushed cores (
