Anal. Chem. 2000, 72, 1039-1044
Quantitative Separation of Aliphatic and Aromatic Hydrocarbons Using Silver Ion-Silica Solid-Phase Extraction B. Bennett* and S. R. Larter
Fossil Fuels and Environmental Geochemistry (Postgraduate Institute): NRG, Drummond Building, The University, Newcastle upon Tyne, NE1 7RU, U.K.
A solid-phase extraction (SPE) method employing silver nitrate impregnated silica has been developed and evaluated for the separation of defined aliphatic and aromatic hydrocarbons from crude oils. The versatility of the SPE method is demonstrated using a light crude oil from the North Sea and a heavy crude oil from Orcutt field (Monterey, California, U.S.A.). The coefficients of variation for a number of geochemical parameters measured on both aliphatic and aromatic hydrocarbons were excellent. The separation efficiency of SPE is demonstrated using quantification of monoaromatic steroid hydrocarbons which are notoriously difficult to efficiently sequester into the aromatic hydrocarbon fraction using traditional liquid chromatographic procedure. The selectivity and efficiency of the SPE technique is comparable with that of silica gel TLC. However, losses of volatile compounds such as naphthalene are limited during SPE since the sample remains in solvent. We conclude that solid-phase extraction affords rapid sample turnover suitable for processing large sample numbers with high reproducibility. Solid-phase extraction (SPE) is a well-established technique used for the analysis of numerous different classes of compounds in a variety of matrixes.1 The benefits derived from SPEslow solvent volumes, rapid preparation, avoiding emulsion formation during liquid/liquid extractionshave resulted in a reappraisal of chromatographic separation methods employed in this laboratory with a view to developing rapid, inexpensive, reproducible methods. For example, Bennett et al.2 employed C18 nonendcapped SPE cartridges for rapid and reproducible isolation of C0-C3-alkylphenols from crude oils in a single step. The separation of petroleum into hydrocarbon groups is important in petroleum geochemistry since a number of compounds exist in trace quantities and require concentration prior to further analysis by GC-FID and GC/MS. Snyder3 established the classical method of column chromatography on silica gel and alumina for separating petroleum into compound groups. Diesel fuel was separated into aliphatic and mono-, di-, and polyaromatic * Corresponding author: (e-mail address)
[email protected]. (1) Berrueta, L. A.; Gallo, B.; Vicente, F. Chromatographia 1995, 40, 474483. (2) Bennett, B.; Bowler, B. F. J.; Larter, S. R. Anal. Chem. 1996, 68, 36973702. (3) Snyder, L. R. Anal. Chem. 1961, 33, 1527-1534. 10.1021/ac9910482 CCC: $19.00 Published on Web 02/01/2000
© 2000 American Chemical Society
hydrocarbons employing silica gel in microglass columns,4 although Wang et al.5 found 10% of the total naphthalene eluted with the aliphatic hydrocarbons. Incomplete separation of low molecular weight compounds and highly alkylated aromatic hydrocarbons has implications for isolation of aromatic compounds with increasing aliphatic character, such as monoaromatic steroid hydrocarbons (and benzohopanes). The isolation and characterization of monoaromatic steroid hydrocarbons is important in petroleum geochemistry6 since they are used for both facies and maturity assessments of sedimentary organic matter.7 Thin-layer chromatography has also been used extensively for the separation of aliphatic and aromatic hydrocarbons from crude oils and core extracts (e.g., Forster et al.8). Although the method is successful, the technique is laborious and time-consuming and utilizes relatively large solvent volumes. Automating the preparative separation of hydrocarbons solves some of the cost problems associated with large-scale LC and has been achieved through using medium-pressure liquid chromatography (MPLC),9 although, in this case, instrument costs and solvent volumes employed may also be discouraging. Increasing costs of environmentally friendly solvent disposal, materials, and time thus led us to consider the development of SPE based methods for application in petroleum geochemistry. Here we describe a SPE method for separating aliphatic and aromatic hydrocarbons from crude oils and rock extracts based on silver ion chromatography. We demonstrate the versatility of the SPE methods for group-type separation employing a lowviscosity North Sea oil (Miller Field) and a highly viscous (API ) 15.8) Californian oil (Orcutt Field). EXPERIMENTAL SECTION Materials. Dichloromethane (CH2Cl2) and n-hexane were purchased from Fisons (U.K.). Isolute C18 nonendcapped (NEC) and Florisil Solid-Phase Extraction (SPE) cartridges (500 mg of (4) Bundt, J.; Herbel, W.; Steinhart, H.; Franke, S.; Francke, W. J. High Resolut. Chromatogr. 1991, 14, 91-98. (5) Wang, Z. D.; Fingas, M.; Li, K. J. Chromatogr. Sci. 1994, 32, 361-366. (6) Seifert, W. K.; Moldowan, J. M. Geochim. Cosmochim. Acta 1978, 42, 7795. (7) Peters, K. E.; Moldowan, J. M. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments; Prentice Hall: Englewood Cliffs, NJ, 1993. (8) Forster, P. G.; Alexander, R.; Kagi, R. I. J. Chromatogr. 1989, 483, 384389. (9) Radke, M.; Willsch, H.; Welte, D. Anal. Chem. 1980, 52, 406-411.
Analytical Chemistry, Vol. 72, No. 5, March 1, 2000 1039
adsorbent/3 mL of reservoir) and empty (3-mL) cartridges and frits were obtained from Jones Chromatography Ltd (U.K.). Silica gel (Kieselgel 60G), silver nitrate, and 1,1-binaphthyl were obtained from Aldrich (U.K.). Preparation of SPE Cartridges. The method involves modification of a silver nitrate impregnated silica thin-layer chromatography technique in use here for many years. The following procedure describes the preparation of silver-impregnated silica SPE cartridges. A 10% silver nitrate/silica (Kieselgel 60G) substrate aqueous slurry was prepared and then placed in an oven at ∼120 °C. Once the substrate is dry, approximately 550 mg of Ag + silica is loaded into an empty sep-pak cartridge with a retaining frit. The Ag + silica is compacted by agitation and packing with a cleaned glass rod, and the substrate is contained by a second frit. The cartridges are precleaned by eluting 5 mL of n-hexane under positive pressure. Isolation of Hydrocarbons: Scheme I. The following scheme was found to be suitable for analysis of low-viscosity mobile crude oils. Typically, 100 mg of oil was loaded onto a preweighed C18 NEC SPE cartridge and allowed to absorb. The hydrocarbon-containing fraction is collected in n-hexane (5 mL). The solvent volume is reduced to ∼1/2 mL under a gentle stream of nitrogen gas at ambient temperature. Scheme II. The following scheme is a modification of Scheme I, to enable recovery of hydrocarbon fractions from heavy, viscous crude oils of low mobility. This method may also be applied to reservoir core and source-rock extracts. An aliquot of oil (∼80 mg) is smeared into a preweighed vial and dissolved in a minimum volume (∼200 µL) of CH2Cl2. The solution is transferred to a Florisil sep-pak and allowed to absorb. The hydrocarbon-containing fraction is eluted in 5 mL of CH2Cl2. Solvent is removed, to complete dryness, by evaporation in a stream of nitrogen gas. The extract is redissolved (agitation by sonication) in n-hexane and transferred to a C18 nonendcapped sep-pak. The hydrocarboncontaining fraction is collected in n-hexane (5 mL). The solvent volume is reduced to ∼1/2 mL. Separation of Hydrocarbon Fractions. An aliquot (50 µL from 1/2 mL) of the n-hexane (hydrocarbon-containing) fraction recovered during C18 NEC SPE of crude oils is added dropwise onto the Ag + silica. The aliphatic hydrocarbons are eluted in ∼2 mL of n-hexane. Aromatic hydrocarbons were subsequently recovered in ∼4 mL of CH2Cl2. The aliphatic and aromatic hydrocarbons were analyzed by GC/MS in selected ion monitoring (SIM) mode. Later et al.10 found the adsorbents’ (silica/alumina) activity, i.e., water content, affected adsorption chromatography of polycyclic aromatic compounds. Chromatographic separation problems may occur from deactivation of substrate if exposed to the atmosphere for several hours. In this case, we found deactivation of the Ag + silica substrate affected the retention of monoaromatic steroid hydrocarbons and naphthalene during n-hexane elution, resulting in their presence in the aliphatic hydrocarbon fraction. This is easily avoided by using freshly prepared substrate. In our experience, the method is highly successful when SPE column preparation and sample separation are performed in a continuous operation.
Contamination. Procedural blanks were used to determine potential problems of contamination from the SPE cartridges. Background arising from polypropylene and polyethylene cartridges may present a problem during trace analysis or sample cleanup.11 Plasticizers were observed in low quantities but rarely interfere with the abundant analytes for the types of samples used (oils and rock extracts). However, in the rare instances when plasticizers do present a problem during analysis, these problems may be reduced by employing glass SPE columns combined with stainless steel frits.11 Thin-Layer Chromatography. For thin layer chromatography (TLC), clean glass plates (20 cm × 20 cm) were coated with an aqueous slurry of silica gel (6 g/12 mL per plate) to provide a 0.5-mm-thick layer. The TLC plates were cleaned by development in ethyl acetate and then activated in an oven at 140 °C for 2 h. An aliquot from the hydrocarbon-enriched n-hexane fractions from C18 NEC SPE of crude oils was applied to the plate. Separation of aliphatic and aromatic hydrocarbons was achieved using
(10) Later, D. W.; Wilson, B. W.; Lee, M. L. Anal. Chem. 1985, 57, 2979-2984.
(11) Theobald, N. Anal. Chim. Acta 1988, 204, 135-144.
1040 Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
Figure 1. Partial gas chromatogram of aliphatic hydrocarbons isolated by (a) SPE and (b) TLC of Miller crude oil hydrocarbon fractions and (c) SPE and (d) TLC of Orcutt crude oil hydrocarbon fraction: i ) isoprenoid, Pr ) pristane, Ph ) phytane.
Table 1. General Geochemical Parameters Measured in Replicate (n ) 6) on Aliphatic Hydrocarbons Isolated from Miller and Orcutt Crude Oil by Silver-Impregnated Silica Solid Phase Extraction (SPE) and Thin Layer Chromatography (TLC) hopanoid parameters
steroid parameters homohopaned index
20Se (20R + 20S)
Rββf (Rββ + RRR)
Miller SPE 0.58 0.01 0.02
0.33 0.01 0.01
0.52 0.01 0.02
0.59 0.01 0.01
0.16 0.03 0.19
Miller TLC 0.57 0.00 0.01
0.30 0.00 0.01
0.56 0.02 0.03
0.49 0.01 0.02
0.25 0.01 0.04
0.56 0.00 0.01
Orcutt SPE 0.49 0.00 0.01
0.30 0.01 0.02
0.40 0.00 0.01
0.43 0.00 0.01
0.25 0.02 0.07
0.55 0.06 0.10
Orcutt TLC 0.48 0.00 0.01
0.29 0.01 0.03
0.41 0.01 0.02
0.47 0.00 0.01
code
Tsa Tm
BNHb (BNH + HOP)
mean sg CVh
1.12 0.11 0.09
0.17 0.01 0.05
mean s CV
1.03 0.05 0.05
mean s CV mean s CV
22Sc (22R + 22S)
a C b 27 18R Trisnorneohopane/C27 17R trisnorhopane measured from peak areas in the m/z 191 chromatogram. C28-Bisnorhopane/(C28bisnorhopane + hopane) measured from peak areas in the m/z 191 chromatogram. c C31 Rβ homohopane 22S/(22R + 22S) measured from peak areas in the m/z 191 chromatogram. d From combined 22S and 22R of Rβ hopanes and extended hopanes C31/(C31 + C32 + C33 + C34 + C35) measured from peak areas in the m/z 191 chromatogram. e C29-RRR-non-rearranged steranes (20S/(20R + 20S)) measured from peak areas in the m/z 217 chromatogram. f C29-(20S + 20R)-non-rearranged sterane isomers RRR and Rββ measured from peak areas in the m/z 217 chromatogram. g s is standard deviation. h CV is coefficient of variation ) s/mean.
n-hexane developer. The aliphatic hydrocarbon fraction, corresponding to a band between Rf values 0.8-0.9 was removed from the plate and transferred to a short column which contained a bed of alumina (1-cm depth) supported on a cotton wool plug. The aliphatic hydrocarbons were recovered with 40 mL of CH2Cl2. The solvent volume was reduced by Buchi evaporation. The same procedure was repeated to recover the remaining aromatic hydrocarbons (Rf ) 0.5-0.6). Gas Chromatography-Flame Ionization Detection. Aliphatic hydrocarbon fractions were analyzed on a Carlo Erba 5160 fitted with a DB-5 fused silica capillary column (30 m × 0.32 mm i.d. × 0.25 µm film thickness) (J&W Scientific) and with flame ionization detection (FID). Samples were injected in cooled oncolumn mode, and the oven temperature program was an initial temperature of 50 °C (2 min) and then 4 °C min-1 to a final temperature of 300 °C which was held for 20 min. Hydrogen was used as the carrier gas at a flow rate of 2 mL min-1. Data were acquired and processed using a Multichrom (VG LabSystems) data system. Gas Chromatography - Mass Spectrometry. Mass spectral characterization of compounds in the hydrocarbon fractions was carried out using combined gas chromatography/mass spectrometry (GC/MS) on a Hewlett-Packard 5890 GC (using splitless injection) interfaced to a HP 5970B quadrupole mass selective detector (electron input energy, 70 eV; source temperature, 250 °C). Aliphatic hydrocarbons and aromatic hydrocarbons were analyzed on a DB-5 fused silica capillary column. Column dimensions were 30 m × 0.32 mm i.d. × 0.25 µm film thickness (J&W Scientific). For aliphatic hydrocarbon analysis, the oven temperature program was 40 °C (initial time, 2 min) to 175 °C at 10 °C min-1 to 225 °C at 6 °C min-1 to 300 °C at 4 °C min-1 and held at 300 °C for 20 min. For aromatic hydrocarbons, the oven temper-
ature program was 40 °C for 2 min, from 40 to 300 °C at 4 °C min-1, and held at 300 °C for 20 min. Quantification. A standard stock solution of 1,1-binaphthyl was prepared in n-hexane. Peak area integration during GC analysis was achieved using Multichrom (VG LabSystems) running on a DEC MicroVax 3100. Peak area integration during GC/ MS analysis was by MASS LAB. The relative response factor (RRF) for individual aromatic compounds versus 1,1-binaphthyl was also assumed to be 1. RESULTS AND DISCUSSION Application of Solid-Phase Extraction for Recovery of Hydrocarbons from Light Crude Oils. Miller crude oil is a typical nondegraded North Sea black oil generated from Upper Jurassic, Kimmeridge Clay Formation source rocks. The oil is mobile, and following sample application to C18 NEC SPE it is readily absorbed in the SPE frit. Aliphatic Hydrocarbons. Figure 1a shows the GC chromatogram of aliphatic hydrocarbons recovered from the SPE of Miller crude oil. The chromatogram shows a distribution typical of a light oil with n-alkanes displaying a front-end bias toward n-C8 tailing off toward n-C37 alkane. Since SPE of light oils avoids subsequent solvent removal, the volatile hydrocarbons (< n-C15) are retained as shown in part a of Figure 1. Part b of Figure 1 shows the effect on aliphatic hydrocarbon distributions when solvent evaporation is complete. The chromatogram shows total loss of hydrocarbons < n-C11 and significant losses of C12-C14 alkanes. The carbon number distribution ranges from n-C12 to n-C37. The loss of volatile compounds presents a problem during application of TLC since solvent evaporation occurs at some stage during sample recovery. The results indicate that SPE has the advantage of retaining volatile compounds during recovery of aliphatic hydrocarbons. Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
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Figure 2. Partial total ion current (30 ions) traces of aromatic hydrocarbon fractions from (a) SPE and (b) TLC of Miller oil and (c) SPE and (d) TLC of Orcutt crude oil: N ) naphthalene, MN ) Methylnaphthalene, DMN ) Dimethylnaphthalene, P ) Phenanthrene, MP ) Methylphenanthrene, MAS ) monoaromatic steroid hydrocarbons, TAS ) triaromatic steroid hydrocarbons.
The precision of typical geochemical ratios determined on aliphatic hydrocarbons isolated during SPE and TLC are very good, indicated by the standard deviations (s) and coefficients of variation (CV) listed in Table 1. However, reproducibility of the bisnorhopane-related parameter is marginally improved during SPE. Aromatic Hydrocarbons. Figure 2 parts a and b show the GC/MS distributions of the aromatic hydrocarbons isolated during SPE and TLC separation, respectively, of Miller crude oil hydrocarbon fractions. The most noticeable differences between the chromatograms are among the early eluting compounds due to low recovery of volatile aromatic hydrocarbon compounds, such as naphthalene, during TLC. The s and CV during replication (n ) 6) of a number of aromatic hydrocarbon maturity ratios are shown in Table 2. The data from both Ag + silica SPE and TLC are highly comparable, although results from SPE were favored. The chromatographic behavior of monoaromatic steroid hydrocarbons (and benzohopanes) challenges the efficiency of aliphatic and aromatic 1042 Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
Figure 3. Partial mass chromatogram (m/z 253) showing monoaromatic steroid hydrocarbons during (a) SPE and (b) TLC of Miller oil and (c) SPE and (d) TLC of Orcutt crude oil. Key: 1 ) C21, 2 ) C22, 3 ) C27β20S, 4 ) C27β20R, 5 ) C28β20S, 6 ) C28R20S + C29β20S, 7 ) C28R20R + C29β20R.
hydrocarbon separation methods due to the combination of an aromatic ring and aliphatic character. Parts a and b of Figure 3 show the distributions of monoaromatic steroid hydrocarbons recovered from both SPE and TLC. The resulting distributions are highly comparable, indicating the success of both SPE and TLC for recovery of monoaromatic steroid hydrocarbons with the aromatic hydrocarbon fraction. The s and CV for replication of monoaromatic steroid hydrocarbon quantification are listed in Table 3. Although quantification of individual compounds from TLC and SPE differ slightly, probably due to changing instrument responses relative to 1,1binaphthyl internal standards, the relative carbon number distributions are similar. In summary, both SPE and TLC gave very good reproducibility of aromatic hydrocarbon maturity ratios and quantitative data. Application of Solid-Phase Extraction for Hydrocarbon Recovery from Heavy Oils and Core Extracts. The versatility of the SPE method with more viscous nonhydrocarbon-rich samples is demonstrated by the analysis of aliphatic and aromatic
Table 2. General Geochemical Parameters Measured in Replicate (n ) 6) on Aromatic Hydrocarbons Isolated by Silver-Impregnated Silica Solid Phase Extraction (SPE) and Thin Layer Chromatography (TLC) of Miller and Orcutt Crude Oil sample code
2-MNa 1-MN
2-MPb 1-MP
1-MPc 9-MP
MPI-1d
DMPe
mean s CV
1.13 0.01 0.01
0.72 0.01 0.01
0.77 0.02 0.01
0.48 0.00 0.01
0.17 0.00 0.02
mean s CV
1.10 0.01 0.01
0.74 0.02 0.03
0.73 0.03 0.04
0.48 0.01 0.02
mean s CV
1.35 0.01 0.01
1.67 0.13 0.07
0.54 0.05 0.09
mean s CV
1.30 0.03 0.02
1.63 0.07 0.05
0.57 0.04 0.06
C20f (C20 + C28)
Tg (T + M)
MeDBTh DBT
4-i 1-
4-EDBTj 4,6-DMDBT
MBpR1k
4-MBPl 2-BP
SPE (Miller) 0.21 0.57 0.01 0.01 0.04 0.01
1.74 0.02 0.01
2.36 0.03 0.01
0.34 0.01 0.02
7.37 0.11 0.02
2.27 0.06 0.03
0.17 0.00 0.01
TLC (Miller) 0.19 0.56 0.01 0.01 0.04 0.03
1.60 0.05 0.03
2.40 0.03 0.01
0.33 0.00 0.01
7.66 0.28 0.04
2.42 0.01 0.04
0.93 0.02 0.02
0.23 0.01 0.02
SPE (Orcutt) 0.13 0.38 0.00 0.01 0.03 0.02
1.72 0.04 0.02
2.03 0.05 0.02
0.38 0.01 0.03
4.71 0.08 0.02
1.87 0.04 0.02
0.94 0.01 0.01
0.22 0.01 0.02
TLC (Orcutt) 0.12 0.34 0.00 0.00 0.02 0.01
1.43 0.03 0.02
2.08 0.03 0.01
0.35 0.01 0.03
4.48 0.13 0.03
1.84 0.06 0.04
a 2-Methylnaphthalene/1-methylnaphthalene ratio, measured from peak areas in the m/z 142 chromatograms. b 2-Methylphenanthrene/1methylphenanthrene measured from peak areas in the m/z 192 chromatograms. c 1-Methylphenanthrene/9-methylphenanthrene measured from peak areas in the m/z 192 chromatograms. d 1.5 (2MP + 3MP)/(P + 1MP + 9MP)sMethylphenanthrene Index measured from peak areas in the m/z 178 and 192 chromatograms for phenanthrene and methylphenanthrene, respectively. No correction was made for the different response factors for the two ions. e 2,6- + 2,7-Dimethylphenanthrenes/1,6-dimethylphenanthrene measured from peak areas in the m/z 206 mass chromatograms. f Apparent carbon-carbon bond cracking in C28 (20R + 20S) to C20 triaromatic steroid hydrocarbons measured from the m/z 231 chromatogram. g Triaromatic/(triaromatic + monoaromatic steroid hydrocarbons) summed C27-C29 monoaromatic steroid hydrocarbons measured from peak areas in the m/z 253 mass chromatogram. Total summed C26-C28 triaromatic steroid hydrocarbons measured from peak areas in the m/z 231 mass chromatograms. h Summed isomers of methyldibenzothiophenes/dibenzothiophene, measured from peak areas in the m/z 184 and 198 mass chromatograms for dibenzothiophene and methyldibenzothiophene, respectively. No correction was made for the different response factors for the two ions. i 4-Methyldibenzothiophene/1-methyldibenzothiophene from peak areas in the m/z 198 chromatogram. j 4-Ethyldibenzothiophenes/4,6-dimethyldibenzothiophene measured from peak areas in the m/z 212 mass chromatogram. k 3-Methyl-biphenyl/2-methyl-biphenyl from peak areas in the m/z 168 chromatogram. l 4-Methyl-biphenyl/2-methyl-biphenyl from peak areas in the m/z 168 chromatogram.
Table 3. Monoaromatic Steroid Hydrocarbon Concentrations (µg g-1 Oil) from SPE and TLC Separation of Hydrocarbon Fractions Isolated from Miller and Orcutt Crude Oils 1 C21
2 C22
3 C27βS
mean s CV
23.15 2.33 0.10
12.42 1.17 0.09
30.32 1.32 0.04
mean s CV
18.55 1.59 0.09
9.88 0.93 0.09
mean s CV
13.48 0.48 0.04
mean s CV
12.91 0.67 0.05
4 C27βR
5 C28βS
6 C28RS + C29βS
7 C29βR + C28RR
Miller SPE 21.82 1.22 0.06
50.02 1.22 0.02
71.99 2.52 0.03
26.74 3.34 0.12
31.54 2.25 0.07
Miller TLC 22.18 2.42 0.11
51.18 5.74 0.11
80.42 8.21 0.10
37.97 3.28 0.09
32.26 1.41 0.04
170.86 6.35 0.04
Orcutt SPE 143.61 5.49 0.04
296.92 11.94 0.04
527.04 17.13 0.03
175.74 6.02 0.03
32.15 1.59 0.05
161.55 9.56 0.06
Orcutt TLC 134.13 8.60 0.06
275.00 21.54 0.08
506.49 28.58 0.06
171.52 8.65 0.05
hydrocarbons from a highly viscous oil (API gravity ) 15.8) from the Monterey Formation reservoir of the Orcutt Field (California, U.S.A.). Because of the viscous nature and lack of solubility of much of the oil in n-hexane, this oil (as well as other Californian oils) was only suitable for C18 NEC SPE after a preparative step using Florisil SPE to clean it up. The fraction isolated during Florisil SPE is ultimately dissolved in n-hexane in preparation for C18 NEC SPE. Aliphatic Hydrocarbons. The GC chromatogram of aliphatic hydrocarbons from Orcutt is shown in part c of Figure 1. The
chromatogram is dominated by n-alkanes ranging from n-C13 to n-C37, with a maximum at n-C18, and abundant isoprenoid alkanes. The absence of volatile hydrocarbons observed in the chromatogram is the result of solvent removal following Florisil SPE, prior to redissolution in n-hexane for application to C18 NEC SPE. This sample preparation has resulted in removal of volatile hydrocarbons up to < n-C15. Conventional molecular parameters for aliphatic hydrocarbons from SPE and TLC are shown in Table 1. The coefficient of variation (n ) 6) from the mean of the molecular maturity parameters shows both SPE and TLC are highly Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
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reproducible, with the parameters Ts/Tm and Bisnorhopane parameter being more consistent following SPE. Aromatic Hydrocarbons. Parts c and d of Figure 2 show the distribution of aromatic hydrocarbons isolated from the Orcutt crude oil. Alkylnaphthalenes and monoaromatic and triaromatic steroid hydrocarbons represent the dominant aromatic compounds. The CV for aromatic hydrocarbon maturity parameters during TLC and SPE are shown in Table 2. The two methods are highly comparable, though in this case TLC is more consistent than SPE. However, parameters utilizing volatile compounds such as methylnaphthalenes (2-MN/1-MN) and biphenyls (MBpR1) achieve marginally better reproducibility following the SPE method. The monoaromatic steroid hydrocarbons are abundant components of the aromatic fractions and representative mass chromatograms are shown in Figs 3c and d from TLC and SPE. The carbon number distributions are more complex than those obtained from Miller oil, due to increased contribution from rearranged monoaromatic steroid hydrocarbons.7 The carbon number distributions are highly comparable and quantification of a number of compounds is highly reproducible (Table 3). The slight differences in quantification of monoaromatic steroid hydrocarbons between TLC and SPE are attributable to changes due to mass spectral responses versus the internal standard, 1,1binaphthyl. The success of SPE for separating aliphatic and aromatic hydrocarbons from a heavy oil has been demonstrated. The same method may be successfully applied to rock extracts. The reproducibility of both TLC and SPE suggests that both these methods are suitable for separating aliphatic and aromatic hydrocarbons; however, the application of SPE presents a number of advantages including processing samples in large batches (10), small solvent volumes (
