Trace Enrichment of Phenolic Compounds from Aqueous Samples by

Anal. Chem. 1997, 69, 5193-5199

Trace Enrichment of Phenolic Compounds from Aqueous Samples by Dynamic Ion-Exchange Solid-Phase Extraction Nanqin Li and Hian Kee Lee*

Department of Chemistry, National University of Singapore, Kent Ridge, Republic of Singapore 119260

A novel trace enrichment procedure for ionogenic compounds in environmental aqueous samples, dynamic ionexchange solid-phase extraction (DIE-SPE), is proposed and evaluated using 15 phenols as model compounds and GC/MS for detection. The procedure is based on the fact that the long-carbon-chain ionic surfactants, such as cetyltrimethylammonium bromide (CTAB), can be steadily adsorbed on the surface of C18-bonded silica, and the ionized functional groups of the adsorbed surfactant molecules can then act as ion-exchange sites to attract the ionized organic analytes from aqueous samples. At the same time, the bonded silica still remains its function as reversed-phase material. Therefore, the mechanism of DIE-SPE has two aspects, i.e., ion exchange and hydrophobic interaction. Sample pH, the loading amount of surfactant on the solid phase, and volume and matrixes of the sample were studied as the factors that affected the extraction efficiency. When applied to the trace enrichment of ionogenic compounds, DIE-SPE was found to be superior in diverse aspects to traditional reversed-phase bonded material SPE, ion-exchange SPE, carbonaceous or polymer adsorbent SPE: the method had higher extraction efficiency and was less affected by the inorganic ions and humic substance in sample solutions, and the analytes could be easily eluted out from the cartridge. It is proved that DIE-SPE can be used for the trace enrichment of ionogenic compounds from different sources of environmental aqueous samples.

and the analytes extracted can be easily eluted out. Other popular characteristics of the material, as mentioned by Fritz et al.,2 are that it is reasonably pure and widely available at low cost in prepacked commercially available cartridges. The shortcoming of reversed-phase bonded silica is that the material can only be efficiently used for the extraction of low or moderately polar compounds in water samples. When used to extract polar compounds, however, the recoveries are low,3 even though the pH and ion strength of the sample are suitably adjusted in order to ensure that the extraction is accomplished under optimized conditions. Many polar organic pollutants including phenols, anilines, and acidic or basic pesticides are ionizable in water. The ion pair technique, as used in HPLC4 and LLE,5 has been introduced to improve the extraction efficiency of ionogenic compounds in water.6-11 This approach can reduce the solubility of the ionogenic analytes in water and is beneficial for their extraction into the apolar phase. On the other hand, this can sometimes lead to the precipitation or increase of adsorption of the analytes onto the vessel walls. Ion-exchange resin is the traditional material for preconcentration of ionogenic compounds from water.1 Conventional ionexchange SPE, however, cannot efficiently extract ionized analytes from high salt content aqueous sample such as seawater,12 and the flow rate of sample through the extraction column is greatly limited owing to the low ion-exchange kinetics on the surface of the solid phase.13 Recently, there were several reports on the use of C18-bonded silica in the extraction of ionic long-carbonchain surfactants from water.14-17 For example, 500 mg of C18-

In general, enrichment or preconcentration is essential to the determination of organic pollutants in environmental water samples, because of the stringent pollutant control requirements and the lack, in many cases, of versatile, sensitive, direct, and accurate instrumental methods for diverse analytes and sample matrixes. The process includes liquid-liquid extraction (LLE) and solidphase extraction (SPE). The latter, being simple, fast, economical, and easy to automate, is now being prefered to LLE. As sorbent materials for SPE, reversed-phase bonded silica (mainly C18) is widely used, although in recent years modified polymer and carboneous materials have been found to have some advantages in extracting polar compounds from water. The attractive quality of the reversed-phase bonded material is based on several aspects. First, reversed-phase bonded silica is a homogeneous and mild material,1 there is little irreversible adsorption or catalytic reaction found in the extraction process,

(2) Fritz, J. S.; Junk, G. A. J. Chromatogr. 1992, 625, 87-90. (3) Crescenzi, C.; Di Corcia, A.; Passariello, G.; Samperi, R.; Turnes Carou, M. I. J. Chromatogr., A 1996, 733, 41-45. (4) Horvath, C. High-Performance Liquid Chromatography, Advances and Perspectives; Academic Press: London, 1980; Vol. 1. (5) Realini, P. A. J. Chromatogr. Sci. 1981, 19, 124-129. (6) Bigley, F. P.; Grob, R. L. J. Chromatogr. 1985, 350, 407-416. (7) Pocurull, E.; Calull, M.; Marce, R. M.; Borrull, F. Chromatographia 1994, 38, 579-584. (8) Balinova, A. J. Chromatogr., A 1996, 728, 319-324. (9) Busto, O.; Guasch, J.; Borrull, F. J. Chromatogr., A 1995, 718, 309-317. (10) Russel, F. G. M.; Creemers, M. C. W.; Tan, Y.; Van Riel, P. L. C. M.; Gribnau, F. W. J. J. Chromatogr., B 1994, 661, 173-177. (11) Lange, F. Th.; Wenz, M.; Brauch, H.-J. J. High Resolut. Chomatogr. 1995, 18, 243-252. (12) Di Corcia, A.; Marchese, S.; Samperi, R. J. Chromatogr. 1993, 642, 175184. (13) Subramanian, G. Quality Assurance in Environmental Monitoring, Instrumental Methods; VCH: Weinheim, 1995. (14) Marcomini, A.; Capri, S.; Giger, W. J. Chromatogr. 1987, 403, 243-252. (15) Marcomini, A.; Di Corcia, A.; Samperi, R.; Capri, S. J. Chromatogr. 1993, 644, 59-71. (16) Scullion, S. D.; Clench, M. R.; Cooke, M.; Ashcroft, A. E. J. Chromatogr., A 1996, 733, 207-216.

(1) Liska, I.; Krupcik, J.; Leclercq, P. A. J. High Resolut. Chromatogr. 1989, 12, 577-590. S0003-2700(97)00602-1 CCC: $14.00

© 1997 American Chemical Society

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bonded silica embedded in a 47 mm PTFE disk (3M, Minneapolis, MN) could efficiently extract linear alkylbenzenesulfonates (LAS); the breakthrough volumes for the LAS were more than 1000 mL.15 This work prompted us to consider further that the ionized functional groups in the surfactant can act as an ion-exchange site after they are adsorbed on the surface of the reversed-phase bonded material, just like the ionized functional group on traditional ion-exchange resin. Based on the foregoing, a novel extraction procedure with a special interaction mechanism may be supposed. In the procedure, the ionic surfactant on the surface of bonded silica is loaded before extraction and eluted out after an extraction; that is, the ion-exchange site is mobile and renewable. Considering the speciality and the similarity of the above extraction procedure relative to the conventional ionexchange SPE, the term “dynamic ion exchange”(DIE) may be suggested to describe this SPE process (DIE-SPE). The objective of this work is to evaluate the effectiveness of DIE-SPE to extract ionogenic compounds from aqueous samples. For this purpose, 15 phenols were chosen as model compounds. Phenols are important pollutants and several (including 6 of the 15 studied here) are listed as priority pollutants by the United States Environmental Protection Agency. Many procedures have been reported to extract the compounds from aqueous samples for determination.18 Besides the traditional LLE19 and ion pair LLE,5 off-line and on-line SPE with ion-exchange resin,20 reversedphase bonded silica,6,7,21-24 carbonaceous12,25-28 and polymer29-35 adsorbents have been studied for their effectiveness in extraction and separation of the phenolic compounds from aqueous samples of different nature. In terms of acidity constant pKa and the hydrophobicity constant, i.e., the octanol-water partition coefficient log Kow, taken together, the 15 phenolic compounds are different from one another (Table 1). Generally, methyl substitution leads to weak ionogenity and hydrophobicity, while chloro (17) Field, J. A.; Field, T. M.; Poiger, T.; Giger, W. Environ. Sci. Technol. 1994, 28, 497-503. (18) Puig, D.; Barcelo, D. Trends Anal. Chem. 1996, 15, 362-375. (19) Hajslova, J.; Kocourek, V.; Zemanova, I.; Pudil, F.; Davidek, J. J. Chromatogr. 1988, 439, 307-316. (20) Chriswell, C. D.; Chang, R. C.; Fritz, J. S. Anal. Chem. 1975, 47, 13251329. (21) Rostad, C. E.; Pereira, W. E.; Ratcliff, S. M. Anal. Chem. 1984, 56, 28562860. (22) Renberg, L.; Lindstrom, K. J. Chromatogr. 1981, 214, 327-334. (23) Werkhoven-Goewie, C. E.; Brinkman, U. A. Th.; Frei, R. W. Anal. Chem. 1981, 53, 2072-2080. (24) Chladek, E.; Marano, R. S. J. Chromatogr. Sci. 1984, 22, 313-320. (25) Borra, C.; Di Corcia, A.; Marchetti, M.; Samperi, R. Anal. Chem. 1986, 58, 2048-2052. (26) Coquart, C.; Hennion, M. C. J. Chromatogr. 1992, 600, 195-201. (27) Di Corcia, A.; Bellioni, A.; Madbouly, M. D.; Marchese, S. J. Chromatogr., A 1996, 733, 383-393. (28) Di Corcia, A.; Marchese, S.; Samperi, R.; Cecchini, G.; Cirilli, L. J. AOAC Int. 1994, 77, 446-453. (29) Puig, D.; Barcelo, D. J. Chromatogr., A. 1996, 733, 371-381. (30) Gawdzik, B.; Gawdzik, J.; Czerwinska-Bil, U. J. Chromatogr. 1990, 509, 135-140. (31) Schmidt, L.; Sun, J. J.; Fritz, J. S.; Hagen, D. F.; Markell, C. G.; Wisted, E. E. J. Chromatogr. 1993, 641, 57-61. (32) Brouwer, E. R.; Brinkman, U. A. Th. J. Chromatogr., A 1994, 678, 223231. (33) Puig, D.; Barcelo, D. Anal. Chim. Acta 1995, 311, 63-69. (34) Puig, D.; Barcelo, D. Chromatographia 1995, 40, 435-444. (35) Puig, D.; Barcelo, D.; Silgoner, I.; Grasserbauer, M. J. Mass Spectrom. 1996, 31, 1297-1307. (36) Dean, J. A. Handbook of Organic Chemistry; McGraw-Hill: New York, 1987. (37) Montgomery, J. H. Groundwater Chemicals Desk Reference; CRC Lewis: Boca Raton, FL, 1996. (38) Sangster, J. J. Phys. Chem. Ref. Data 1989, 18, 1111-1229.

5194 Analytical Chemistry, Vol. 69, No. 24, December 15, 1997

Table 1. List of Phenolic Compounds with Their pKa and log Kow Values Considered in This Work and Their Retention Times and Ions (m/z) for GC/MS Quantification compound

pKaa

log Kowb

phenol o-cresol m-cresol p-cresol 2-chlorophenol 3-chlorophenol 4-chlorophenol 2,5-xylenol 2,4-xylenol 3,5-xylenol 2-bromophenol (IS) 2,4,6-trimethylphenol 4-chloro-3-methylphenol 2,4-dichlorophenol 2,4,6-trichlorophenol 2-naphthol

9.99 10.26 10.00 10.26 8.55 9.10 9.43 10.22 10.58 10.15

1.50 1.98 1.98 1.97 2.15 2.50 2.41 2.34 2.35 2.35

10.88 9.55 7.85 7.42 9.57

2.73 3.10 3.08 3.69 2.70

a

retention time (min)

m/z

6.72 9.90 10.26 11.44 12.81 14.14 14.40 14.77 14.94 16.02 16.27 17.42 17.89 18.23 19.33 20.68

94 108 108 108 128 128 128 122 122 122 172 136 142 162 196 144

Values taken from refs 36 and 37. b Values taken from refs 38-40.

substitution leads to strong ionogenity and high hydrophobicity. Phenol itself is a weakly ionogenic and highly hydrophilic and has good solubility in water. It usually has the lowest recovery among phenolic compounds in almost all LLE and SPE methods for aqueous environmental samples.5,7,21,25,29 It has been proven that DIE-SPE is a effective approach for the extraction of all the phenolic compounds, even phenol itself. For this work, the phenolic compounds were detected as their acetyl derivatives with gas chromatography/mass spectrometry (GC/MS). EXPERIMENTAL SECTION Reagents and Chemicals. Phenol standards were purchased from various sources; the purity was more than 98%. All the phenolic compounds used in this work are listed in Table 1, along with their pKa and log Kow values. Individual standard solutions were prepared by dissolving 50 mg of each of the phenols in 10 mL of acetone. A working composite standard solutions of the compounds was prepared by mixing 100 µL of each of the phenolic standard solution and diluting to 25 mL with acetone. 2-Bromophenol was used as the internal standard (IS); the solution was prepared separately at a concentration of 2 ppm. Acetone, methanol, and hexane were of pesticide grade and purchased from Labscan (Dublin, Ireland). Tetrabutylammonium bromide (TBA), tetraoctylammonium bromide (TOA), and cetyltrimethylammonium bromide (CTAB) were of analytical-reagent grade, and supplied by Sigma (St. Louis, MO). Humic acid was in its sodium form and obtained from Aldrich (Milwaukee, WI). All other chemicals were of analytical reagent grade. MilliQ (Millipore, Milford, MA) water was used throughout. Tap water and seawater were obtained locally. Before use, seawater was filtered with Whatman (Maidstone, England) GMF150 glass fiber pads. Sample Extraction and Derivatization. The SPE cartridges were obtained from J. T. Baker (Phillipsburg, NJ); each had a (39) Makovskaya, V.; Dean, J. R.; Tomlinson, W. R.; Canber, M. Anal. Chim. Acta 1995, 315, 193-200. (40) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525-616.

total volume of 6 mL and contained 500 mg of end-capped C18bonded silica. Before extraction, sodium tetraborate or potassium dihydrogen phosphate was added to the aqueous sample as buffer. The pH of the sample was adjusted to 9.5. For recovery experiments, the samples were fortified with known volumes of the working composite standard solution of the phenolic compounds. Before use, the cartridge was conditioned with 5 mL each of acetone, methanol, and pure water sequentially. After that, known amounts of the cationic surfactant (TBA, TOA, or CTAB) solution was percolated through the cartridge to form a layer of ion-exchange site on the stationary-phase surface. The conditioning process was carried out with a vacuum manifold (Supelco, Bellefonte, PA). The flow rate of the solvent or solution through the cartridge was set at 1-1.5 mL/min. During the conditioning and the subsequent extraction process, the cartridge was not allowed to go dry. The percolation of aqueous sample through the cartridge was carried out with a positive pressure (nitrogen). A separatory funnel was used as the reservoir for the sample. The cartridge was tightly and directly connected to the separatory funnel by a rubber hose. For all the extraction experiments, the flow rate of sample through the cartridge was controlled at ∼15 mL/min. Initially, the effect of flow rate on extraction efficiency was tested at two levels, 8 and 15 mL/min; we found that the flow rate had no effect on the recoveries of the phenols. Since the high pressure in the apparatus was undesirable for safety reasons, flow rates of >15 mL/min were not tested. After the sample had been percolated, the cartridge was transferred back to the vacuum manifold and air-dried for 10 min before the analytes on the cartridge were eluted with 5 mL of acetone acidified with 50 µL of glacial acetic acid. This elution process was based on the method of Pocurull et al.7 In our work, it was proven effective and was not optimized further. The procedure for the derivatization of phenolic compounds was based on that of Renberg et al.22 A slight modification was that a little more potassium hydroxide was used to neutralize the acetic acid in the eluate. The entire method was as follows: A 100 µL sample of 2 ppm 2-bromophenol (the internal standard) was added and mixed with the eluate and then the eluate was concentrated to ∼1 mL under a gentle stream of nitrogen. A 3 mL sample of 0.1 M aqueous potassium carbonate and potassium hydroxide solution was added and well mixed with the extracted sample solution. After that, 2 mL of hexane containing 50 µL of acetic anhydride was added and the solution was immediately shaken for 1 min to derivatize and extract the phenolic compounds. When the two phases were well-separated in the solution, the hexane layer was transferred to a 1.5 mL sample vial, into which a few crystals of anhydrous sodium sulfate were added. For recovery experiments, extraction was performed in triplicate for each sample. Average recoveries and relative standard deviations (RSD) were calculated for each of the compounds. Instrumental Analytical Procedure. All prepared samples were identified and quantified with a Shimadzu QP5000 gas chromatograph/mass spectrometer equipped with an AOC-17 autosampler (Shimadzu, Tokyo, Japan). A 30 m × 0.32 mm i.d. × 0.25 mm film thickness DB-1 column (J&W Scientific, Folsom, CA) was used to perform gas chromatographic separation. The temperature program included an initial oven temperature of 60 °C for 5 min, which was increased to 90 °C at 3 °C/min and held for 1 min and then another ramp of 20 °C/min to 250 °C (held

for 1 min) for a 25 min run time. The carrier gas (helium) was set at a flow rate of 2.5 mL/min with a split ratio of 1:20. Injections were performed in splitless mode into a glass insert packed with silanized glass wool and with an injection time of 1 min and an injection temperature of 250 °C. The sample injection volume was 1 µL. Mass spectrometric conditions included electron impact ionization, an interface temperature of 250 °C, sampling rate of 0.20, and detector voltage of 1.20 kV. For each phenolic compound, three characteristic ions were selected under SIM mode for identification, while one ion was used for quantitation. The quantitation ions as well as the retention time for each phenolic compounds are listed in Table 1. RESULTS AND DISCUSSION Fifteen phenolic compounds were selected for the work and 2-bromophenol as internal standard (Table 1). Under the GC/ MS conditions described above, all the 15 phenols and 2-bromophenol were well separated as their acetyl derivatives. For quantitation, calibration curves were constructed for the concentration range 10∼1000 ppb. Good linearity was obtained for all the analytes, the correlation coefficients were more than 0.999. The detection limits for the 15 phenols were between 2.0-10.0 pg (signal to noise ratio, 3). If 500 mL of aqueous sample were used, the lowest detectable concentrations for each of the compounds were between 0.01 and 0.04 ppb. For the establishment of DIE-SPE, the first step is essentially to load the surfactant to the original solid adsorbent to form a layer of artificial ion-exchange sites on the surface. For the anionogenic phenolic compounds, three universal quaternary ammonium surfactants were considered for the purpose, viz., TBA, CTAB, and TOA. The last named, however, is a big molecule and has low solubility in water, which makes it inconvenient to load the surfactant to the solid-phase surface. Thus, only TBA and CTAB were used. After each of the cartridges was conditioned with TBA and CTAB solutions, respectively, pure water samples spiked with the mixture of phenolic compounds were extracted. Recovery data are shown in Table 2. Although TBA and CTAB have similar molecular mass, the single alkyl chain of CTAB is much longer than that of TBA. As one can see in Table 2, when CTAB was used to condition the cartridge, satisfactory recoveries for all the phenolic compounds could be achieved. In contrast, low recoveries were obtained when TBA was used. Obviously, these results were due to the low retention ability of TBA on the C18-bonded silica. CTAB has a C16 alkyl chain as part of its molecular structure; it is naturally an advantage for it to be steadily adsorbed onto the C18-bonded silica. The phenomenon is not difficult to understand from the principle of “like adsorbs like”. In fact, CTAB has excellent retention ability in the C18-bonded silica, as shown in the experiment described below. The recoveries for the phenolic compounds remained satisfactory even after 1000 mL water samples were percolated through the cartridge (refer to Table 5). The effect of the amount of CTAB used to condition the cartridge on the recovery of phenols was evaluated. Prior to application of the aqueous samples, the cartridges were conditioned with 0, 2, 4, 6, 8, and 10 mL of 0.025 M CTAB solution, respectively; the recoveries for the phenols in each circumstance are shown in Table 3. Functionally, each CTAB molecule adsorbed on the surface of the solid bonded silica acts as an ion-exchange site. Thus, a Analytical Chemistry, Vol. 69, No. 24, December 15, 1997

5195

Table 2. Comparison of Percentage Recoveries of Phenolic Compounds with TBA and CTAB as Conditioning Reagents recovery (mean + RSD %)a compounds

TBA

CTAB

phenol o-cresol m-cresol p-cresol 2-chlorophenol 3-chlorophenol 4-chlorophenol 2,5-xylenol 2,4-xylenol 3,5-xylenol 2,4,6-trimethylphenol 4-chloro-3-methylphenol 2,4-dichlorophenol 2,4,6-trichlorophenol 2-naphthol

21 (3.6) 15 (6.1) 18 (10.2) 14 (9.3) 8 (21.4) 15 (22.9) 27 (14.1) 56 (2.9) 29 (10.6) 56 (8.2) 53 (5.0) 94 (0.7) 68 (9.3) 156 (2.8)b 83 (12.5)

86 (3.0) 91 (5.5) 92 (4.9) 93 (5.5) 88 (1.4) 94 (3.5) 88 (3.2) 98 (1.9) 95 (4.2) 98 (4.4) 126 (22.8) 98 (5.9) 102 (3.4) 100 (14.0) 102 (5.1)

a Cartridge conditioned with 8 mL of 0.025 M TBA or CTAB solution before extraction. Sample: 500 mL of pure water dissolved with 0.0025 M Na2B4O7, pH 9.50, and spiked with 20 µL of composite phenolic compounds working standard; spiking level 0.2 ppb. b Interference from the ion pair reagent.

large amount of CTAB is certainly beneficial to the extraction of the ionized analyte molecule from the aqueous sample. This reasoning is well supported by the recovery data in Table 3. When applied to a spiked pure water sample, 6 mL of the above CTABconditioned cartridge can extract all the phenolic compounds in a 500 mL volume satisfactorily. Moreover, as indicated in the following experiments, the change in the nature of the sample, such as the presence of large amounts of inorganic salt or humic substance, or the increase of sample volume, will lower the recoveries of the ionogenic analytes. These negative effects can be compensated for to some extent by the increase of the loading amount of the ionic surfactant on the cartridge. On the other hand, CTAB adsorbed on the cartridge will increase the resistance of water sample through it, so a higher nitrogen pressure will be needed to maintain the flow rate. Nevertheless, the increase in the amount of CTAB cannot be sustained practically, so a balance

must be struck between extraction efficiency and the loading amount of CTAB. Further analysis of the data in Table 3 is helpful to understand the mechanism of DIE-SPE. Unlike conventional ion-exchange SPE, where ion electrostatic interaction governs the whole extraction process and the analytes are extracted in the ionized form, in DIE-SPE, analytes can be extracted in ionic or neutral form through electrostatic and hydrophobic interaction. In a sample solution of pH 9.5, the chloro-substituted phenols are largely ionized and the methyl-substituted phenols partly ionized. As can be read from the recovery data in Table 3, when no CTAB solution was used to condition the cartridge, the phenolic compounds had varying recoveries. The methyl-substituted phenols had generally larger recoveries than the chlorophenols. This may be because under pH 9.50 the methylphenols were less ionized and had more affinity for the nonpolar solid phase when hydrophobic interaction was the primary mechanism of analyte retention. The evidence for the reasoning is the very low recovery of 2,4,6-trichlorophenol (pKa 7.42), which was nearly completely ionized under pH 9.5. This is because retention of this compound was conceivably low during the sample loading step, and most of it was lost from the cartridge. The introduction of CTAB to the cartridge was undoubtedly beneficial to the retention of the more readily ionizable phenols. After 2 mL of the above CTAB solution was used to condition the cartridge, all the chlorophenols in 500 mL of water could be satisfactorily recovered. The recoveries for the less ionized methylphenols and phenol itself increased gradually with the increase of the loading amount of CTAB until 6 mL of the latter solution was used. At that point, the most recalcitrant analyte, phenol itself, could be satisfactorily extracted. The mechanism of DIE-SPE is supported by the observed recoveries of the phenolic compounds in samples of different pH, as presented in Table 4. Under the same experimental conditions, each 500 mL sample of pH 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 was extracted. The recoveries of the phenolic compounds under two extreme pH conditions, i.e, 5.0 and 10.0, allow us to understand the characteristics of the extraction process. Under pH 5.0, almost all the phenols were neutral. Thus, hydrophobic interaction was

Table 3. Percentage Recoveries of Phenolic Compounds with Cartridges Conditioned with Different Volumes of 0.025 M CTAB recovery (mean + RSD %)a compound

0 mL

2 mL

4 mL

6 mL

8 mL

10 mL

phenol o-cresol m-cresol p-cresol 2-chlorophenol 3-chlorophenol 4-chlorophenol 2,5-xylenol 2,4-xylenol 3,5-xylenol 2,4,6-trimethylphenol 4-chloro-3-cresol 2,4-dichlorophenol 2,4,6-trichlorophenol 2-naphthol

19 (3.7) 18 (2.0) 15 (3.7) 12 (8.8) 4 (182) 10 (25.0) 14 (20.9) 69 (3.6) 68 (1.1) 59 (2.4) 90 (5.8) 73 (2.9) 8 (11.7) 10 (23.5) 92 (5.0)

42 (1.5) 60 (5.9) 75 (2.8) 68 (8.8) 90 (1.6) 94 (8.3) 95 (1.9) 97 (3.0) 90 (6.5) 90 (5.0) 73 (18.6) 103 (3.1) 101 (3.7) 125 (8.1) 99 (7.3)

66 (1.4) 93 (5.7) 94 (3.2) 88 (1.6) 94 (2.4) 96 (5.3) 88 (7.4) 101 (1.5) 97 (2.2) 96 (3.3) 112 (4.9) 100 (4.9) 99 (2.4) 120 (4.4) 105 (5.0)

83 (0.2) 99 (3.1) 98 (1.5) 96 (7.7) 94 (1.9) 95 (1.1) 89 (3.1) 104 (2.9) 99 (2.7) 91 (1.0) 122 (8.2) 100 (1.9) 103 (3.5) 123 (14.2) 102 (2.4)

102 (1.5) 92 (0.5) 95 (0.4) 98 (4.4) 91 (2.7) 94 (2.2) 88 (2.2) 102 (1.3) 96 (4.6) 91 (2.8) 122 (4.8) 96 (1.7) 99 (0.8) 139 (7.7) 97 (1.1)

106 (3.8) 99 (2.0) 99 (0.8) 92 (5.6) 96 (3.2) 97 (1.7) 93 (2.7) 108 (2.8) 106 (2.6) 95 (3.3) 150 (2.7) 102 (2.0) 105 (3.2) 112 (6.6) 101 (3.4)

a Sample: 500 mL of pure water dissolved 0.0025 M Na B O , pH 9.50, spiked with 20 µL of working composite working standard solution; 2 4 7 spiking level of 0.2 ppb.

5196 Analytical Chemistry, Vol. 69, No. 24, December 15, 1997

Table 4. Percentage Recoveries of Phenolic Compounds Extracted from Water Samples at Different pHs recovery (mean + RSD %)a compound

pH 5.0

pH 6.0

pH 7.0

pH 8.0

pH 9.0

pH 10.0


24 (1.7) 39 (4.8) 44 (3.7) 41 (8.9) 88 (1.9) 95 (1.5) 94 (5.2) 94 (4.6) 91 (5.0) 89 (2.5) 120 (13.1) 100 (6.1) 102 (2.0) 108 (8.7) 110 (9.5)

29 (8.0) 52 (4.3) 52 (4.6) 48 (4.6) 86 (2.8) 89 (5.7) 86 (3.1) 95 (2.9) 94 (4.3) 84 (1.0) 128(12.0) 93 (1.9) 92 (3.1) 91 (28.8) 97 (2.7)

32 (1.7) 49 (3.8) 60 (3.6) 57 (4.4) 80 (7.3) 85 (5.4) 79 (3.8) 94 (2.6) 93 (3.2) 84 (1.0) 136 (8.4) 93 (4.8) 92 (12.7) 128 (18.1) 96 (5.5)

43 (1.9) 64 (2.0) 78 (4.5) 76 (3.9) 104 (14.6) 87 (0.7) 83 (2.6) 94 (1.4) 89 (3.8) 83 (1.4) 133 (7.3) 93 (1.3) 95 (2.7) 120 (21.9) 94 (0.6)

54 (5.1) 72 (2.1) 84 (4.9) 82 (2.4) 112 (12.7) 89 (2.7) 85 (4.2) 97 (1.2) 93 (1.0) 87 (2.8) 129 (2.1) 97 (5.9) 99 (2.6) 118 (30.5) 102 (9.0)

99 (6.0) 95 (1.8) 95 (14.8) 91 (6.9) 125 (13.0) 93 (0.9) 91 (1.8) 103 (14.0) 100 (3.4) 92 (0.4) 124 (7.5) 104 (1.5) 103 (2.2) 136 (5.8) 106 (2.4)

a Cartridge conditioned with 8 mL of 0.025 M CTAB. Sample: 500 mL of pure water spiked with 20 µL of working composite standard solution; spiking level 0.2 ppb. pH 5.0-9.0 samples contained 0.01 M NaH2PO4; pH 10.0 sample contained 0.0025 M Na2B4O7.

mainly responsible for the retention and subsequent extraction of the analytes. For the relatively hydrophobic phenols (based on log Kow), satisfactory recoveries were obtained, while for the less hydrophobic cresols and phenol itself, recoveries were low. In contrast, at pH 10.0, all the phenolic compounds would be mostly ionized and could be well extracted, mainly through ionexchange electrostatic interaction. Based on the above experimental evidence, two conclusions may be derived: (1) Hydrophobic interaction and ion exchange contributed to the extraction process simultaneously. (2) When ion exchange governed the extraction process, the breakthrough volume was much larger than that of the process solely through the reversed-phase hydrophobic interaction. The above statement is supported by the variation of recoveries of the phenolic compounds with the gradual change of sample pH. It should be mentioned that the addition of buffer to the sample is essential in DIE-SPE in order to keep a constant pH and to guarantee that the analyte can largely exist in the ionized form, as a compensation for the fact that the silica material cannot be used under very basic environment. (In conventional ionexchange SPE of acidic compounds, the pH of the sample should be 2 pH units above the pKa of the analytes.5 For the silica-bonded material in an HPLC column, it is normally recommended that the maximum pH not be more than 8.0. For the solid-phase material in SPE, it is recommended that the maximum pH be no more than 10.0. It was observed in this work that prelonged repetitive percolation of basic samples through the cartridge might cause a slight shrinking of the packing material in the cartridge; this, however, did not affect the extraction efficiency significantly. In this work, the samples were kept at pH 9.5, and under such conditions, there were no significant variations in recoveries after percolating 5 × 500 mL samples continuously through the cartridge.) The extraction efficiency of DIE-SPE at increasing water volumes sampled was evaluated. Each 250, 500, and 1000 mL pure water sample was buffered, pH adjusted, spiked with the same amount of phenols standards, and then extracted. The results are shown in Table 5. As can be seen, good recoveries were obtained for all the analytes even for the 1000 mL sample; only phenol itself exhibited a slightly low recovery (66%) for this volume. This means that the breakthrough volume for phenol in

Table 5. Percentage Recoveries of Phenolic Compounds Extracted from Spiked Pure Water Samples of Different Volumes recovery (mean + RSD %)a compounds

250 mL

500 mL

1000 mL


105 (6.4) 103 (7.8) 101 (5.5) 97 (3.8) 96 (1.6) 100 (2.2) 97 (3.2) 98 (6.6) 80 (10.8) 93 (1.9) 98 (11.2) 105 (2.2) 103 (2.3) 99 (6.6) 93 (4.2)

102 (1.5) 92 (0.5) 95 (0.4) 98 (4.4) 92 (2.7) 94 (2.2) 88 (2.2) 102 (1.3) 96 (4.8) 91 (2.8) 122 (4.8) 96 (1.7) 99 (0.8) 139 (7.7) 97 (1.1)

66 (1.9) 92 (2.1) 98 (3.8) 92 (0.8) 98 (3.2) 97 (1.7) 95 (6.8) 100 (5.0) 89 (7.8) 90 (1.2) 86 (14.9) 99 (1.4) 102 (0.9) 73 (14.7) 94 (10.2)

a Cartridge conditioned with 8 mL of 0.025 M CTAB. Each sample spiked with 20 µL of working composite standard solution; spiking levels for the 250, 500, 1000 mL water samples were 0.4, 0.2, 0.1 ppb, respectively.

a reasonably clean water sample was between 500 and 1000 mL while the breakthrough volumes for other phenols were more than 1000 mL. The lower breakthrough volume of phenol compared to the substituted phenols provides the evidence that the retention of analytes is dependent on the synergistic effect of electrostatic attraction and hydrophobic interaction, because ion exchange or hydrophobic interaction solely cannot explain this great difference in breakthrough volume between the cresols and phenol. In the SPE of real environmental aqueous samples, the recoveries are generally lower than those of pure water samples. Two main factors contribute to the difference, viz., the dissolved organics (largely humic acid) and the inorganic ions. While most surface waters have high humic substance content, the marine water possesses high ionic strength. In comparison, tap water and groundwater are the cleanest. In this work, we assessed the influence of matrix factors to the recoveries in the DIE-SPE of phenolic compounds by extraction of spiked 500 mL seawater and two 500 mL pure water samples dissolved with 35 g/L sodium Analytical Chemistry, Vol. 69, No. 24, December 15, 1997

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Table 6. Percentage Recoveries of Phenolic Compounds Extracted from 500 mL of Pure Water Dissolved with Humic Acid and Sodium Chloride and 500 and 250 mL of Filtered Seawater Samples recovery (mean + RSD %)a compound

water with humic acidb

water with NaClc

seawater 500 mLd

seawater 250 mLd


71 (11.4) 95 (6.4) 93 (4.3) 95 (1.2) 90 (2.5) 90 (2.9) 88 (5.1) 99 (4.0) 97 (6.1) 88 (4.4) 136 (6.1) 100 (2.7) 104 (2.1) 125 (4.2) 98 (3.4)

41 (2.3) 69 (1.9) 78 (2.6) 76 (2.2) 88 (1.6) 97 (2.0) 102 (1.1) 121 (10.1) 96 (4.2) 93 (1.7) 69 (27.0) 109 (2.4) 116 (3.1) 120 (5.0) 116 (3.0)

47 (2.4) 67 (4.9) 73 (1.4) 74 (8.4) 88 (2.5) 97 (1.9) 89 (1.6) 112 (5.8) 92 (6.0) 93 (3.4) 116 (8.1) 105.7 (4.5) 110 (2.6) 91 (22.5) 102 (2.5)

60 (2.0) 83 (2.9) 88 (1.0) 86 (3.0) 89 (1.4) 92 (1.8) 92 (2.2) 101 (2.4) 103 (4.5) 88 (0.7) 98 (11.2) 99 (13.4) 104 (1.5) 96 (9.9) 86 (6.3)

a Cartridge conditioned with 10 mL of 0.025 M CTAB. Each sample spiked with 20 µL of working composite standard solution; the final spiking level was 0.2 (500 mL) and 0.4 ppb (250 mL). b 500 mL pure water with dissolved 20 mg/L humic acid sodium and 0.0025 M Na2B4O7, pH 9.50. c 500 mL of pure water dissolved with 35 mg/L NaCl and 0.0025 M Na B O , pH 9.50. d Filtered seawater dissolved 0.0025 M Na B O , pH 9.50. 2 4 7 2 4 7 Several phenols could be detected in the blank seawater sample. For these phenols, the recoveries were calculated from analyses of the original and spiked samples.

chloride and 20 mg/L humic acid (sodium salt), respectively. These solutions may be viewed as having the largest inorganic salt content in seawater and the highest humic substance content in surface water. The recovery data are presented in Table 6. From the knowledge of conventional ion-exchange SPE and reversed-phase SPE, the presence of inorganic ions lowers the extraction efficiency of ionized analytes in ion-exchange SPE by competing with the extractants for the ion-exchange sites on the solid-phase surface,13 but the high ionic strength has some positive effect in reversed-phase SPE.24 There is experimental evidence that conventional ion-exchange SPE fails to extract some of the ionogenic organic analytes in aqueous samples of ionic strength as high as seawater.12 The reason is that the ion electrostatic attraction is the main approach for the ion-exchange solid phase to extract analytes. Also for this reason, the inorganic ions possess the capability to contest with the ionized organic analytes for adsorption sites on the surface of the ion exchanger. In this case, the ion exchanger does not have much selectivity for most of the ionized organic analytes relative to the inorganic ions. It seems to be an advantage of DIE-SPE that the ionogenic organic analytes can be extracted in the ionized and unionized forms and through ion electrostatic attraction or hydrophobic interaction at the same time. Therefore, DIE-SPE has exceptional selectivity for ionogenic organic compounds relative to the inorganic ions. Although the presence of large amounts of Cl- in the sample could lessen the ion electrostatic interaction between the analyte ions and the ionized functional group of the surfactant adsorbed on the C18bonded silica surface, there were no apparent breakthroughs of all the phenolic compounds except the cresols and phenol itself when 500 mL of seawater sample was percolated through the cartridge. On the other hand, the lower recoveries of cresols and phenol may be viewed as the breakthrough of the analytes which may be compensated by reducing the sample volume. To verify this, a 250 mL seawater sample spiked with the phenols mixture was extracted. Satisfactory recoveries for the cresols were achieved, although the recovery for phenol itself was still rather low (Table 6). 5198 Analytical Chemistry, Vol. 69, No. 24, December 15, 1997

The possibility of applying DIE-SPE to surface waters was evaluated by extracting water with dissolved humic acid instead of the real sample for the reason stated by Crescenzi et al.3 The dissolved humic acid in water has been assumed to be the cause of lowered recoveries of organic compounds in reversed-phase or other SPE procedures. The recovery data in Table 6 illustrate that as high as 20 mg/L humic acid in water had little influence on the extraction of the phenolic compounds in water in DIESPE. As a matter of fact, the negative effect of humic acid in reversed-phase or other SPE methods is due to the adsorption of the analytes on this substance,41,42 during which the analytes are not extracted and pass through the stationary phase along with the humic acid. In DIE-SPE, however, the dissolved humic acid could well be adsorbed onto the solid phase, as indicated by the appearance of a brownish color (the specific color of humic acid) on the cartridge and the disappearance of it from the water. It was also an additional advantage that the adsorbed humic acid on the DIE-SPE cartridge was not easily eluted out with the solvent used. There was thus less interference in the analysis. From this point of view, the DIE-SPE method is a potential alternative to trace enrichment of ionogenic analytes from surface water samples. CONCLUSION Dynamic ion-exchange solid-phase extraction has been developed and evaluated. Extraction of 15 phenolic compounds using DIE-SPE was carried out by applying the technique to tap water and seawater. Several phenols were identified and quantified in each of the samples, although their concentrations were far below the control standard for the specific sample. DIE-SPE is a potentially useful trace enrichment procedure for ionogenic organic compounds because of its special mechanism and wide suitability for different sample matrixes. Although conventional (41) Johnson, W. E.; Fendinger, N. J.; Plimmer, J. R. Anal. Chem. 1991, 63, 1510-1513. (42) Di Corcia, A.; Samperi, A.; Marcomini, A.; Stelluto, S. Anal. Chem. 1993, 65, 907-912.

C18-bonded silica cartridges were used, DIE-SPE was found to improve significantly the extraction efficiency of ionogenic compounds in comparison with conventional reversed-phase SPE through ion suppression or ion pairing. In other words, currently available and affordable SPE cartridges from numerous manufacturers and suppliers can be used for DIE-SPE of such compounds. Exploiting the homogeneous and mild characteristics of chemically bonded materials, DIE-SPE obviates the irreversible adsorption and catalytic reactions in the extraction process that normally occurs on the surface of carbonaceous materials and other adsorbents. DIE-SPE is superior to conventional ion-exchange SPE in that the method can extract ionogenic organic compounds from highly ionic aqueous samples. Humic acid in natural surface waters has little influence on the extraction efficiency.

ACKNOWLEDGMENT The authors thank the National University of Singapore for financial support and wish to express their appreciation to Shimadzu Asia Pacific for the loan of the Shimadzu QP5000 gas chromatograph/mass spectrometer.

Received for review June 11, 1997. Accepted October 7, 1997.X AC970602+

X

Abstract published in Advance ACS Abstracts, November 15, 1997.

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Trace Enrichment of Phenolic Compounds from Aqueous Samples by

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