Anal. Chem. lS86, 58,3261-3263
that this approach might be a valuable tool for neurochemical studies of dopamine release. The improved selectivity is accompanied by enhanced sensitivity. The preparation of the bilayer coating requires no surface pretreatment and can be accomplished in a few minutes. We are currently evaluating the utility of the perfluorinated ionomer/cellulose acetate bilayer coating for combining the selective response toward neurotransmitters with effective discrimination against surface-active organic materials, as applied to amperometric detection in flowing streams. The strategy of multifunctional operation, based on combining the properties of different polymers, thus shows great promise for many practical applications. Bilayer coatings, prepared from other polymeric substances, may be designed to promote other analytical advantages. For example, we have recently demonstrated that composite and bilayer polymer electrode coatings, based on cellulose acetate and poly(vinylpyridine), exhibit properties superior to those of the two components alone (12). While the analytical utility of cellulosic coatings has been clearly illustrated in this and previous studies, additional fundamental work is desired to further elucidate the various factors affecting their transport characteristics.
3261
Registry No. Carbon, 7440-44-0; Nafion, 39464-59-0; cellulose acetate, 9004-35-7; dopamine, 51-61-6; epinephrine, 51-43-4; norepinephrine, 51-41-2; serotonin, 50-67-9. LITERATURE CITED Cox, J. A.; Kulesza, P. J. Anal. Chem. 1984, 56, 1021. Sittampalam, G.; Wilson, G. S.Anal. Chem. 1983, 55, 1608. Wang, J.; Hutchins-Kumar, L. D. Anal. Chem. 1986, 58. 402. Izutsu, K.; Nakamura, T.: Taklzawa, R.: Hanawa, H. Anal. Chim, Acta 1983, 149, 147. Cox, J. A.; Kulesza, P. J. Anal. Chim. Acta 1984, 758,335. Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984, 56, 1898. Gerhardt, G. A.; Oke, A. F.; Nagy, F.: Moghaddam, B.; Adams, R. N. Brain Res. 1984, 290, 390. Nagy, F.; Gerhardt, G. A.; Oke, A. F.; Rice, M. E.; Adams. R. N.: Moore, R. B.; Szentirmay, M. N.; Martin, C. R . J. Electroanal. Chem. 1985, 788, 85. Wang, J.; Hutchins, L. D. Anal. Chem. 1985, 57, 1536. Morrison, R. T.; Boyd, R . N. Organic Chemistry, 3rd ed.; Allyn and Bacon: Boston, MA, 1973; p 1127. Schneider, J. R.; Murray, R. W. Anal. Chem. 1982, 54, 1508. Wang, J.; Tuzhi, P. J . Electrochem. Soc., in press.
RECEIVED for review May 23,1986. Accepted August 4,1986. The financial support provided by the National Institutes of Health (GM-30913-3) is acknowledged.
Stationary Phase for the Gas Chromatographic Determination of Phenols at the Nanogram Level F. Mangani, A. Fabbri, G . Crescentini, and F. Bruner* Istituto di Scienze Chimiche, Universitci di Urbino, Piazza Rinascimento, 6, 61029 Urbino, Italy The GC analysis of phenolic compounds has been the object of several papers in the past years. Chriswell et al. ( I ) obtained acceptable results using a column packed with Tenax, the well-known porous polymer that, in spite of the low resolution yielded, has to be considered suitable for the elution of highly polar compounds. Morever, Tenax has the good property of eluting water very fast ( Z ) , which is an important factor in environmental analysis. In the paper cited, particular attention was devoted to develop an effective method for isolating and concentrating phenols from water and no particular attention was devoted to the GC column. More recently, Di Corcia et al. ( 3 ) developed a column able to separate the phenolic compounds included in the list of the priority pollutants. Although the separation obtained is satisfactory, some problems still remain, and their solution would help the analysts involved in the determination of phenols. In particular, when the absolute amounts of the phenols injected into the GC column become lower than 80-100 ng, two problems arise that make separation and quantitative analysis impossible: (i) The peaks of the most polar and chemically active compounds, such as 2,4-dinitrophenol and 2-methyl-4,6-dinitrophenol exhibit significant tailing and are not eluted at those concentrations. This is due both to the low response factor of the FID toward these compounds and to the irreversible adsorption that they undergo. (ii) The first three compounds eluted, i.e., 2-chlorophenol and 2-nitrophenol, are partially hidden by the solvent tail. Furthermore, in these conditions, a solvent effect on the stationary phase system takes place, so that the separation becomes incomplete. In this paper we describe a new GC column that overcomes these problems and allows quantitative analysis of the 11 compounds included in the list of priority pollutants. For this purpose we have exploited the technique of gas-liquid-solid
chromatography ( 4 ) ,which has been proved to be highly selective and particularly suitable for the elution of polar compounds ( 5 ) . The solid absorbent is a particular type of graphitized carbon black, which shows a low surface area (about 7.0 m2g-l) and is now available from Supelco, Inc., Bellefonte, PA. This material, used for a long time by several research groups (6-8) and known as Sterling MT, is obtained from natural gas. The main chromatographic characteristics of this adsorbent have been described elsewhere ( 5 ) .
EXPERIMENTAL SECTION Apparatus and Materials. Graphitized carbon blacks of the Sterling MT type, in stock in our laboratory, and obtained several years ago from different sources (Cabot Corp., Bellirica, MA) have been compared with Carbopack F, supplied by Supelco, Inc., Bellefonte, PA. No substantial differences in the chemical behavior have been found among these materials, but Carbopack F appears to be somewhat harder, probably due to a better pelletization. Stationary phases and testing compounds have been obtained from various sources and all solvents used are HPLC grade. A DAN1 gas chromatograph, Model 3800, equipped with a flame ionization detector and a Shimadzu integrator, Model C-RlA, are used. Preparation of the Stationary Phases and Chromatographic Conditions. Fifty grams of graphitized carbon black is washed in a Gooch filter with 1.5M aqueous solution of H3P04 and then with distilled water until the pH of the effluent is about 6.5. This procedure has been used by Di Corcia and co-workers and by our group in previous studies (3,5)when acidic compounds are to be eluted on graphitized carbon blacks. Then 0.24,0.24, and 0.70% (w/w) of trimesic acid, Carbowax 20 M, and Apiezon L, respectively, are deposited on the carbon black by using the following procedure: First, trimesic acid in methanol is placed on carbon black. After solvent evaporation both Carbowax 20M and Apiezon L were added in a CH2C12solution. This step by step procedure has been
0003-2700/86/0358-3261$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
50
1W
IS0
ZOO
150
ne
Flgwe 1. Calibration curves for the 11 priority pollutants phenols: (1) 2-chlorophenol; (2) 4-chloro-3-methylphenol; (3) 2-nitrophenol: (4) 4nitrophenol; (5) 2,4,6-trichlorophenol: (6) pentachlorophenol: (7) 2methyl-4,6dinitrophenol; (8) 2,edinitrophenol;(9) phenol: (10) 2,4dimethylphenol; (11) 2,4dichlorophenol.
found necessary because of the difficulties encountered in dissolving trimesic acid in CHzClzand Apiezon L in methanol. Glass columns 2 m X 6 mm o.d., 2 mm i.d., have been used in all experiments. Graphitized carbon black is in the 60-80 mesh range and helium is used as carrier gas. Programmed temperature is from 115 "C to 220 "C at 7 "C/min in all chromatograms. Column Conditioning. The column is conditioned under helium flow for 10 h at 220 "C. Afterward, the temperature is set again at 115 "C and a 5-pL injection of a solution of 2,4-diis made. Then nitrophenol in methylene chloride (about 1%/A) the temperature is raised at 220 "C according to the program above. This operation is repeated again. Then the column is conditioned and it may be used routinely without the need of further injections. The treatment of the column is necessary to obtain a linear elution of 2,4-dinitrophenol and 2-methyl-4,6-dinitrophenol at low concentrations.
RESULTS AND DISCUSSION The three liquid modifier system, though rather complicated, is necessary if all the requirements for phenol separation are to be met. In fact, the column should have the following characteristics: (1)linear elution of all the 11phenols included in the list of priority pollutants down to less than 50 ng; (2) analysis time should be limited to less than 20 min, to avoid excessive time in routine analysis; (3) the first compounds of the phenol series should be eluted well after the solvent front to avoid interferences; (4) column efficiency and selectivity should be high enough to obtain a suitable separation of the compounds of interest. Gas-liquid-solid chromatography employing graphitized carbon black as adsorbent allows a very selective and linear elution of polar compounds. Moreover, the low surface area of Sterling M T and similar materials allows a much faster elution of relatively high boiling compounds ( 5 ) . However, the adsorbent still shows very high retention times compared to gas-liquid chromatography. This problem has been overcome by coating the packing material with some monolayers of a nonpolar liquid phase such as Apiezon L. The problem of the presence of some chemically active adsorption sites has been overcome by coating the adsorbent with an acidic phase. The choice of trimesic acid (1,3,5-benzenetricarboxylic acid) is due to the fact that this compound is strongly adsorbed on the graphitic surface because of its aromatic structure, and at the same time is able to deactivate the alkaline active sites with its three carboxyl groups (3). Carbowax 20M is used to increase the polarity of the overall liquid phase system and this helps in the separation of organic compounds of similar structure (4). The loading of 0.24% was chosen because a t this concentration a full coverage of the
Flgure 2. Chromatogram of the 11 phenolic priority pollutants at the minimum concentration of the calibration curves. Sample: 1 pL of
a phenols mixture in methylene chloride. Peak identification: (1) 2-chlorophenol, 18 ng; (2) %-nitrophenol,18 ng; (3) phenol, 11 ng; (4) 2,4dymethylphenol, 11 ng; (5) 2,4dichlorophenol, 19 ng; (6) 2,4,6trichlorophenol, 37 ng; (7) 4-chloro-3-methylphenol, 19 ng; (8) 2,4dinitrophenol, 46 ng; (9) 2-methyl-4,6dinitrophenol, 47 ng; (10) 4nitrophenol, 38 ng; (11) pentachlorophenol, 39 ng.
4
8
12mln.
Figure 3. Direct analysis of an industrial water from an olive oil extraction plant: sample size, 4 pL of water: (1) guaiacol, (2) phenol, (3) o-cresol, (4) m-cresol, (5) p-cresol, (6) and (7) unknown. surface by polar and acidic groups occurs. Figure 1shows the calibration curves for the 11 compounds of interest. In Figure 2 a chromatogram obtained with the concentration of the single compounds at the first point of the calibration curves is reported. The analysis of an industrial water from an olive oil extraction plant is shown in Figure.3. The chromatogram is obtained by direct injection of 4 p L of this industrial water polluted by some phenols. One particular feature of the column described is that the elution of water is very fast and does not affect the column performance. More than 30 injections of polluted water were made without any significant loss of efficiency or change in retention times. Direct analysis may be performed at the parts-per-million level. Finally, a comparison with the packed column officially used in the USA (9) for the analysis of the 11 priority pollutant phenols is made. This column (1.8 m, 2 mm i.d. glass, packed with 1%SP1240 DA on Supelcoport 80-100 mesh) shows on overall analysis time of about 26 min against the 17 min required with our column. Further, 2,4-dinitrophenol is practically overlapping with 2-methyl-4,6-dinitrophenol, while
Anal. Chem. 1986,58,3263-3265
they are completely separated with the column described here. The separation of 2,4-dimethylphenol and 2,4-dichlorophenol is also less effective. Although is it not clear in ref 10 which is the minimum amount of phenols that can be injected for linear elution, from the context of the overall analytical method described, it can be inferred that several hundred nanograms should be injected. A similar separation can also be obtained by using a capillary column (10) with some differences in favor of the packed column described here. The overall analysis time is 50% higher. Although the column is able to elute linearly 5-25 ng of each compound, the minimum amount of the phenols to be injected is 500 ng because of the necessity to split 1:lOO. On the other hand the first compounds of the phenol series are eluted a t such a low temperature that on column injection appears to be scarcely effective. These results show that packed columns in some cases show a better analytical capability with respect to capillary columns. This is particularly true when the number of compounds to be identified and quantitatively analyzed is limited to 10-15. Registry No. 2-Chlorophenol,95-57-8; 2-nitrophenol,88-75-5; phenol, 108-95-2;2,4-dimethylphenol,105-67-92,4-dichlorophenol,
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120-83-2;2,4,6-trichlorophenol,88-06-2;4-chlor0-3-methylpheno1, 59-50-7;2,4-dinitrophenol, 51-28-5;2-methyl-4,6-dinitrophenol, 534-52-1; 4-nitrophenol, 100-02-7; pentachlorophenol, 87-86-5; guaiacol, 90-05-1;o-cresol, 95-48-7;m-cresol, 108-39-4;p-cresol, 106-44-5;water, 7732-18-5;trimesic acid, 554-95-0;Carbowax 20M, 56592-21-3;Apiezon L, 12678-02-3.
LITERATURE CITED (1) Chriswell, C. D.; Chang, R. C.; Fritz, J. S. Anal. Chem. 1975, 47, 1325-1329. (2) Novotny, M.;Hayes, J. M.;Bruner, F.;Simmonds, P. G. Science 1975, 189,215-216. (3) Di Corcia, A.; Samperi, R.; Sebastiani, E.; Severini. C. Chromatographia 1981, 14, 86-88. (4) Bruner, F.; Ciccioli, P.; Crescentini, G.; Pistolesi, M. T. Anal. Chem. 1973, 45, 1851-1859. (5) Mangani, F.; Bruner, F. J. Chromatogr. 1984,289, 85-94. (6) Kiselev, A. V.; Yashin, Y. I . Gas Adsorption Chromatography;Plenum Press: New York, 1969. (7) Vidal Madjar, C.; Ganansia, G.; Guiochon, G. I n Gas Chromatography 1970;Stock, M. Ed.; Butterworths: London, 1971;p 20. (8) Halasz, I.; Horvath, C. Anal. Chem. 1964,36, 1178-1181. (9) Fed. Regist. 1984. 49(209),43297. (IO) Supeico Catalog 1985,23, 29.
RECEIVED for review February 27, 1986. Accepted May 14, 1986.
Versatile Slot Burner Design for Atomic Absorption Spectrometry Robert J. Krupa,' Lori A. Davis, Timothy F. Culbreth, Benjamin W. Smith, and James D. Winefordner*
Department of Chemistry, University of Florida, Gainesuille, Florida 32611 Burners that produce diffusion flames have been used for a number of years in industrial applications and in atomic spectrometry. A design for a "blow pipe" diffusion flame burner was patented as early as 1889 ( I ) . Various modifications of this burner have appeared over the years, including the popular ribbon burners used for glass blowing and welding applications (2). Slot-type diffusion flame burners have also found many applications in the field of combustion diagnostics where explosive gas mixtures are employed (3). Since 1955, when Walsh laid the foundation for analytical atomic absorption spectrometry (AAS) ( 4 ) ,there have been many publications on the development of long-path-length burners for AAS (5-9). With the recent popularity of coherent forward scattering (CFS)and laser-enhanced ionization (LEI), slot burners are finding increased use as atom reservoirs in atomic spectrometry. The conventional slot burners, however, can only burn relatively low-burning-velocity (SJ, low-temperature premixed flames, such as those employing air or nitrous oxide as the oxidant. This restricts the types of flames that can be safely burned, without the flame flashing back. By using high-temperature flames, the atomization efficiencies of several elements can be dramatically improved ( 1 0 , l l ) . The problem with producing high-temperature premixed flames is that the high S, places very stringent requirements on the design and operation of these burners. Diffusion flames, on the other hand, are immune to flashbacks, no matter what fuel and oxidant combination is chosen. We have designed a long-path-length burner for spectroscopic methods which depend upon the absorption process, namely, AAS, CFS, and LEI. The burner is capable of producing premixed and dif'Present address: Baird Corp., 125 Middlesex Turnpike, Bedford, MA 01730.
fusion flames which employ a variety of flame gases.
EXPERIMENTAL SECTION The burner system studied employed a commercially available Perkin-Elmer spray chamber (Model 004-0146) equipped with a pneumatic nebulizer (Model 0303-0352). The conventional slot burner was replaced with a surface-mix slot burner designed to produce both premixed and diffusion flames. This surface burner, constructed from brass, is illustrated in Figures 1 and 2. The central slot, through which the fuel, nebulizer gas, and sample aerosol pass, is lined on both sides by a row of holes for the addition of an auxiliary oxidant. The oxidant holes (0.79 mm in diameter) are connected to two chambers which lie parallel to the 10-cm slot (0.61 mm wide). The auxiliary oxidant is supplied to the burner by a copper tube with a flow spoiler within the oxidant chamber. The end of the tube is capped, and four holes are drilled in the sides of the tube in order to diffuse the gas entering the oxidant chamber. Without the flow spoiler, the oxidant flow out of the center portion of holes is much higher than at the ends, making the flame inhomogeneous along the path length. Because the fuel, entering through the slot, and the oxidant, entering through the side holes, are not premixed in the spray chamber, the diffusion flame cannot flashback, no matter what flame gases are employed. The gases mix on the surface of the burner by diffusion, producing flames with temperatures up to 3300 K. These high-temperature flames require the burner t o be water-cooled to prevent warping of the burner slot and melting of the plastic tubing which supplies the oxidant t o the burner. The performance of the burner was evaluated by determining limits of detection (LODs) and matrix effectsusing a Perkin-Elmer atomic absorption spectrometer (Model 303). The output of the AA was fed into a laboratory-constructed 10-5 integrator, which was connected to a Keithley digital voltmeter (Model 175) for displaying the absorbance signals. LODs were determined by taking the standard deviation of 16 consecutive blank readings, multiplied by 3, and dividing by the slope of the calibration curve.
0003-2700/86/0358-3263$01.50/00 1986 American Chemical Society
