Technical Notes Anal. Chem. 1996, 68, 3697-3702
Determination of C0-C3 Alkylphenols in Crude Oils and Waters Barry Bennett, Bernard F. J. Bowler, and Stephen R. Larter*
Fossil Fuels and Environmental Geochemistry (Postgraduate Institute): NRG, Drummond Building, The University, Newcastle upon Tyne NE1 7RU, U.K.
Three rapid methods for the isolation and determination of alkylphenols from crude oils are described. Two of the methods are based on a novel approach involving the preconcentration of crude oil phenols using non-endcapped C18 solid-phase extraction and a normal-phase solvent program. To our knowledge, this is the first report of a C18 sorbent being used in “normal-phase mode” to isolate relatively polar compounds from a more nonpolar matrix. Determination of alkylphenols is performed by reversed-phase high-performance liquid chromatography (RP-HPLC), combined with oxidative electrochemical detection (ED), or by gas chromatography/flame ionization detection (GC/FID) and gas chromatography/mass spectrometry (GC/MS) after chemical derivatization. A direct alkaline extraction RP-HPLC/ED approach is also described. The reversed-phase HPLC method affords rapid sample turnovers with accurate quantitation of phenol, cresols, and major dimethylphenols and is suitable for screening large numbers of samples. Alternatively, the GC/FID-GC/MS method allows the determination of phenol, cresols, dimethylphenols, ethylphenols, and trimethylphenols at levels suitable for routine use in petroleum geochemistry. We illustrate the methods with two petroleum geochemical examples: determination of the partition coefficients of alkylphenols in oil/brine systems under high pressure and temperature conditions, and study of alkylphenol distributions in a migration case history from the North Sea. Low molecular weight alkylphenols (C0-C3) occur widely in fossil fuels and their derivative products (e.g., crude oils,1 shale oils,2 and coal liquefaction products3) and in the aqueous environment (e.g., potable water,4 oil field formation water, and discharge waters5,6). In addition to being listed pollutants, alkylphenols have been proposed as tracers of the petroleum migration process7,8 (1) MacCrehan, W. A.; Brown-Thomas, J. M. Anal. Chem. 1987, 59, 477479. (2) Foley, J. P. J. Chromatogr. 1988, 441, 347-354. (3) Pauls, R. E.; Bambacht, M. E.; Bradley, C.; Scheppele, S. E.; Cronauer, D. C. Energy Fuels 1990, 4, 236-242. (4) Rennie, P. J.; Mitchell, S. F. Chromatographia 1987, 24, 319-323. (5) Produced water: Technological/Environmental Issues and Solutions; Ray, J. P., Engelhardt, F. R., Eds.; Environmental Science Research 46; Plenum Press: New York, 1992. S0003-2700(96)00299-5 CCC: $12.00
© 1996 American Chemical Society
and, when present in oil field waters, have been used as petroleum proximity indicators.9,10 With increasing interest from the petroleum geochemical community in these compounds, we have evaluated existing analytical methods and developed several new methods for the rapid quantitative determination of alkylphenols in petroleum systems (oils and waters). Numerous published methods exist for the determination of alkylphenols in organic matrices. Most analytical schemes primarily employ a combination of aqueous alkaline extraction of the matrix, followed by acidification and back-extraction into organic phases to prepare a phenol concentrate for LC or GC analysis.1,3,8,11-15 Reversed-phase solid-phase extraction (SPE) using C18 alkylbonded silica has been employed after alkaline extraction for the removal of nonpolar material and particulates.1,15,16 The determination of alkylphenol distributions in aqueous samples is relatively simple, generally involving filtration prior to RP-HPLC analysis (cf. also refs 16 and 17). The separation and quantitation of alkylphenols is most commonly achieved by HPLC, GC, and/or GC/MS. With many published RP-HPLC methods, there is a problem in achieving complete resolution of the m- and p-cresols.1 The use of a polystyrene-divinylbenzene column in combination with an alkaline mobile phase enables baseline separation of m- and (6) Macleod, G.; Taylor, P. N.; Larter, S. R.; Aplin, A. C. In Geofluids 93; Parnell, J., et al., Eds.; Geol. Soc. Spec. Publ. 1993, 18-20. (7) Larter, S. R.; Aplin, A. C. In Reservoir Geochemistry; England, W. A., Cubitt, J., Eds.; Geol. Soc. Spec. Publ. 1995, 5-32. (8) Taylor, P. Ph.D. Thesis, The University of Newcastle upon Tyne, 1994. (9) Rustamov, R. I. Azerb. Neft. Khoz. 1989, 1, 5-10. (10) Kudryakov, V. A. Uzb. Geol. 1987, 1, 52-58. (11) Hertz, H. S.; Brown, J. M.; Chesler, S. N.; Guenther, F. R.; Hilpert, L. R.; May, W. E.; Parris, R. M.; Wise, S. A. Anal. Chem. 1980, 52, 1650-1657. (12) Guenther, F. R.; Parris, R. M.; Chesler, S. N.; Hilpert, L. R. J. Chromatogr. 1981, 207, 256-261. (13) Ogan, K.; Katz, E. Anal. Chem. 1981, 53, 160-163. (14) Shoup, R. E.; Mayer, G. S. Anal. Chem. 1982, 54, 1164-1169. (15) Ioppolo, M.; Alexander, R.; Kagi, R. I. Org. Geochem. 1992, 18 (5), 603609. (16) Chao, G. K.-J.; Suatoni, J. C. J. Chromatogr. Sci. 1982, 20, 436-440. (17) Dale, J. D.; Larter, S. R.; Aplin, A. C.; Macleod, G. M. In Organic Geochemistry: Developments and applications to energy, climate, environment and human history; Grimalt, J. O., Dorronsoro, C., Eds.; Proceedings of the 17th International Meeting on Organic Geochemistry, Donostia-San Sebastia´n, The Basque Country, Spain, 4-8 September 1995, A.I.G.O.A. Spain, 1995; pp 396-398.
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p-cresols and the separation of xylenols in samples prepared from workplace air18 and shale oils.2 Capillary GC analysis of alkylphenol concentrates enables a relatively high degree of isomeric resolution to be obtained. Ioppolo et al.15 identified 19 individual alkylphenols in crude oils by capillary GC/FID and GC/MS, reporting concentrations ranging from 29 000 ng/g down to the limit of detection of 20 ng/g (ppb). Taylor8 studied the distribution of 20 C0-C3 alkylphenol isomers in petroleum and rock extracts using GC/MS. Macleod et al.6 reported total alkylphenol concentrations for a North Sea crude oil of around 50 mg/L, and for related North Sea formation waters, concentrations were close to 1 mg/L. The aims of this paper are to describe novel SPE-based methods which allow the rapid quantitative analysis of alkylphenols in petroleum and petroleum-related waters and to give a brief illustration of their practical application to petroleum geochemistry. EXPERIMENTAL SECTION Materials. High purity grade (Distol) diethyl ether was purchased from Fisher Scientific and purified before use by passing it through a short column of pre-extracted and activated neutral alumina (BDH). Acetonitrile suitable for HPLC/ED was obtained from Fisher. Methanol and petroleum ether (bp 40-60 °C) and other organic solvents were purchased as technical grade reagents and redistilled before use. Water was distilled and deionized. Magnesium sulfate (AnalaR, Fisher Scientific) was preextracted with dichloromethane and dehydrated in an oven at 150 °C. Orthophosphoric acid was prepared as a 50% solution in distilled water and extracted with dichloromethane prior to use. Isolute C18 and C8 non-end-capped (NEC) and C18 end-capped (EC) SPE cartridges (500 mg/3 mL) were obtained from Jones Chromatography Ltd. 2-Naphthol was purchased from Aldrich. Alkylphenol standards were obtained from Aldrich and BDH. AnalaR grade sodium acetate, sodium chloride (NaCl), and sodium hydroxide (NaOH) were obtained from BDH. BSTFA containing 1% TMCS was obtained from Sigma Chemical Co. Sample Processing. SPE Isolation of Alkylphenols. The isolation of an alkylphenol-enriched fraction from crude oils (Miller Oil, North Sea) was achieved by a novel approach involving a NEC C18 SPE sorbent in combination with a nonpolar solvent as the initial eluant. The C18 bonded silica sorbent was initially solvated (conditioned) using 3 mL of petroleum ether under gravity flow. The bulk of the remaining solvent was removed by a gentle air flush. Typically, for HPLC analysis of C0-C2 alkylphenols, 0.5 mL (∼400 mg) of oil was loaded directly onto the top of the SPE column. For GC analysis of C0-C3 alkylphenols the sample capacity with respect to a typical North Sea oil was 150-200 mg of oil/500 mg of adsorbent. 2-Naphthol (15 µg, internal standard) was added to the oil prior to loading onto the column. Positive pressure (applied manually, using a syringe and column adapter) was usually required to transfer the sample to the sorbent and to obtain suitable flow rates (∼0.5 mL/min) in subsequent elution steps. In contrast to the conventional approach, where polar eluents are used to initially develop reversedphase columns, the first eluent in this case is petroleum ether (4.5 mL) (n-hexane may also be used), and the eluate obtained is comprised mainly of nonpolar compounds, principally hydrocar(18) Nieminen, E.; Heikkila, P. J. Chromatogr. 1986, 360, 271-278. (19) Bennett, B.; Larter, S. R. In ref 17, pp 348-350. (20) Bennett, B.; Larter, S. R. Geochim. Cosmochim. Acta, submitted.
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bons. Alkylphenols are subsequently eluted (flow rate > 1 mL/ min) with 5 mL of methanol/water (1:1) and diluted to an accurate volume (usually 10 mL), again with methanol/water (1:1). At this stage, the extracts can be analyzed by RP-HPLC/ED. For GC analysis, the alkylphenols eluted with 4.5 mL of methanol/water (1:1) were collected in a glass-stoppered test tube (10 mL capacity) and extracted using a scaled-down version of the method described by Taylor.8 The methanol-water-alkylphenols mixture was diluted with 2 mL of distilled water and acidified with 1 drop of 50% orthophosphoric acid. The aqueous phase was then extracted with 2 × 2 mL of diethyl ether and then with 2 × 2 mL of petroleum ether/diethyl ether (4:1). The organic phase extracts were combined in a round-bottomed flask, and ∼5 mL dichloromethane was added. The solvent volume was reduced to ∼1 mL under vacuum using a Buchi rotary evaporator. Residual water was removed by passing the extract through a short column of anhydrous magnesium sulfate, rinsing the flask and the column with ∼2 mL total volume dichloromethane. The extract was collected in a vial, and the solvent volume was reduced to ∼200 µL under nitrogen. The alkylphenols were derivatized using BSTFA + TMCS (100 µL) and heating at 60 °C in an oven for 1 h minimum and were subsequently analyzed by GC/FID and/or GC/MS. Rapid Alkaline Extract Determination of Alkylphenols in Oils. For comparison with the SPE-based separation, direct aqueous alkaline extraction of oil samples was carried out as follows. An aliquot of oil (0.5 mL) was shaken with 10 mL of 1 M aqueous NaOH solution, and the mixture was allowed to separate into oil and water phases. The phenolate ions formed under alkaline conditions are readily extracted into the aqueous layer. The alkaline extract is then analyzed by RP-HPLC/ED. Rapid Determination of Alkylphenols in Oil Field Waters and in Brines from Crude Oil/Brine Equilibration Experiments. Water samples can be injected directly into the RPHPLC/ED system without prior treatment. HPLC Conditions. A Hewlett-Packard HP1050 HPLC system was used for the analysis of the C0-C2 alkylphenols. The column was a Hamilton PRP-1 (150 mm × 4.1 mm i.d.) with particle diameter 10 µm. Samples were injected onto the column in 20 µL injection volumes using an autosampler. HPLC conditions were adopted from Nieminen and Heikkila.18 The mobile phase comprised 17% acetonitrile and 83% aqueous 0.1 M sodium acetate adjusted to pH 11.5-11.6 with 1 M NaOH. pH measurements were monitored with an ATC pH meter (Piccolo 2 by Hanna). Flow rate was 2 mL/min. Alkylphenols were detected using an Antec Decade electrochemical detector (Presearch U.K.). The detector cell was equipped with a glassy carbon electrode and a silver/silver chloride reference electrode. The applied potential used for alkylphenol analysis was +0.6 V. For quantitation purposes, a standard stock solution of alkylphenols was prepared in distilled and deionized water, with concentrations ranging from 253 (o-cresol) to 36 ppm (3,5-DMP). Aliquots were taken from the stock solution and diluted to the levels expected in samples (∼0.1-5 ppm). Alkylphenol standard weights and peak areas obtained from a minimum of three calibration runs were plotted to determine calibration curves. Typically, five samples were analyzed, with three calibration standard samples injected at the beginning and at the end of the sample run sequence providing external standard calibration and checks on detector stability. Quantitation of all alkylphenols
detected in standard solutions and samples was based on peak area measurements obtained using Multichrom (VG LabSystems) running on a DEC MicroVax 3100. Alkylphenols concentrations in samples are reported in micrograms per liter (ppb). GC/FID and GC/MS Conditions. The alkylphenols were analyzed as the trimethylsilyl ethers using a Carlo Erba HRGC 5160 gas chromatograph fitted with a cooled on-column injector and flame ionization detector. The components of the SPE alkylphenol fraction were separated on a DB-5 fused silica capillary column (30 m × 0.32 mm i.d. × 0.25 µm film thickness; J&W Scientific). Hydrogen was used as carrier gas at a flow rate of 2 mL/min. The oven temperature program was 35 °C (initial hold 10 min), then 2 °C/min to 150 °C, then 8 °C/min to 300 °C (final hold 20 min). Phenol and the C1-C3 alkylphenol isomers were identified on the basis of retention time compared to authentic standards. Quantitation was based on the measurement of peak height using Multichrom. While the quantitation of many C1-C3 alkylphenol isomers in oils is possible using GC/FID analysis, coelution interference by nonphenolic compounds can cause problems in some instances. In such cases, GC/MS can be used in the single-ion monitoring (SIM) mode, monitoring the molecular ion and the M - 15 ion for phenol and the C1-C3 alkylphenols (as the TMS ether derivatives). GC/MS analysis was performed using an HP-1 fused silica capillary column (25 m × 0.25 mm i.d. × 0.17 µm film thickness) with an oven heating program as described for GC/ FID analysis. RESULTS AND DISCUSSION SPE Method Validation. Sorbent Selection. Alkylphenols are characterized by a relatively wide range in ability to form both polar (H-bonding) and nonpolar interactions. Isomers in which the phenolic hydroxyl group is sterically shielded, either fully as with 2,6-dimethylphenol (2,6-DMP) and 2,4,6-trimethylphenol (2,4,6-TMP), or partially as with 2,4-DMP and 2-isopropylphenol, exhibit lower affinity for H-bonding than isomers in which the functional group is exposed. Similarly, the polarity of the molecule would tend to decrease with increasing chain length of the alkyl substituents. This range in physicochemical characteristics presents one of the main problems in the isolation and analysis of C0-C3+ alkylphenols by chromatographic methods and is reflected in differences in the selectivity and sorptive/retentive capacity of the sorbent for different alkylphenols in a given elution system. The OH-shielded isomers are relatively weakly retained by silica sorbents, and one of the main aims of the C18 SPE procedure is to remove as many interfering compounds as possible without losing these components. This is particularly important since the abundance of these isomers relative to that of unshielded isomers forms the basis of a molecular parameter of potential value in assessing petroleum secondary migration distance, as discussed later. We have evaluated a number of SPE sorbent phases and solvent programs for the isolation of C0-C3 alkylphenols from crude oils. The importance of both the chain length of the alkyl group bonded to the silica and the presence of free silanol groups on the sorbent surface is illustrated in Figure 1. A comparison of the petroleum ether elution profiles for a mixture of C0-C3 alkylphenols on C18 NEC (Figure 1a) and C18 end-capped (EC) (Figure 1b) sorbents clearly shows a significant increase in the retention of shielded isomers by the NEC sorbent (Figure 1a).
Figure 1. Petroleum ether elution profiles of alkylphenol standards on (a) C18 NEC, (b) C18 EC, and (c) C8 NEC SPE sorbents, showing the concentrations of alkylphenols in sequentially eluted 1 mL aliquots of petroleum ether. Key: solid bars, 2,6-DMP; striped bars, 2-propyl; open bars, 2,4,6-TMP; dotted bars, 2,3,6-TMP.
The use of the NEC version of the C18 sorbent thus allows a greater quantity of solvent to be used in the interference elution, resulting in a cleaner alkylphenol-enriched fraction at the analyte elution stage. Similarly, a comparison of the elution profile for the C0-C3 alkylphenols on C8 NEC (Figure 1c) and C18 NEC (Figure 1a) sorbents illustrates the importance of the alkyl chain length in providing increased retention of the shielded isomers, such as 2,6-DMP. It is apparent from these elution profile experiments that the volume of the interference elution must be controlled in order to prevent breakthrough losses of the OH-shielded alkylphenol isomers. By using non-end-capped C18 bonded silica, the interference elution volume could be more than doubled without causing breakthrough loss of target compounds (Figure 1a). Of the sorbents tested in this study, the non-end-capped octadecyl bonded silica proved the most satisfactory, allowing the isolation of a relatively clean alkylphenol-enriched fraction without loss of OH-shielded alkylphenol isomers. Reproducibility and Recovery. The standard deviation (n ) 6) for replication of the quantitation achieved by the analytical method was evaluated by GC/MS for C0-C3 alkylphenols as 4.35.5% for nonshielded isomers and 6.4-8.6% for shielded isomers. Recovery of individual standard compounds was in the range 88.3-93.6% for nonshielded isomers and 73.7-82.1% for shielded isomers. The greater standard deviation and lower recovery values for the OH-shielded alkylphenols are considered to reflect slight differences in elution rate, since loading and elution volumes were kept constant in these experiments. The experiments Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
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Table 1. Replicate Comparisons of Alkylphenol Concentrations (µg/L) Determined by RP-HPLC/ED after Isolation by SPE and by Alkaline Extraction of Miller Oil solid-phase extraction
alkaline extraction
compound
run 1 A
run 2 B
run 3 C
mean
SD
(1) phenol (2) m-cresol (3) p-cresol (4) o-cresol (5) 3,5-DMPa (6) 3,4-DMPa (7) 2,5-DMP (8) 2,3-DMPa (9) 2,4-DMP (10) 2,6-DMP
2913 1988 2387 8869 1811 1568 2317 3709 6808 6699
2941 2021 2379 8910 1987 1585 2344 3720 7010 6872
2852 1947 2258 8673 1908 1502 2440 3808 6992 6723
2920 1985 2341 8817 1902 1551 2367 3746 6937 6765
37 30 59 104 72 36 53 44 91 77
% SD
run 4 D
run 5 E
run 6 F
mean
SD
% SD
1.3 1.5 2.5 1.2 3.8 2.3 2.2 1.2 1.3 1.1
2385 1993 2152 9016 1723 1301 2223 2744 5583 8343
2332 1951 2176 8986 1678 1283 2304 2586 5469 8044
2360 1951 2098 8783 1622 1252 2300 2596 5458 7985
2359 1965 2142 8928 1674 1279 2276 2642 5503 8124
22 19 33 103 41 21 37 72 57 157
0.9 1.0 1.5 1.2 2.5 1.6 1.6 2.7 1.0 1.9
a Coelution of compounds determined by cochromatography with authentic standards: 3,5-DMP + 4-ethylphenol; 3,4-DMP + 3-ethylphenol; 2,3-DMP + 2-ethylphenol.
described in this paper involved manual elution of the SPE columns, with inherent problems in controlling elution rate. The associated variability with respect to recovery of OH-shielded alkylphenol isomers would probably be reduced through the use of a sample processing station using fixed-rate eluant delivery. Nonetheless, even with manual elution, the method is adequately reproducible with excellent recovery. Mechanism of Sorption. The retention of alkylphenols by the NEC C18 bonded silica probably reflects a combination of (1) nonpolar (van der Waals) interaction between the alkylphenol and the octadecyl chains bonded to the silica substrate and (2) polar interaction (hydrogen bonding) between the phenolic hydroxyl group and the silanol groups on the substrate surface. The low retention capacity of an EC SPE sorbent for OHshielded alkylphenol isomers reflects both the low availability of reactive silanol groups on the sorbent surfaces due to end-capping and the shielding effect of the alkyl substituents on the hydroxyl group of the phenol molecule. The much higher abundance of silanol groups on a non-end-capped substrate increases the opportunity for polar interactions and results in the increased retention of the OH-shielded alkylphenol isomers. If polar interaction was the only mechanism involved in the retention of alkylphenols then it should be possible to obtain a similar separation using natural (nonalkylated and non-endcapped) silica and the same eluants as described above. Under these conditions, however, the breakthrough volume is very small, and the more highly alkylated phenol isomers are eluted with the first nonpolar eluant, together with a variety of hydrocarbons and heteroatom compounds. The octadecyl chain bonded to the silica thus appears to play an important role in enhancing the retention of the alkylphenols, probably through van der Waals interactions. The NEC C18 bonded sorbents possess both nonpolar and polar characteristics, reflecting the presence of both the alkyl groups and the reactive silanol groups. The SPE system described here is perhaps more accurately described as a mixed mode or multiple active group interaction chromatography (MAGIC) system, rather than as a system in which a “reverse-phase sorbent is used in the normal-phase mode”. Sorbents with a cyanopropyl (CN) functional group are commonly used nonpolar sorbents which exhibit both polar and nonpolar interactions depending on the polarity of the conditioning medium and the availability of surface silanol groups for polar secondary interaction. Thus, this is probably not the first report 3700 Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
of a nonpolar sorbent/nonpolar eluant application of SPE. However, to our knowledge, this publication represents the first report of an alkyl-bonded sorbent being used in this way. We envisage that this novel approach will be of particular significance in furthering the analysis of polar compounds in petroleum and rock extracts as it mimics natural chromatographic processes in petroleum migration. Extraction Efficiency of the SPE Cartridges. The extraction efficiency of the SPE/RP-HPLC/ED method for various C0-C2 alkylphenols was determined through a comparison with the results of analyses made on the same samples using alkaline extraction followed by direct RP-HPLC/ED analysis. Triplicate determinations of alkylphenol concentrations after isolation by SPE and also by direct alkaline extraction were carried out using crude oil from the Miller field (North Sea). Inspection of the data in Table 1 shows that the results from the two methods are reproducible and comparable for most of the alkylphenols. However, the data also show that phenol is extracted more efficiently by SPE, whereas 2,6-DMP is recovered more efficiently using the alkaline extraction method. By using the SPE method described here, an alkylphenol concentrate from an oil sample is ready for analysis by RP-HPLC/ ED in ∼15 min with few problems, even with oils having high asphaltene content. Alkaline extraction, however, may be influenced by the ability of the mixed oil and water to separate, since emulsions may form during the shaking/extraction stage.14 More recently reported analytical schemes have attempted to overcome the problems of emulsion formation by centrifugation15 or the use of a “milder” alkaline extraction method combined with SPE.1 However, as a consequence, analysis time is increased, and additional stages are introduced into the analytical scheme. In contrast, the isolation of alkylphenols by SPE requires only one (relatively short) preparative stage prior to analysis by HPLC. The combination of SPE and RP-HPLC/ED provides a rapid (∼25 min), efficient, relatively problem-free, reproducible, and highly selective method for the quantitative determination of alkylphenols in crude oil. Application 1. Determination of Alkylphenol Partition Coefficients in North Sea Crude Oil/Brine Systems. To demonstrate an application of the rapid SPE-based method for alkylphenol analysis, we have applied it to an investigation of the oil/water partition coefficients of alkylphenols in crude oil/brine systems using a flow injection device in which alternate segments
Figure 2. Reversed-phase HPLC/ED chromatograms of alkylphenols (peaks numbered as indicated in Table 1) from (a) extract of Miller oil, (b) extract of Miller oil partitioned 1:1 with brine (2% NaCl), and (c) brine (2% NaCl) sample after partitioning with Miller oil at 25 °C and 25 bar.
of oil and brine are equilibrated during flow through a capillary tube under specific conditions of temperature and pressure.19 Figure 2a shows a typical RP-HPLC/ED chromatogram of the alkylphenols in a Miller field crude oil; the distribution is dominated by phenol and cresols, and there are appreciable quantities of DMPs present. The RP-HPLC/ED chromatograms in Figure 2b,c show the alkylphenol distributions in oil and brine samples, respectively, after equilibration with oil/water (1:1 v/v). In comparison to the starting oil composition, partitioned oil samples (compare Figure 2a and b) are relatively depleted in phenol and (to a lesser extent) cresols, while the abundances of the DMPs appear almost unchanged. Conversely, the equivalent brine samples analyzed by direct injection RP-HPLC/ED are characterized by a strong predominance of phenol, with abundant cresols (Figure 2c); the DMPs are present in much lower relative abundances. These results apparently reflect the relative aqueous solubilities of alkylphenols, where solubilities decrease in the order phenol > cresols > DMPs. The full study of the effect of pressure and temperature, as well as salinity, on alkylphenol partition coefficients involved the analysis of over 200 samples and was only made possible by the speed of the SPE-based method. Full details of the partition study will appear elsewhere.20 In summary, the quantitative determination of phenol, cresols, and 2,4-, 2,5-, and 2,6-DMPs in crude oils can be rapidly and efficiently achieved using SPE/RP-HPLC/ED, while brine samples can be analyzed directly by RP-HPLC/ED. The methods are reliable, involve a minimum of sample handling/processing, and
allow an analysis time of ∼25 min/sample. The main disadvantage is the lack of resolution of the ethylphenols and coeluting DMPs and the inability to quantify the C3 alkylphenols. If such a degree of resolution is required of an alkylphenol mixture, then either GC or GC/MS must be used. Application 2. Variations of Alkylphenol Concentrations with Petroleum Migration Range in North Sea Oils. Low molecular weight alkylphenols are found in petroleums,15 in reservoir waters,17 and on sediment surfaces.21,22 The degree and position of alkyl substitution on the phenol nucleus determine the solubility/sorptive properties of these compounds and hence their distribution between petroleum and the organic, aqueous, and mineral phases of subsurface environments.7,8 Consequently, it might be expected that there would be a change in both the relative and absolute abundances of the various alkylphenols during the movement of petroleum from source to trap, with the degree of change reflecting the extent of interaction of the migrating petroleum with water, organic matter, and the mineral matrix in the carrier system. As a class of compounds, the alkylphenols are thus of potential value in providing assessments of secondary migration distance.23 Taylor8 analyzed the alkylphenols in oils sampled along a proposed fill-spill sequence in the Tampen Spur area of the North Sea, and more recently Chen24 analyzed these compounds in oils sampled across the Miller reservoir, also in the North Sea. Petroleum secondary migration distances for these studies were ∼25 km for the Tampen Spur and ∼8 km for the Miller field. In both cases, with increasing migration distance, there was a decrease in the concentration of C0-C3 alkylphenols and some increase in the relative abundances of isomers in which the phenolic hydroxyl group is shielded compared to those in which this group is exposed. To test the SPE-GC/FID method within the context of a geochemical application, we have repeated the analysis of alkylphenols in the Tampen Spur oils. A GC/FID chromatogram of the alkylphenols obtained through SPE isolation from one of the Tampen Spur oils (Statfjord East) is shown in Figure 3a. GC/FID analysis alone is often suitable for the determination of many of the alkylphenols listed in Table 2, but in some cases problems with coelution interference require that GC/MS-SIM be used to provide a more reliable and accurate quantitation of the C0-C3 alkylphenols. Absolute quantitation by GC/MS-SIM should involve correction for response factors. As shown in Figure 4, there is a general trend for the concentration of C0-C3 alkylphenols in the Tampen Spur oils to decrease with increasing migration distance, as found earlier using the alkaline extraction method.8 The alkylphenol content of the sample at migration distance 14 km (34/7-12, Tordis), a sample not analyzed by Taylor,8 is high with respect to the general trend, suggesting that this oil may be from another source/migration route or may have been subjected to alternative process(es) prior to or during trapping. The SPE-based method for the determination of alkylphenol distributions in oils appears to provide a combination of speed and precision, making it suitable for use in routine (21) Isaacson, P. J.; Frink, C. R. Environ. Sci. Technol. 1984, 18, 43-48. (22) Laquer, F. C.; Manahan, S. E. Chemosphere 1987, 16, 1431-1445. (23) Larter, S. R.; Taylor, P. N.; Chen, M.; Bowler, B. F. J.; Ringrose, P.; Horstad, I. Europe’s Petroleum Resource; University of Aberdeen’s Quincentennial Symposium; Geological Society: London, 1995 (in press). (24) Chen, M. Ph.D. Thesis, The University of Newcastle upon Tyne, 1995.
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Table 2. Key to Identification of C0-C3 Alkylphenol Isomers in GC/FID and GC/MS Chromatograms in Figure 3
Figure 3. (a) Partial gas chromatogram (GC/FID) of C0-C3 alkylphenols in an SPE alkylphenol fraction from a Statfjord East (Tampen Spur) crude oil. (b) GC/MS chromatogram of the same sample. The numbers refer to the peak assignments listed in Table 2. Alkylphenols were analyzed as the TMS derivatives.
Figure 4. Variation in the total concentration of measured C0-C3 alkylphenols in oils from the Tampen Spur area (North Sea) with relative migration distance, assuming a filling point at 34/7-5 (0 km, Statfjord East). Key: b, data from this study; O, data from Taylor.8 At 25 km, the open circle is under the solid circle.
geochemical analysis, and provides data comparable to those obtained by established methods. CONCLUSIONS We have developed and validated a suite of methods for the quantitative isolation and analysis of alkylphenols from crude oils.
3702 Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
peak no.
compound
peak no.
compound
1 2 3 4 5 6 7 8 9 10
phenol o-cresol m-cresol p-cresol 2-ethylphenol 2,5-DMP 2,4-DMP 2,6-DMP 3,5-DMP 4-ethylphenol
11 12 13 14 15 16 17 18 19
2,3-DMP 3,4-DMP 2-propylphenol 3-isopropylphenol 4-isopropylphenol 2,4,6-TMP 2,3,5-TMP 2,3,6-TMP 3,4,5-TMP
Two of the methods for the isolation of an alkylphenol-enriched fraction are based on a novel application of SPE using a non-endcapped C18 bonded silica sorbent. The selectivity of the SPE method reflects a multiple-mode interaction (both polar and nonpolar) between the sorbent and analyte. When combined with analysis by reversed-phase HPLC and electrochemical detection, the methods enable the quantitation of phenol, cresols, and several C2 alkylphenols in crude oils within an analysis time of about 25 min/sample, making possible the analysis of large sample numbers generated during a comprehensive study of partition behavior of alkylphenols in crude oil/brine systems or as part of large reservoir geochemical studies. The SPE-GC/FID-GC/MS procedure is necessary when information regarding individual (C0-C3) alkylphenols is required. We have illustrated this method with a small secondary migration case history from the North Sea. Again, alkylphenol isolation by SPE has led to a reduction in analysis time in comparison to other published methods. The two analytical techniques are highly complementary, RP-HPLC providing a rapid screening method by which samples of interest may be selected for the determination of more detailed isomeric information obtainable through the use of SPE-GC/FID-GC/MS. ACKNOWLEDGMENT We thank the European Community JOULE II research program for funding. We thank Gordon MacLeod, Paul Taylor, Jason Dale, Brian Horsfield, and Lorenz Schwark for useful discussions. We thank Idar Horstad (Saga Petroleum a.s., Norway) for providing the Tampen Spur oil samples and relevant discussion, and we thank Andrew Fleet (B.P.) for kindly donating the Miller crude oil.
Received for review March 26, 1996. Accepted July 19, 1996.X AC960299X X
Abstract published in Advance ACS Abstracts, September 1, 1996.
