Anal. Chem. 1986, 58, 3257-3261
:o;
:0 '
to 0.9844 for ratios of 02+ and N+ peaks, and finally to 0.9959 when cross-correlation ratios are used. The increase from 0.9726 to 0.9844 is due to the fact N+ is more intense than NO+; hence, more precise values are obtained. Further improvement to 0.9959 by using cross-correlation values for ratioing is due to the fact that in this method all the available data (i.e., all the peaks in the spectrum) are utilized in computing ratios as opposed to a part of the data being used in sample ratios. Application to eight different liquid mixtures of CCll and cyclohexane has given more dramatic improvements in precisiion due to the increase in the number of peaks in their spectra. In this case, the correlation coefficient increases from 0.9482 for ratios of C+ (12) and CH3+(15) peaks to 0.9819 for ratios of C1+ (35) and CzH3+(27) peaks and finally to 0.9957 when cross-correlation ratios are used. This method is expected to be applicable to any kind of spectroscopic measurements of mixtures for improving precision. However, the cross-correlation of the spectra of a mixture and a pure component may have a finite value even if the mixture does not contain the component; hence, caution must be used as has already been pointed out by Mann et a1
(a)
;I
3257
:k
3
N,O+
(9).
LITERATURE CITED ;ai
2 i' 15
20
25
-
30
35
;1 L5
LO
Mass(or m l z )
(C)
(1) Black, W. W. Nucl. Instrum. Methods 1979, 71, 317-327. (2) Hieftje, G. M. Anal. Chem. 1972,4 4 , 81A-88A. (3) Hleftje, 0. M.; Bystroff, I . R.; Lim, R. Anal. Chem. 1973, 4 5 , 253-258. (4) Powell, L. A.; Hieftje, G. M. Anal. Chim. Acta 1978, 100, 313-327. (5) Horlick, G. Anal. Chem. 1973,45, 319-324. (6) Kelly, P. C. and Horlick, G. Anal. Chem. 1973,45, 518-527. (7) Ng, R. C. L.; Horlick, G. Spectrochim. Acta, Part B 1981, 366, 529-547. (8) Ng, R. C. L.; Horllck, G. Spectrochim. Acta. Part B 1981, 368, 543-551 . .. (9) Mann. C. K.; Golenlevskl, J. R.; Slsmanidls, C. A. Appl. Spectrosc. 1982,36, 223-227. (10) Tyson, L. L.; Vickers, T. J.; Mann, C. K. Appi. Spectrosc. 1984,38, 663-668. (11) Tyson, L. L.; Ling, Y. C.; Mann, C. K. Appl. Spectrosc. 1984,38, 697-700. (12) Bingham, 0.;Burton, C. H. Appl. Spectrosc. 1984,38, 705-709. (13) Vickers. T. J.; Mann. C. K.; Moriev, N. A,; Kina, T. H. I n t . Lab. 1984 (Nov.-Dec.) 12-26. (14) Ng, R. C. L.; Horlick, G. Appl. Spectrosc. 1985,39, 834-840. (15) Ng, R. C. L.; Horlick, G. Appl. Spectrosc. 1985,39, 841-846. (16) Brigham, 0. The Fast Fourier Transform; Prentice-Hall: Englwood Cliffs-, NJ. 1974 (17) Bryant, Wm. F.; Trlvedi. M.; Hinchman, B.; Sofranka, S.; Mitacek, P., Jr. Anal. Chem. 1980, 5 2 , 38-43. (18) Schorr, W. K.; Duschner, H.; Starke, K. Anal. Chem. 1982, 5 4 , 671-674. (19) Brown, C . W.; Lynch, P. F.; Obremski, R. J.: Lavery, D. S. Anal. Chem. lS82,5 4 , 1472-1479.
- - --
L
. : .. , *
. ..
::
15
:s
,uUi
W
20
25
.
35
30
I
40
45 Mass(or m l z )
Flgure 1. Mass spectra of (a) pure 02,(b) pure N,O, and (c) a mixture of N20 and 02.
are obtained by using peak areas of 02+ (32) and NO+ (30), for the second, areas of 02+ (32) and N+ (14) are used, and for the last, cross-correlation values at zero displacement are used. The same set of data is used for computing ratios in all three graphs. The correlation coefficient (R) of the calibration lines is one measure of the precision of the method and NO+ peaks and increases from 0.9726 for ratios of 02+
-.
RECEIVED for review February 28, 1986. Accepted July 16, 1986. We wish to acknowledge the Volkswagen Foundation of the Federal Republic of Germany for the generous grant which supplied the mass spectrometer and the vacuum system.
Selectivity and Sensitivity Improvements at Perfluorinated Ionomer/Celiulose Acetate Bilayer Electrodes Joseph Wang* and Peng Tuzhi Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 Polymer-modified electrodes are of considerable recent interest. In addition to electrocatalytic applications, such electrodes show great promise for electroanalysis. Analysis with polymer-coated electrodes can benefit from their catalytic (1) and discriminative (transport) (2,3)properties, as well as from their use as preconcentrating surfaces (4,5). As a result,
substantial improvements in the selectivity, sensitivity, versatility, and reproducibility of voltammetric measurements can be achieved. One useful strategy involves electrostatic binding of ionic analytes at electrodes coated with polymeric ion exchangers. In particular, the use of Du Pont's Nafion perfluorinated films
0003-2700/86/0358-3257$01.50/00 1986 Amerlcan Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
charge rectifying properties (analogous to semiconductor interfaces). Even though the concept is presented in the context of neurotransmitter measurements, it could be extended to the determination of other ionic analytes by an appropriate choice of the inner polymer. The results of our investigation into the behavior and advantages of glassy carbon electrodes coated with a perfluorinated ionomer/cellulose acetate bilayer assembly are described in the following sections.
Flgure 1. Schematic depiction of glassy carbon electrode modified with a perfluorinated ionomer/cellulose acetate bilayer coating.
offers tremendous ion-exchange affinity for organic cations, relative to simple inorganic cations (6). This characteristic of perfluorinated ionomers, coupled with their impermeability to anions, has been successfully utilized for in vivo monitoring of primary catecholamine neurotransmitters (7, 8). In this case, the permselectivity is particularly advantageous because major interferences due to ascorbic acid and various anionic biogenic amines are eliminated. However, the practical utility of perfluorinated film-based sensors for neurochemical studies is limited by interferences due to other hydrophobic cations (whether electroactive or not) that compete for exchange sites. In particular, it is difficult to distinguish between cationic neurotransmitters possessing similar redox properties. For example, attempts to monitor dopamine (DA) release in response to various brain stimuli are complicated by an oversignal. (Nonelectroactive organic lapping norepinephrine (NE) cations cause only some decrease in the slope of calibration curves when brain samples are concerned @).) The lack of resolving power between oxidizable organic cations is thus the major shortcoming of in vivo measurements at microelectrodes coated with perfluorinated ionomers. The present paper describes the analytical advantages and characteristics of a new bilayer polymeric coating with a cellulose acetate layer atop of the Du Pont's Nafion film (Figure 1). We have demonstrated recently (3,9) that the permeability of cellulose acetate coatings can be controlled by subjecting the polymer to hydrolysis in alkaline media. Such a process increases the porosity of cellulose acetate by breaking its chain into small fragments (IO),yielding permselectivity based primarily on size. The discriminative properties of electrodes modified with single-domain cellulosic coatings have shown to offer substantial improvements in the stability and selectivity of anodic stripping measurements of trace metals (3) and amperometric detection of organic compounds (9). With the perfluorinated ionomer/cellulose acetate bilayer assembly described in the present work, the outermost cellulose acetate coating serves as a separator film (that separates the bulk solution from the perfluorinated layer). With careful control of the permeability of the outer layer, the dopamine ions (of interest) rapidly diffuse toward the inner perfluorinated film, while the larger (and otherwise interfering) norepinephrine or epinephrine (EP) ions are excluded. As a result, a selective scheme for dopamine detection is obtained. Additionally, the bilayer arrangement displays enhanced sensitivity compared to a single-domain perfluorinated coating. This concept differs from previously described bilayer electrode assemblies ( I I ) , based on two overlaid electroactive polymers, that display current and
EXPERIMENTAL SECTION Apparatus. A Bioanalytical Systems Model VC-2 electrochemical cell was employed. The working electrode (3-mm-diameter glassy carbon disk, Model MF 2012, Bioanalytical Systems), reference electrode (Ag/AgCl, Model RE-1, Bioanalytical Systems), and platinum wire auxiliary electrode joined the cell through holes in its Teflon cover. Current-voltage data were recorded with a EG&G PAR Model 264A voltammetric analyzer and a EG&G PAR Model 0073 X-Y recorder. Reagents. A 5% solution of Nafion perfluorinated ionomer (1100 EW) was obtained from Solution Technology, Inc. (Mendenhall, PA). Cellulose acetate (39.8% acetyl content) was purchased from Aldrich Chemical Co. Millimolar stock solutions of dopamine, epinephrine, norepinephrine, and serotonin (Sigma Chemical Co.) were prepared each day. The supporting electrolyte was 0.05 M phosphate buffer, adjusted to pH 4.0 with phosphoric acid. All chemicals were used without further purification. Coating the Glassy Carbon Disk with the Perfluorinated Ionomer/Cellulose Acetate Bilayer. Prior to its coating, the electrode was polished with 0.05-pm alumina particles,rinsed with deionized water, sonicated for 5 min, and allowed to air-dry. The bilayer coating was prepared in two steps. First, the electrode and its surrounding were coated with 4 pL of the 5% Nafion solution; the perfluorinated film was allowed to dry (for 1 min under a flow of air). The outer cellulosic layer was placed by syringing 4 pL of the 5% cellulose acetate solution (in a 1:l mixture of acetone and cyclohexanone) on top of the perfluorinated film; the solvent was then allowed to evaporate in air. Following this the coated electrode was placed in a stirred 0.07 M KOH solution, where hydrolysis proceeded for the desired time. The electrode was immersed in a phosphate buffer (pH 4.0) solution for 10 min prior to the cyclic voltammetric experiment. This was performed by scanning the potential between -0.2 V and +0.9 V at 50 mV s-'. Charge was measured by integrating the area under the anodic peak of the loading voltammogram, after the electrode was immersed in the fully equilibrated solution. Voltammograms presented represent the response after full equilibration. Experiments were conducted at room temperature. RESULTS AND DISCUSSION Differentiation between Cationic Neurotransmitters. The combined structural/mechanical and electrostatic functions of the perfluorinated ionomer/cellulose acetate bilayer assembly are demonstrated in Figure 2; this compares cyclic voltammograms (after equilibration) for 5 X 10%M DA in the absence (a) and presence (b) of 2 X M EP, as obtained at the perfluorinated polymer (A) and bilayer (B) coated electrodes. The addition of E P to the DA solution results in a substantial increase of the voltammetric peaks a t the ionomer-coated electrode; the two compounds oxidize at about the same potential, thus yielding an overlapping response. In contrast, essentially the same voltammetric response is observed at the bilayer-coated electrode before and after the E P addition. It is apparent that the outer cellulosic layer excludes the E P from reaching the inner perfluorinated film. As a result, selective detection of DA is feasible in the presence of higher levels of EP. Successful functioning of the perfluorinated ionomer/cellulose acetate assembly requires knowledge of the effect of the hydrolysis time (Le., permeability of the outer layer) on the voltammetric response. Figure 3 shows the dependence of the oxidation peak charge (for cyclic voltammograms after equilibration) on the hydrolysis time for epinephrine (a) and dopamine (b). Hydrolysis times shorter than 10 min make
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
3259
A
I
= I
I
,I
0.6
0.2
-0.2
I
0.6
0.2
-a2
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0.6
0.2
-0.2
E(v) 0.6
0.2
-0.2
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-0.2
0.2
(VJ
Flgure 2. Cyclic voltammograms at perfluorinated polymer (A) and bilayer (B) coated glassy carbon electrodes: (a) 5 X 10" M dopamine and (b) same as in a but after addition of 2 X lo-' M epinephrine; hydrolysis time (B), 30 min; scan rate, 50 mV s-'; electrolyte, 0.05 M phosphate buffer (pH 4). Voltammograms were obtained after allowing electrodes to fully equilibrate with solute solutions.
4 7
I
I 0
10
I
I
20
30
40
50
Hydrolysts tlme ( mi n) Flgure 3. Dependence of the peak charge on the hydrolysis time for epinephrine (a) and dopamine (b): solute concentration, 5 X lo-' M. The charge was estimated from cyclic voltammetty loading experiments. Other conditions are as in Figure 2.
the inner perfluorinated layer inaccessible to both ions. A gradual increase of the DA peak is observed upon hydrolyzing the film for 20,30,40, and 50 min. The larger E P ion diffuses through the outer cellulosic layer only for hydrolysis times longer than 30 min. (The two transmitters differ only by hydroxyl and methyl groups; MW = 153 (DA) vs. 183 (EP).) Thus,a f i e control of the permeability-via a judicious choice of the hydrolysis time-provides the basis for the analytical utility of the bilayer-coated electrode. Optimization of the hydrolysis time should be based also on sensitivity considerations (as obvious from Figure 3). Any change in hydrolysis conditions (e.g., KOH concentration or rate of mass transport) will require reexamination of the response vs. hydrolysis time profiles. Plain perfluorinated polymer-coated electrodes are not affected by hydrolysis in alkaline media. For example, similar DA' "loading" voltammograms were obtained for the nonhydrolyzed and 40-min-hydrolyzed ionomer-coated electrodes. Attempts to use a composite polymeric layer, based on a mixture of perfluorinated ionomer and cellulose acetate, yielded inferior voltammetric performance. The potential of the permselective outer cellulosic layer for selective detection of DA was evaluated in the presence of various cationic neurotransmitters (Figure 4). For this purpose the bilayer electrode WBB "challenged" first by exposure to solutions containing 5 x 10" M of possible interfering ions (a); this was followed by spiking DA to a similar
Flgure 4. Voltammetric measurement of dopamine in the presence of various neurotransmitters (a): 5 X lo-' M epinephrine (A), norepinephrine (B), and serotonin (C); (b) same as in a but after addition of 5 X 10" M dopamine; hydrolysis time, 30 (A), 20 (B), and 25 (C) min. Voltammograms presented represent the response after full equilibration. Other conditions are as in Figure 2.
level (b). In addition to the elimination of E P interference (A), expected from the data described earlier, the bilayer coating allows selective measurement of DA in the presence of NE (B). A 20-min hydrolysis period was used for this mixture; longer periods result in transport of NE ions through the cellulosic layer. Thus, a fine control of hydrolysis conditions permits differentiation between structurally similar species (DA and NE differ only by a hydroxyl group). In contrast, exclusion of serotonin is not feasible under conditions allowing diffusion of DA through the cellulosic coating (C); both ions yield voltammetric peaks with hydrolysis times longer than 10 min. Fortunately, differences in redox properties can be exploited for simultaneous measurements of dopamine and serotonin (C (b)). The above data indicate that molecular weight (i.e., pore size) is not the sole factor affecting transport through the outer cellulosic layer. While for the catecholamine quinones DA, NE, and EP, which differ only by substitution in the side chain, the trend in permeability (DA > NE > EP) is in good agreement with the variation in molecular weight, this is not the case with serotonin (3-(aminoethyl)-5-hydroxyindole),which constitutes a chemically distinct transmitter. The latter (MW = 178), for example, exhibits larger permeability than NE (MW = 169). Other changes (in hydrophobicity, conformation, or water swelling) can occur as a result of the facile base hydrolysis of cellulose acetate (an ester) to the corresponding alcohol. For example, the thickness of the cellulosic layer increased from 6 to 10 gm upon hydrolyzing it for 40 min, indicating significant water swelling. Such changes may be responsible, at least in part, to the changes in permeability. Overall-and as applied to neurochemical studies-the quantitation of DA is feasible in the presence of the three transmitters tested in Figure 4. Similarly, NE may be detected in the presence of E P (using 30-min hydrolysis time). It is not possible, however, to measure E P or NE in the presence of DA. To the best of our knowledge, no earlier reports of selective voltammetric measurement of DA in the presence of NE or E P are available. (When in vivo measurements are concerned, this problem can be reduced by judicious placement of the electrode into certain brain regions.) Similar improvements in the selectivity were obtained in analogous experiments using a pH 7.4 phosphate buffer solution; however, a pH 4 phosphate buffer electrolyte was used throughout this study because it yielded a larger voltammetric response. Response Characteristics. While the improvements in selectivity represent the major advantage of the bilayer electrode arrangement, it offers an additional advantage of enhanced sensitivity. For example, Figure 5 compares cyclic voltammograms for 40-min-hydrolyzed bilayer (a) and per-
3260
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 I
I
‘-,
0.8
,
b
0.3
-
0.2
E(v) Flgure 5. Voltammograms for 7.5 X lo-‘ M dopamine at bilayer (a) and perfluorinated polymer (b) coated electrodes: hydrolysis time (a), 40 min. Other conditions are as in Figure 2.
fluorinated polymer (b) coated electrodes after equilibration with 7.5 X lo4 M dopamine. The bilayer assembly exhibits a 3-fold enhancement of the anodic peak current. Obviously (and as expected from Figure 2), the relative sensitivity depends on the hydrolysis time of the bilayer assembly. For example, a 30-min-hydrolyzed bilayer coating yields a 1.5-fold peak enhancement compared to plain perfluorinated polymer-coated electrode (Figure 2a). A simple explanation for the increased response of the bilayer electrode is not available a t present time. From a thermodynamic point of view, no differences in the quantity of immobilized DA+ are expected from the presence of the outer cellulosic layer. T o examine the role of the cellulosic layer in the observed behavior, single-domain (40-min-hydrolyzed) cellulosic films were exposed to solutions of the various neurotransmitters. Small peaks, substantially lower than those of the bilayer assembly, were observed. The peaks did not increase upon repetitive scans; no peaks were observed at the nonhydrolyzed cellulosic-coatid electrode. It might be possible that the outer cellulosic layer excludes nonelectroactive cations (present in solution) from reaching the perfluorinated layer, thus minimizing competition for the ionomer-S03- sites. Hence, this unique factor remains to be elucidated. Overall, the bilayer electrode shows great promise for trace analysis of organic cations. Well-defined voltammetric peaks were obtained throughout this study for solutions containing micromolar and submicromolar levels of dopamine. The quantity of DA incorporated into the bilayer coating, and the resulting voltammetric peak, depends on the volume of the perfluorinated polymer solution used for preparing the inner coating. For example, bilayers made by depositing 2, 4, and 6 pL of the ionomer solution (about 5,10, and 15 wm thickness, respectively), as well as 4 MLof cellulose acetate, yielded anodic peak charges of 4.27, 5.83, and 6.92 p C , respectively, after equilibration in a 7.5 x 10% M dopamine solution (conditions as in Figure 5). As such, typical coverages range from 3 x to 5 x mol cm-2. An analogous experiment with a plain (4 pL) perfluorinated polymer-coated electrode yielded a charge of 2.49 p C . No significant change in the charge was observed upon changing the scan rate over the 10-100 mV s-l range, indicating that the above charge values correspond to complete oxidation of dopamine in the film. The change in the composition of the bilayer assembly also affected the reversibility of the DA cyclic voltammetric response. Peak potential separations (Ep,* - E,,J of 153, 202, and 237 mV were observed a t the 2-, 4-, and 6-pL ionomercontaining bilayers, respectively. At the different bilayer
0.8
0.3
-
0.2
E (v) Flgure 6. Voltammograms for dopamine solutions of increasing concentration: 2 X lo-’ M (a),4 X lo-’ M (b), 6 X lo-‘ M (c). Other conditions are as in Figure 2. assemblies used in this experiment, the ratio of the DA anodic/cathodic peak currents was close to unity, as predicted by the theory. The plain perfluorinated polymer-coated electrode yielded a larger (266 mV) peak potential separation. Unless otherwise stated, experiments were undertaken with the 4-pL ionomer bilayer assembly. The reproducibility of the results bears an important factor on the utility of such modified electrodes. This was estimated from a series of repetitive cyclic voltammetry loading experiments ( n = 6) using a 7.5 X lo4 M DA solution and fresh coatings (conditions as in Figure 5). The mean charge value (full equilibration) was 5.83 wC, with a range of 4.90-6.11 p C and a relative standard deviation of 8%. These data indicate a quite reproducible and easily controllable deposition of the bilayer coating. Also important for practical analytical applications is the effect of solute concentration on the voltammetric response. Figure 6 shows loading voltammograms (full equilibration) for bilayer electrodes that had been immersed M (a), in DA solutions of increasing concentration: 2 x 4 X lo4 M (b), and 6 X lo4 M (c). The resulting plot of the anodic peak charge vs. DA concentration was linear, with a slope of 0.61 p C / p M , intercept of -0.08 p C , and correlation coefficient of 0.999. The equilibrium time is another parameter of practical interest. This had been shown to be strongly affected by the hydrolysis time. For example, the time required to attain a steady-state DA response decreases from 80 to 50 min upon extending the hydrolysis time from 30 to 50 min (conditions as in Figure 2). Apparently, the increase in average pore dimension of the cellulosic layer, associated with the increase of the hydrolysis time, results in a faster rate of mass transport toward the inner perfluorinated film. Slow attainment of equilibrium also characterizes plain perfluorinated polymer-coated electrodes (6). As applied to routine analytical applications, such dynamics problems can be alleviated by using forced-convection systems, or may require an appropriate compromise between sensitivity and speed. In conclusion, we have presented data which demonstrate that perfluorinated ionomer/cellulose acetate bilayer coated electrodes can be used to differentiate between various organic cations. For example, the electrode will measure concentration changes of dopamine without responding to concomitant changes in norepinephrine concentration. Thus, it appears
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
