Anal. Chem. 1987, 59, 477-479
477
Determination of Phenols in Petroleum Crude Oils Using Liquid Chromatography with Electrochemical Detection W. A. MacCrehan* and J. M. Brown-Thomas
Organic Analytical Research Division, Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
A new method for the determination of phenolic compounds in unrefined petroleum products is presented. The method is based on a k a h solvent extraction of the dl, purification wlth a solid-phase extraction column, and determination by reverse-phase liquid chromatography with oxidative eiectrochemical detection. Phenol and 2-methyiphenoi, as well as 1- and 2-naphthol, have been determined in two National Bureau of Standards (NBS) Standard Reference Materials: SRM 1582 petroleum crude oil and SRM 1580 organics in shale oll. Agreement between the new HquM chromatography (LC) method and Independent (gas chromatography (GC) and gas chromatography/mass spectrometry (GCIMS)) methods is good. The detection limits are below 100 ng/g for the phenoWnaphthois in the oil.
There are many methods for the determination of phenolic compounds in unrefined petroleum products, including gas chromatography/mass spectrometry (I),gas chromatography following derivatization (2), and colorimetry (3). Because of its sensitivity and selectivity, one particularly advantageous approach for this type of determination is liquid chromatography with oxidative electrochemical detection, LCEC (4). The LCEC method provides two major advantages: the sample preparation is simple, a solvent extraction is followed by direct determination, no derivatizations are required; and the sensitivity is quite good, the detection limit is in the sub-microgram-per-gram range. range. In the previous LCEC work (4), the sample preparation consists of the extraction of the phenols from the oil sample with strong aqueous base. However, an emulsion often results from this procedure, necessitating centrifugation of the two layers. In the new sample preparation described here, the extraction solution does not form an emulsion with the oil. If only the simple alkaline extraction is used, the extract contains, in addition to the desired phenols, unwanted sample components that adsorb t o and eventually destroy bondedphase silica columns. In the new method, the sample is clarified by a simple filtration through an octadecylsilyl solid-phase extraction column, removing the strongly adsorbed sample components but passing the phenols. In addition to phenol and 2-methylphenol, the number of phenolic compounds that may be simultaneously determined by the new LCEC method has been increased to include 1-and 2-naphthol by the use of a gradient-elution solvent program. The new method has been tested on two Standard Reference Materials: organics in shale oil (SRM 1580) and petroleum crude oil (SRM 1582). The determined values in SRM 1580 for phenol and 2-methylphenol are in excellent agreement with the certified values. Additionally, 1-and 2-naphthol are determined in the shale oil.
EXPERIMENTAL SECTION Apparatus. The liquid chromatograph consisted of a dualpiston solvent delivery system with pressure feedback compensation, a low pressure solvent gradient programmer, a loop-type
(fixed 2O-pL volume) injector, and constant-potentialpotentiostat. The electrochemical detector cell was a modified TL-6A (Bioanalytical Systems, West Lafayette, IN), where the working electrode block was fabricated in-house. The working electrode was a 1.2 mm diameter glassy carbon rod press-fit into an undersized hole in a poly(tetrafluoroethy1ene) block. The electrode was positioned in the center of the gasketed flow spacer but not directly in opposition to the auxiliary electrode. This configuration provides a relatively low overall cell impedance without the signal-to-noise problems associated with opposed electrodes (5). All detector potentials are referenced to a silver/silver chloride, 3 mol/L potassium chloride reference electrode. Extraction of Phenols from Oils. Phenols and naphthols may be quantitatively extracted from oil samples (dissolved in organic solvent) into a strongly basic aqueous solution (4) because of the ease of formation of the corresponding phenolate anions (pK, values are 9.89 for phenol, 10.20 for 2-methylphenol, 9.34 for 1-naphthol,and 9.51 for 2-naphthol (6)). However, an emulsion often forms upon extraction of the oil sample ( 4 ) ,probably due in part to the surface-active character of the phenolate anions. We investigated many combinations of organic and aqueous solvents and have found that if the oil sample is dissolved in hexane and extracted with a 60% methanol/water (v/v) mixture, containing 0.1 mol/L sodium hydroxide, no emulsion forms. The addition of this proportion of methanol was crucial (&lo%),to avoid the formation of an emulsion. The proportion of oil sample to hexane was determined by the limited solubility in hexane, not by the formation of an emulsion. For the extraction of the phenols/naphthols from the oils, a weighed portion of the oil sample (0.1 g for the shale, 0.5 g for the crude) was dissolved with shaking into 3 mL of hexane in a 30-mL separatory funnel. Three milliliters of the methanol/ water/base extractant was then added and the mixture was shaken vigorously for a minimum of 3 min (shorter equilibrations resulted in erratic and low recoveries). After the layers were allowed to separate (about 10 min), the lower layer was drawn off for fdtration through a solid-phase extraction column-a (2-18 Sep-PAK cartridge (Millipore, Inc., Bedford, MA). This substantially clarifies the sample, removing coextracted material that would strongly adsorb on a (2-18 analytical LC column, as well as removing most particulate material in these extracts. With this composition of extractant,the phenols/naphthols are not retained by the cartridge since they are present as the conjugate-base anions in a strongly methanolic solvent. After filtration, 20 p L of glacial acetic acid is added to the sample to neutralize the sodium hydroxide and convert the phenolate anions back to the phenolic form. If the samples could not be analyzed immediately, we found no losses in the phenols when stored at -4 "C for periods up to 1week. After being thawed with ultrasonic agitation, a 20-pL sample was injected into the LCEC system for the final determination. The extraction procedure, although only tested on these two oil samples, should be applicable to other unrefined petroleum products. LC Separation of Phenols/Naphthols. For the separation of the phenolic extracts, we employed a monomeric, octadecylsilane-modified silica column (Supelcosil LC-18, Supelco, Bellefonte, PA), with 5-pm spherical particles and 0.46 x 25 cm bed dimensions. For the separation of the phenols, the mobile phase was a 30% acetonitrle/water (v/v) mixture containing 0.04 mol/L of ammonium acetate buffer at pH 4.3. The buffer was prepared from acetic acid and ammonium hydroxide (both Suprapur grade, Merck, Darmstadt, West Germany) by neutralizing the diluted acid to the desired pH, monitored with a glass-electrode pH meter. The flow rate for all work presented was 1.5 mL/min.
This article not subject to U.S. Copyright. Published 1987 by the American Chemical Society
478
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
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Fhwe 2. Reverse-phase separation of phenols In petroleum crude oil extract. Table I. Determination of Phenols in SRM 1582 Petroleum Crude Oil (Values in pg/g & 1 Standard Deviation) Applied Potential in V
Flgure 1. Hydrodynamic currentipotential curves for the phenols. For the determination of the phenol and 2-methylphenol in the crude oil, a step gradient was used with an initial isocratic composition of 30% acetonitrile/water/buffer. After 20 min the solvent was switched to 100% acetonitrile for 1 min to wash off long-retained components in the extract. For the determination of the phenols and naphthols in the shale oil, a gradient from 20% acetonitrile/3% 1-propanol/water to 40% acetonitrile/3% 1propanol/water (with the constant 0.01 mol/L acetate buffer) over 30 rnin was used. After 45 rnin the solvent could be returned to the original, giving a total cycle time of 55 min between injections. This much longer gradient separation was required to separate the naphthols from interfering components.
RESULTS AND DISCUSSION Crude Oil. The hydrodynamic current/potential response curves for phenol, 2-methylphenol, and 1-and 2-naphthol are shown in Figure 1. The current response areas were normalized to the response obtained at +1.05 V (which was nearly the maximum signal for all compounds). The potential used for the phenol determination was +1.00 V and +0.90 V for the naphthols, since larger potentials produced large increases in base line noise without much gain in signal. Many combinations of solvents and columns were tried to separate 3- and 4-methylphenol (which would also be of interest in these samples), but none were successful. We also examined the possibility of utilizing the control of the applied potential in the amperometric detector to provide resolution of the 3- and 4-methylphenol signals. However, the electrofor 3-methylphenol chemical waves are not well resolved (E1/* is +0.85 V and for 4-methylphenol is +0.76 V) and are quite irreversible (and hence quite broad). Since there was such a large overlap of the signals, we did not feel it was possible to achieve good precision in the quantitative results for this chromatographically unresolved pair by using two current measurements at two applied potentials in conjunction with two equations in two unknowns. In order to determine the phenols in the crude oil (at the sub-microgram-per-gram level), high sensitivity was required. Thus, an isocratic separation, with an acetonitrile flush, was employed. A chromatogram from the analysis of the petroleum crude extract is shown in Figure 2. Phenol and 2methylphenol are clearly resolved from other matrix constituents. One anomalous observation is the base line dip that occurs just before the elution of phenol in these extracts. This
compound
LCEC (this work)
GC/FID
phenol 2-methylphenol
0.258 f 0.18 0.416 i 0.057
0.26 0.53
dip is not observed in the chromatography of pure standards in this concentration range and is thus presumably caused by adsorption of an electroinactive sample matrix constituent. This results in a momentary decrease in the standing residual current and hence the dip. In light of this difficulty, quantitation was based on a peak height measurement, drawing a straight line from the base line before the dip to the base line at the end of the phenol peak. A standard additions procedure was used to quantitate the phenols in the crude oil. For each measurement, five aliquots of the oil were taken: two were unspiked and three were spiked with increasing increments of a standard solution dissolved in methylene chloride. All five solutions were then run through the extraction procedure and determined by the LCEC procedure using duplicate injections from each extract. Linearity of the standard additions curves was quite good (the least-squares correlation coefficient was always greater than 0.98) and the x intercepts were taken as the values (Table I). Agreement with a single measurement by gas chromatography/flame ionization (GC/FID) values (7) is good. Shale Oil. For the determination of the phenols/naphthols in the shale oil, a nonlinear gradient-elution solvent program was used. "he small shift of the electrochemical detector base line that results from solvent changes (8) did not pose a problem because of the relatively high concentrations of these compounds and the correspondingly low sensitivities required (see Figure 3). In order to avoid electrode-fouling reactions often observed for high concentrations of phenol at glassy carbon electrodes, a smaller proportion of shale oil to hexane was used than for the crude oil. Initially, for the shale-oil determination, gradients consisting of acetonitrile and water only were used. However, in order to determine 1-and 2-naphthol, additional chromatographic selectivity was required. In our experience, and in the work of others (9, IO), the addition of a third solvent in the reverse-phase separation provides a change in selectivity. In particular, the addition of a solvent that possesses one polar end as well as an aliphatic end to the molecule (such as 1propanol, tetrahydrofuran, and methyl tert-butyl ether) can provide useful changes in chromatographic selectivity. Figure 4 shows the difference in the separation of the shale-oil extract
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987 46%CH3CN,3% n-Prop
T a b l e 11. C o n c e n t r a t i o n s of P h e n o l s / N a p h t h o l s in S h a l e Oil SRM 1580 ( V a l u e s in pg/g f 1 Standard D e v i a t i o n )
28% CH3CN.3% n-Prop
compound
LCEC (this work)
certified (11)
phenol 2-methylphenol I-naphthol 2-naphthol
375 f 15 391 f 5 19.7 f 6.1 18.9 f 3.2
407 f 50 385 f 50
T a b l e 111. D e t e c t i o n L i m i t s in ng/g (pg i n j e c t e d ) ]
5
10
15
20
25
30
35
48
Figure 3. Separation of phenols/naphthols in shale oll extract.
25
30
35
for P h e n o l s / N a p h t h o l s [Value
compound
as injected
in the crude oil
phenol 2-methylphenol I-naphthol 2-naphthol
5.7 (110) 13 (270) 20 (390) 13 (260)
34 80 120 79
45
Minutes
20
479
40
Minutes
solved shoulder on the 1-naphthol peak). Determination of this compound by independent techniques would resolve this question. The detection limits (defined as a signal 2 times the peak-to-peak base line noise) for the phenols/naphthols as injected and as present in the oils (allowing for the dilution in the sample extraction used for the crude oil) are presented in Table 111,both in concentration units of nanogram per gram and in absolute amount in picograms (listed in parentheses). This new method represents an improvement on the existing LCEC approach for the determination of phenolic compounds and may be easily extended to include the measurement of di- and trimethylphenols and other phenolic compounds extracted by this solvent. R e g i s t r y No. phe, 108-95-2;2-m phe, 95-48-7; 2-nap, 90-15-3; 1-nap, 135-19-3.
LITERATURE CITED
Minutes
Flgure 4. Comparison of selectivity gained by the addition of 1propanol to the mobile phase: upper trace, acetonitrile/water only: lower trace, acetonitrWwater with the addition of 3% (v/v) lgropanol.
obtained without and with the addition of a constant amount (3% v/v) of 1-propanol to the acetonitrile gradient. The solvent strengths in these two chromatograms were adjusted in their acetonitrile content so that the retention times were similar. In this case the different selectivity observed when using 1-propanol allows a cleaner separation of the naphthols. Even with the addition of the alcohol, a small, coeluting shoulder may still be seen on the peak for 1-naphthol. For the quantitation of the phenols/naphthols in the shale oil, the same standard additions protocol was used as for the crude oil. The determined values of phenol, 2-methylphenol, and 1- and 2-naphthol, as well as the uncertainties, are presented in Table 11. The results for the phenols are in excellent agreement with the certified values. No comparison data exist for the naphthols in this SRM. It should be noted that the result for 1-naphthol has a rather large relative uncertainty, and this may reflect the interference of a coeluting detectable matrix component (that appears as a poorly re-
(1) Guenther, F. R.; Parris, R. M.; Chesler, S. N.; Hilpert. L. R. J . Chromatogr. 1981, 207, 256. (2) 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, 5 2 , 1650. (3) Rand, M. C.; Greenberg, A. E.; Taras, M. J. Standard Methods for the Examination of Waste and Wastewater, 14th ed.; American Public Health Association: Washington, DC, 1976. (4) Shoup, R. E.; Mayer, G. S. Anal. Chem. 1982, 5 4 , 1164. (5) Weber, S. G.; Purdy, W. C. Anal. Chem. 1982, 5 4 , 1757. (6) Handbook of Chemlsfty and Physlcs; CRC Press: Cleveland, OH, 1970; p D120. (7) Certificate of Analysis SRM 1582 Petroleum Crude Oil, National Bureau of Standards, 1984. (6) Khaledi, M. G.; Dorsey, J. G. Anal. Chem. 1985, 5 7 , 2190. (9) Roggendorf, E.; Spatz, R. J . Chromatogr. 1981, 204, 263. (IO) Schoenmakers, R. J.; Billiet, H. A. H.; De Galan, L. J . Chromatogr. 1981, 218, 261. (1 1) Certificate of Analysis SRM 1580 Organics in Shale Oil, National Bureau of Standards, 1980.
RECEIVED for review May 13,1986. Accepted September 19, 1986. This work was supported in part by the United States Department of Energy Office of Health and Environmental Research. Certain commerical materials are identified in this report to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials identified are necessarily the best available for the purpose.
