Anal. Chem. 1980, 52, 1345-1350
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Acid-Washed Graphitized Carbon Black for Gas Chromatography Antonio Di Corcia,
Roberto Samperi, Eiigio Sebastiani, and Claudio Severini
Istituto di Chimica Analitica, Universits di Roma, 00 185 Rome, It&
The surfaces of two well-known examples of graphitized carbon blacks (GCB), Vulcan (Carbopack B) and P-33 (Carbopack C ) were washed with an aqueous solution of H,PO,. After this chemical treatment, the GCB surfaces were no longer contaminated by sulfur- and oxygen-carbon complexes which cause chemisorption of acidic compounds. Indeed, acid-washed Vulcan allowed the separation of C2-C, free fatty acids to be performed with untailed peaks at the ppm level. Symmetrical peaks were obtained for phenol and cresols at the subnanogram level with acid-washed P-33. The high performace of acid-washed P-33 modified with 1 % Apiezon and 0.1 YO polyethylene glycol (PEG 20 M) was exploited in the elution of m e high-bolling free fatty acids, such as cerotic acid. The analysis of a complex, dllute aqueous solution of biological interest Containing acids and alcohols was made possible by using acid-washed Vulcan modified with 5 % PEG 20 M.
on both uncoated and liquid-coated GCB surfaces. T h e addition of an acidic deactivating agent only partially resolves the problem. In fact, after some days of continuous use, the deterioration of the gas chromatographic column appears to be due t o neutralization of H3P04by basic surface groups. Moreover, it is known that H3P04is thermally unstable a t temperatures above 200 "C. This is a limiting factor for the use of GCB in the analysis of high.boiling acids. Finally, the presence of H3P04on the carbon surface results in serious tailings for peaks corresponding to eluates having an alcoholic group. In the present work, attempts were made to identify the nature of basic surface groups and to determine their surface concentrations. In order t o improve their gas lchromatographic performance, GCB's have been washed with an acidic solution. After the washing operation, both P-33 and Vulcan were shown to give excellent performances in the elution of acids. EXPERIMENTAL
I t is known that graphitized carbon black (GCB), although considered to be a homogeneous adsorbent, contains on its surface some heterogeneities. These are both geometric ( I ) and chemical ( 2 ) in nature. As far as chemical impurities on the GCB surface are concerned, very few investigations have been made (3-5) mainly because of the difficulties in determining very small quantities of functional groups. Kraus (6) reported that the oxygen and sulfur present in the structure of Graphon, which is an example of GCB, correspond to only 0.12% and 0.270, respectively, of the weight of Graphon. Zettlemoyer (7) and other authors supposed that oxygen surface complexes are a burnt-off residue left over from the heating a t 3000 "C of carbon blacks in producing graphitic carbons. Since 1863, surface oxides on carbon blacks have been studied by a large number of investigators (8). Two kinds of surface oxides were distinguished: basic and acidic surface oxides. As t o basic surface groups, their structure is not yet elucidated t o an entirely satisfactory degree. Several years ago Wiegand (9) showed that surface groups responsible for acidity or basicity of carbon blacks could be extracted with water. In addition, he noted t h a t a carbon black with an original p H of 3.6 gave a final p H value of about 10.7 after being heated a t 1100 "C in an inert atmosphere. In our laboratory, we have observed that if one part of graphitized carbon black is immersed in 20 parts of distilled water and continuously stirred for 24 h, and then the supernatant liquid is separated by filtration, and the water placed in contact with the glass electrode of a p H electrometer, a reading is obtained which corresponds to a p H of 10.5. Such a result is surprisingly in agreement with the p H value reported above for a carbon black heated above 1000 "C. From the gas chromatographic point of veiw, GCB's with basic character obviously make the analysis of acidic compounds very difficult. Chemisorption of acids can be expected 0003-2700/80/0352-1345$01 .OO/O
Vdcan (surface area: IO0 mZ/g)and P-33 (surfacearea: 9 m'/g) are graphitized carbon blacks commercially known as Carbopack B and C, respectively. They were kindly supplied by Supelco, Bellefonte Pa. Electrometric Titration. Exploratory work by the authors demonstrated that about 24 h of contact of the GCB's with the acidic solution was needed to obtain reproducible pH values, and that if the glass electrode was in the presence of GCB particles, anomalous pH values were obtained. The technique used was therefore to make ii suspension of 5 g for Vulcan and P-33 by adding 100 mL of water containing a given amount of HC1. The suspension was then stirred for 24 h a t room temperature by an electromagnetic stirrer. In order to avoid contact of COz with the suspension, a si.ream of nitrogen was passed through the cell. The suspension was then filtered and the liquid was placed in contact with the glass electrode of a pH electrometer. The same procedure was then repeated each time using a fresh sample of GCB and varying the amount of HC1. Determination of Total S u l f u r Content. Weighed samples of the two GCB's were placed in a tubular oven kept at 900 "C where a stream of oxygen passed. Sulfur dioxide coming out of the oven was fixed by a solution containing the tetrachloromercurate ion and reacted with formaldehyde and pararosaniline in acidic solution to form rosanilin-methylsulfonic acid which is able to absorb at a wavelength of 550 nm. Then, measurements carried out by the spectrophotometric technique allowed the determination of the sulfur concentration in GCG's. Determination of S u l f u r as Sulfide. The surface concentration of sulfide present on the two carbons under examination was determined by using weighed samples of the two carbons and immersing them in a strongly acidic solution The hydrogen sulfide formed was carried by a stream of nitrogen into a solution containing a suspension of cadmium hydroxide in water which serves to fix H2S. Then, FeCl, and paminodimethylaniline were added to form methyl blue which was determined by spectrophotometry a t a wavelength of 655 nm. Acid-Washing of P-33 and Vulcan. Preliminary experiments by the authors showed that basic surface groups on Vulcan can be more easily erased by acid-washing than those present on the P-33 surface. In fact, the Vulcan surface was shown to be ilmost 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
B
t
+: 01
0
0.2
0 0
0 4
mpq
HCI
Figure 1. Electrometric titration curves of two graphitized carbon blacks: Vulcan, 0 ; P-33, x
Table I. Sulfur Analysis for Vulcan and P-33
GCB
Vulcan P-3 3
total sulfur content, ppm surface (w/w) area, before after m'/g acid-washing 100
44
*
9
27
i
2 3
8 i: 3 9i4
sulfur sulfides, ppm (w/w) before after acid-washing 4.5 I 0.4 3.5 i 0.4
0.0 0.0
completely free of basic groups simply by treating it with a dilute acetic acid aqueous solution for 15 min with continuous stirring. The same procedure gave unsatisfactory results for P-33. Especially for P-33, at any rate, the effectiveness of the acid-washing was shown to increase if, after decantation of the supernatant liquid, the washing procedure was repeated with a fresh acidic solution. The technique used, therefore, for eliminating surface basicity of P-33 and Vulcan was to fill a glass column having a diameter of 6 cm with 30 g of carbon in the 100-120 mesh range. Then, a total of 400 mL of aqueous 1.5 M H3P04was allowed to pass through the carbon particles, taking care to regulate the flow rate of the liquid to 100 mL per h and constantly maintain the upper level of water about 1 cm higher than that of carbon. Then, the carbon was placed in a funnel with a paper filter and washed with distilled water until the pH of water used to wash the carbon was equal to that of the distilled water. Then, the carbon was washed with some portions of acetone and placed in the oven until it was dry. The effectiveness of the acid-washing was evaluated by the gas chromatographic technique. The dried material was resieved to maintain a mesh range of 100-120 and was packed into a 0.6 m X 1.5 cm i.d. glass column. This column was then heated to 250 "C and maintained at this temperature under flow for 24 h. At this temperature, then, calibrated test-solutions containing dodecane, as internal standard, caprinic acid and 2,4-dinitrophenol of varying concentrations were repeatedly injected into the column. These measurements were made in order to establish whether after acid-washing, chemisorption of acidic compounds still occurs. Partial chemisorption of a given compound by the stationary phase can be adequately recognized by injecting variable amounts of the compound examined together with an inert eluate and measuring changes of the response factor at any given concentration. Coating of the Carbopack C surface was carried out in the usual way ( I O ) , whereas coating of Carbopack B was carried out as recently shown elsewhere (11). The packing operation, which is very critical when Carbopack is used, has been described elsewhere ( I O ) . Columns packed with uncoated Carbopack B and C were conditioned about 24 h at 250 "C. Columns packed with Carbopack B + 5% PEG 20 M and Carbopack C + 1% Apiezon + 0.2% PEG 20 M were both conditioned 12 h a t 245 "C. A Model GI gas chromatoqraph (Carlo Erba) equipped with
32
Figure 2. Chromatograms of the elution of caprinic acid, dodecane, and 2,4-dinitrophenol on P-33 before (A) and after (B) acid-washing. Column, 0.5 m X 1.5 mm; sample size of each component, -200 ng; carrier gas, hydrogen; dead time 2.6 s; temperature, 250 OC
Table 11. Response Factors for Caprinic Acid and 2,4-Dinitrophenol a t Various Concentrations on Acid-Washed P-33 concentration 1:500
1:2.500 1:5.000 1:10.000
response factors caprinic acid 2,4-dinitrophenol 0.74 i: 0.76 * 0.72 k 0.73 *
0.02
0.32
0.02 0.02
0.25 0.12
0.04
0.05
*
0.02
i.
0.03
i:
0.02 0.01
*
a flame-ionization detector was used. When hydrogen was used as the carrier gas, it was necessary to dilute it with nitrogen at the column outlet. This was done by introducing nitrogen in the line of the chromatographic apparatus designed for hydrogen with a flow-rate twice that of hydrogen.
RESULTS AND DISCUSSION Determination of Basic Surface Groups. For the two GCB's under consideration, titration curves plotted as p H vs. milliequivalents of HC1 are given in Figure 1. Duplicate measurements for a given amount of HC1 showed a reproducibility within &0.10 p H values. I t m a y be seen that Vulcan and P-33 contain both qualitatively and quantitatively the same basic surface groups. Titration curves show two distinct inflection points, thus indicating the presence of two different groups of characteristic basicities on the two GCB surfaces. T h e amount of HC1 needed t o produce the second inflection point was not very different from that needed to produce the first inflection point. This implies t h a t the two different surface complexes occur roughly in equivalent amounts. Data concerning the total sulfur content and sulfur as surface sulfide (or sulfhydryl) for Vulcan and P-33 before and after acid-washing are reported in Table I. As can be seen, Vulcan contains an original amount of sulfur almost twice that of P-33. On the other hand, P-33 has a surface concentration of sulfide complexes much larger than Vulcan. This is in contrast with the generally accepted belief that the surface of P-33 has a higher degree of surface homogeneity than Vulcan. From the data reported here, it can be noted that acid-washing is effective in completely removing sulfides from the carbon surface.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
e
1 qao
..
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0, L
1
1-0
eo 0
El0
eeo
eso
eso
e40
e60
e70
Figure 3. Dependence of the ionic current of a flame ionization detector on the column temperature for: untreated P-33 P-33 f 1 % DEGS, A
Acid-washing eliminates about 80% and 70% of the original sulfur contained in Vulcan and P-33, respectively. On the other hand, surface sulfides represent only about 10% and 1370 of the total sulfur content for Vulcan and P-33, respectively. Therefore, it can be deduced that attack by acid on the GCB surfaces eliminates not only sulfur present as sulfide but also other unknown sulfur-carbon complexes. Attempts to identify basic surface groups as sulfur-carbon complexes failed since the HCl consumption was about 180 times higher than that theoretically needed to neutralize surface sulfides. I t can be concluded, therefore, that sulfurcarbon complexes are only a small fraction of the chemical impurities which contaminate the GCB surfaces. Evaluation of Acid-Washed Graphitized Carbon Black. The two chromatograms in Figure 2 show the elution on the untreated and chemically treated surface of P-33 of a dilute solution containing two acids, caprinic acid and 2,4dinitrophenol, and dodecane as an internal standard. As can be seen, only a weak electrical signal was obtained for elution of about 100 ng of a fatty acid on the untreated Carbopack surface. For 2,4-dinitrophenol, no significant signal could be observed even by injecting amounts of this compound 100 times larger than those reported. This fact can be accounted for by considering that surface sulfides of carbon are able to reduce 2,4-dinitrophenol to 2-amino-4-nitrophenol (12). After acid-washing of the carbon surface, both the fatty acid and the phenol derivative are eluted as symmetrical peaks. To substantiate this result, quantitative measurements carried out by eluting these compounds at various concentrations are reported in Table 11. For each solution, peak areas for the two acidic compounds were measured relative to the peak area of the paraffin. By the examination of the response factors relative to the fatty acid, it can be deduced that acid-washing of the carbon surface is effective in cancelling out basic surface groups. Chemisorption of 2,4-dinitrophenol is to some extent still present after elimination of basic carbon-oxygen and carbon-sulfur complexes. This can be explained by taking into account that weak Lewis basic-type compounds, which are able to form stable molecular complexes with 2,4-dinitrophenol (12),are present in traces on the carbon surface and cannot be removed by acid-washing.
T,?
+1
280
DEGS, 0;acid-treated
1+2
I
B
A
ry
0
2
0
L
2 T I41 t , m i n u t e s
Figure 4. Chromatograms of the elution of an aqueous solution containing some phenols. Packing material: (A) untreated P-33 1 YO DEGS; (B) acid-treated P-33 + 1 YO DEGS; column, 0.5 rn X 1.5 rnm; injected amount, 0.6 pL of water containing about 25 ppm of each component; termperature, 205 O C ; carrier gas, nitrogen; dead time, 5 s; (1)water; (2)phenol; (3)2,44methylphenol; (4) 2,4dichlorophenol
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
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1
1
2
I
I
1
I
1
0
2
4
fi
8
I
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Flgure 5. Chromatogram of the elution of a water solution of C,-C, free fatty acids on acid-washed Vulcan. Column, 0.5 m X 1.5 mm; injected amount, 0.4 pL of water containing about 30 ppm of each component; temperature, 160 to 230 O C at 7 "C/min: carrier gas, nitrogen; dead time, 5.6 s; (1) acetic acid; (2) propionic acid; (3) isobutyric acid; (4) butyric acid; (5) 2-methylbutyric acid; (6) 3methylbutyric acid; (7) valerianic acid
Parameters which can affect the neutralization of basic surface groups of the solid material have been considered in order to optimize the acid-washing. Temperature was found to have no influence in the acid-washing process. The same is true for time of contact of the acidic solution with the solid material. For Vulcan, a free-base surface was obtained by shaking the suspension for only 15 min. It was also found that the concentration of the acidic solution does not remarkably affect the neutralization process of basic surface groups, providing the concentration is maintained within the 0.3-1.5 M H3P04range. However, we observed that the nature of the acid played an important role in determining the quality of the acid-washed adsorbing material. Treatments with CH3COOH and HC1 were shown to give a low-quality adsorbing material for gas chromatography. On the contrary, good results were obtained by treating the carbon surface with solutions containing HC104 or H3P04. This result seems to indicate either that the anion of the acid is chemically bonded to the carbon surface or that the anion participates in secondary reactions with GCB surface complexes. According to both of these hypotheses, adsorption characteristics of an acid-washed carbon can be assumed to be affected by the type of acid used. Thermal stability and chemical inertness of a stationary phase are to some extent dependent on the activity of the supporting solid material surface used. In Figure 3, the ionic current of the flame ionization detector (FID) is plotted as a function of the column temperature for two columns: 1YG diethyleneglycolsuccinate (DEGS) on untreated GCB and 1YG DEGS on H3P04-washedP-33. For the sake of simplicity, an arbitrary scale for the ionic current of the FID was used. By holding the sensitivity of the electrometer a t 1/16 of the
1
I
0
I
I
I
2
4
6
TIME, minutes
Figure 6. Chromatogram of the elution of a very dilute aqueous solution of phenol and cresols on acid-treated P-33. Column, 0.5 m X 1.5 mm; injected amount, 1 pL of water containing 0.5 ppm of each component; temperature, 165 OC; carrier gas, nitrogen; dead time, 5.8 s; (1) phenol; (2) o-cresol: (3) rn-cresol; (4) p-cresol
Figure 7. Chromatogram of the elution of some C,,-C* free fatly acids. 1 % Apiezon Column, 0.5 m X 1.5 mm; packing material, A.W. P-33 and 0.1% PEG 20 M: injected amount, 0.6 p L of dichloromethane containing about 150 ppm of each component; termperature, 245 "C: carrier gas, hydrogen: dead time, 2.2 s; (1) oleic acid; (2) arachidonic acid; (3) erucic acid: (4) tetracosanoic acid; (5) cerotic acid
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980 1
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4
'
,
'
1 -
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2
4
6
8
10
12
14
1s
18
20
22
24
I IY t
26
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2s
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Figure 8. Chromatogram of the elution of an aqueous complex mixture of acids and alcohols of biological interest. Column, 1.8 m X 1.5 mm; packing material, A.W. Vulcan f 5 % PEG 20 M; injected amount, 1 pL of water containing 50 ppm of each component; temperature, 80 to 220 "C at 5 "C/min; carrier gas, hydrogen; dead time, 15 s: (1) acetaldehyde: (2) methanol: (3)ethanol; (4) ethylacetate: (5) propanol: (6) isobutanol: (7) butanol; (8) acetic acid; (9) 2-methyl-1-butanol; (10) 3-methyl-1-butanol; (1 1) 1-pentanol; (12) propionic acid; (13) isobutyric acid: (14) butyric acid; (15) 2-rnethylbutyric acid; (16) 3-methylbutyric acid; (17) valerianic acid: (18)isocapronic acid; (19) capronic acid; (20) enantic
acid maximum sensitivity, a t any column temperature the increase of the ionic current was determined by measuring in centimeters the raise of the base line on the chart of the recorder operating with 1-mV, full-scale response. The zero value of this relative scale was chosen when for two different temperatures no increment in the background current was noted. Measurements were carried out after the two columns under consideration were conditioned for two days at 230 "C under flow. Also, background current measurements were carried out after maintaining the column for 3 h at any given temperature. From the plot in Figure 3, it is evident that column "bleeding" is considerably reduced when the polyester liquid phase is deposited on an adsorbing surface which is no longer contaminated by basic chemical groups. The fact that polymeric molecules containing ester functional groups have high thermal stability on a neutral adsorbing surface can be easily explained by considering that hydrolysis of an ester is catalyzed by the presence of acids or bases. I t was reported elsewhere (13-15) that liquid phases deposited on GCB have good tolerance to water injections. This is probably due to the absence of high surface concentrations of chemical groups which are able to catalyze decomposition of stationary phases by water. The tolerance of a stationary phase to repeated water injections was evaluated by depositing i t on both unwashed and acid-washed GCB. The two chromatograms in Figure 4 show the elution of a dilute aqueous solution of phenols on the same two columns mentioned above. It is apparent that the water interfering peak is considerably decreased after elimination of basic groups present on the carbon surface. In addition, no significant variation in the chromatographic characteristics of the column packed with DEGS-coated acid-washed GCB was found even after continuous injections of aqueous solutions for some days. In conclusion, a GCB surface completely free of basic surface groups can be considered an ideal chromatographic material for the direct analysis of water samples for contaminants a t the ppm level. Analytical Applications. T o give experimental evidence of the usefulness of acid-washed (A.W.) GCB in gas chromatographic analyses, some separations of interest were performed. In Figure 5 the chromatogram shows the elution of a water solution containing about 30 ppm of each C2-C5 acid on the bare surface of A.W. Vulcan (Carbopack B). As can be seen, perfectly symmetrical peaks can be obtained even for very
polar compounds, such as acetic and propionic acids. Also, the water interfering peak is negligible because of the absence of any liquid phase. The large separation of 2-niethylbutyric and 3-methylbutyric acid obtained here cannot be obtained even on low-polarity chromatographic columns. In Figure 6 the chromatogram shows the elution of a very dilute water solution containing phenol and cresols each a t a concentration of 0.5 ppm, on A.W. P-33 (Carhopack C). It is clear that 0.4 ng of a phenolic compound can be eluted as an untailed peak. On unwashed GCB, the peaks for the phenols appear to be slightly tailed, thus preventing analysis of these compounds a t the nanogram level. Deactivating the GCB surface with a suitable liquid phase eliminates tailing of peaks for phenols. However, the analysis of a very dilute aqueous solution of phenol is hindered by the water disturbance which overlaps the peak for phenol. Figure 7 shows the elution of tiny amounts of some C18-C26 free fatty acids eluted on A.W. P-33 modified with 1%Apiezon and 0.1% P E G 20 M. Hydrogen was used as the carrier gas in order to reduce the elution time. This result is significant considering t h a t on conventional chromatographic columns free fatty acids higher than C20cannot be analyzed owing to the thermal stabilities of the polar stationary phase and the H3P04. Acid-washing the GCB surface allows the acidic deactivating agent to be eliminated and increases the thermal stability of the stationary phase. It has to be emphasized that a low-surface area GCB, such as P-33, can be successfully used even as a supporting material of stationary phases for the analysis of high-boiling, polar compounds. Finally, Figure 8 shows a chromatogram of an aqueous solution containing both acids and alcohols which can be encountered in the analysis of bacterial fermentation products. For this analysis, A.W. Vulcan was suitably modified with 5% PEG 20 M in order to obtain the desired separation. The role of PEG 20 M is also to eliminate nonlinear adsorption of very small quantities of alcohols occurring on the surface of Vulcan. T h e source of this anomalous adsorption for alcohols can be traced to the presence of surface chemical groups which are not erased by acid-washing. At the present time, the structure of these groups is unknown to us. In conclusion, further investigations are needed to obtain a definitively homogeneous adsorbing surface.
ACKNOWLEDGMENT Thanks are expressed to A. Liberti for helpful advice while
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Anal. Chem. 1980, 52, 1350-1351
the work of the authors was being carried out and to V. Di Palo for the sulfur determination. LITERATURE CITED (1) Graham, Donald. J . Phys. Chem. 1957, 61 1310-1313. (2) Healey, F. H.; Yu, Y . F.; Chessick, J. J. J . Phys. Chem. 1955, 59, 399-404. (3) Diehl. E. Diplom-Arbeit, Technische Hochschule, Darmstadt, Germany 1960. (4) Laine, N. R.; Vastola. F. J.; Walker, P. L. "Proceedings of the 5th Conference on Carbon", 1961: Peraamon Press: Oxford, Enahnd, 1963: Vel. 11, p 211. (5) Laine, N. R.; Vastola, F. J.: Walker, P. L. J . Phys. Chem. 1963, 6 7 , 2030-2033.
(6) Kraus, Gerard. J . Phys. Chem. 1955, 59,343-345. (7) Zettlemoyer, A. C. J . Colloid Interface Sci. 1968. 28, 343-348. (8) Boehm, H. P. "Advances in Catalysis"; Academic Press: New York, 1966; Vol. 16, p 179. (9) Wiegand, William 5. Ind. Eng. Chem. 1937, 29,953-956. (10) Di Corcia, Antonio; Liberti, Arnaldo; Samperi, Roberto. J . Chromatog. 1976, 122,149-153. ( 1 1) Di Corcia, Antonio: Samperi, Roberto; Severini, Ciaudio. J . Chromatog. 1979, 170, 325-327. (12j Beilstein, S. "Handbuch der Organischen Chemie", Vol. VI: SpringerVerlaa: Berlin. 1965: 3rd ed.: D 857. (13) Di Cokia, Antonio: Liberti, Arnaldo; Samperi, Roberto. Anal. Chem. 1973, 45, 1228-1232. (14) Di Corcia, Antonio; Samperi, Roberto. Anal. Chem. 1974, 46, 140-142. (15) Di Corch, Antonio; Samperi, Roberto. Anal. Chem. 1974, 46, 977-979.
RECEIVED for review August 2, 1979. Accepted April 2 , 1980.
CORRESPONDENCE Direct Determination of Polynuclear Aromatic Hydrocarbons in Coal Liquids and Shale Oil by Laser Excited Shpol'skii Spectrometry Sir: Coal liquefaction products (coal liquids) and crude shale oil are known to contain substantial concentrations of potentially carcinogenic and mutagenic polynuclear aromatic hydrocarbons (PAHs). The extremely complex nature of these liquid fuels, which are known to contain hundreds of low and high molecular weight organic compounds, has made i t difficult to directly analyze the liquid fuels for PAH compounds. Presently, the PAHs are usually isolated as a compound class followed by the isolation and characterization of the individual molecules by various chromatographic techniques. The most widely used techniques are high resolution, capillary-column, gas chromatography combined with mass spectroscopic characterization or high performance liquid chromatography with fluorescence detection (1-7). These techniques are often inadequate for resolving the complex mixtures of PAHs present in energy related materials ( I , 2, 5 , 8 ) , and they are not readily adaptable to the direct determination of selected high potency species without resorting to time consuming prior separations. In this short communication we show that tunable, dye laser excitation of Shpol'skii effect spectra provides a potentially useful means of determining PAH compounds directly in coal liquids and shale oil without prior isolation of the PAH fraction by chromatographic or other techniques. Shpol'skii Effect, S h a r p - L i n e Luminescence Spectra. The characteristic sharp-line, low-temperature (
