Anal. Chem. 1984, 56. 2434-2441
intensity of light scattered by a particle light-scattering intensity functions particle refractive index number of particles in the detector a t any instant particle size distribution function nebulizer gas and liquid volumetric flow rates detector response corrected for density and refractive index effects average molar refractivity of units in a copolymer average detector response instantaneous detector response time nebulizer gas and liquid velocities sample weight particle diameter diameter of a nebulized drop largest drop diameter formed by a nebulizer number average particle diameter (ENXIEN) constant in the particle size distribution function related to the standard deviation light-scattering angle wavelength liquid viscosity solute density liquid density liquid surface tension
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12)
Chariesworth, J. M. Anal. Chem. 1978, 5 0 , 1414. Morris, C. E. M.; Grabovac, I. J . Chromatogr. 1980, 789, 259. Macrae, R.; Dick, J. J. Chromatogr. 1981, 270, 138. Macrae, R.; Trugo, L. C.; Dick, J. Chromatographla 1982, 15, 476. Rabinowitz, I. N.; Kou, 0.; King, D. K.; Kunberger, G. LC, Llq. Chromatogr. HPLC Mag. 1983, 7 , 496. Stoiyhwo, A.; Colin, H.; Guichon, G. J. Chromatogr. 1983, 265, 1. Ford, D. L.; Kennard, W. J . Oil Colour Chem. Assoc. 1968, 4 9 , 299. Brandup, J., Immergut, E. H., Eds. “Polymer Handbook”; Wiiey: New York, 1975. Denman, H. H.; Heiier, W.; Pangonis, W. J. “Angular Scattering Functions for Spheres”; Wayne State University Press: Detroit, MI, 1966. Kerker, M. “The Scattering of Light and other Electromagnetic Radiation”; Academic Press: New York, 1989. Nukiyama, S.; Tanasawa, Y. Trans. SOC. Mech. Eng., Tokyo 1938, 4 , 86. Nukiyama, S.; Tanasawa. Y. Trans. SOC. Mech. Eng. Tokyo 1938, 4 ,
138. (13) Nukiyama, S.; Tanasawa, Y. Trans. SOC.Mech. Eng. Tokyo 1939, 5 , 63. (14) Nukiyama, S.; Tanasawa, Y. Trans. SOC.Mech. Eng., Tokyo 1939, 5 , 88. (15) Nukiyama, S.; Tanasawa, Y. Trans. SOC. Mech. Eng., Tokyo 1940, 6, 117. (16) Bitron, M. D. Ind. Eng. Chem. 1955, 4 7 , 23. (17) Mugeie, R. A.; Evans, H. D. Ind. Eng. Chem. 1951, 43, 1317. (18) Oppenheimer, L. E.; Mourey, T. H. J. Chromatogr. 1984, 298, 217.
RECEIVED for review February 27, 1984. Accepted June 6, 1984.
Separation of Aromatic Hydrocarbons Using Bonded-Phase Charge-Transfer Liquid Chromatography Jane S.Thomaon* and James W. Reynolds Bartlesuille Energy Technology Center,’ P.O. Box 2128, Bartlesville, Oklahoma 74005
Five charge-transfer bonded phases, (triamine)siiica (TA), [8-(2,4-dinitroaniiino)octyl]slllca (DNAO), [8-(2,4,6-trinltroanliino)octyi]silica (TNAO), [3-( 2,4-dinltroaniiino)propyl]slllca (DNAP), and [3-( 2,4,6-trinitroaniiino)propyi]siiica (TNAP), were compared for the compound class high-performance liquid chromatographic separation of aromatic hydrocarbons. The importance of structural and substituent effects on grouping of compounds by ring number was determined by using 85 model compounds typically found In fossil fuels. Although model compound studies predicted few differences between the columns, analysis of three liquid fossil fuel Sampies showed the DNAP column grouped hydrocarbons most like earlier separations on alumina. Increasing the distance of the charge transfer group from the surface of the silica in the octyinltroaryi columns offered little advantage in grouping hydrocarbons by ring number, but may be of interest for specialized separation technlques.
Characterization of heavy crude oils and synthetic fuels has been receiving increased attention in the fossil fuel industry. In part, this is because refining changes necessary for processing of heavy API gravity crudes or synfuel feedstocks depend on a knowledge of pure component properties such as vapor-liquid equilibrium, density, and mass transport data. These properties may be calculated if the compositional Now National Institute for Petroleum and Energy Research.
differences of these newer feedstocks are known. In addition, the isolation of carcinogenic and mutagenic compounds is of considerable interest. A number of approaches to the chawcterization of these heavy feedstocks have been taken. These include a preliminary separation, either by boiling point, as in work based on an extension of the API Project 60 method, or separation by solvent or supercritical extraction. Recent work has been reviewed by several authors (1-6). Before examination of the hydrocarbons by instrumental methods such as mass or nuclear magnetic resonance spectrometry, further separation by ring number or number of double bonds is desirable. There has been considerable recent work in normal phase separations using silica-based bonded-phase charge-transfer chromatography. Boduszynski et al. (7)used a preparative diamine column to fractionate hydrocarbons by the number of double bonds. Karlesky and co-workers (8) studied amine column separation mechanisms. Multiple column studies have been done by Matsunaga (9) and Lockmiiller, who studied the effects of degree of nitration of bonded groups on retention of aromatic hydrocarbons (10). Matlin and others synthesized a picramidopropylsilica ([3(2,4,6-trinitroanilino)propyl]silica) column and compared its separation with an amine column (11). Hemetsberger et al. (12) studied the temperature dependence of retention on a 2,4,5,7-tetranitrofluorenimine column. Mourey and others (13) examined pyrrolidone-bonded silica in both normal and reverse phase modes. An extensive evaluation of surface adsorption on nitroaryl bonded-phase materials was made by Eppert and Schinke (14).
This article not subject to US. Copyright. Published 1984 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1984
Scheme I
2435
Table I. Mobile Phases Used in Column Studies 2,4 . Olnltroanlllnooclylslllca [DNA01
TAb DNAO" TNAO" DNAP" TNAP"
NO2 = SI.CH~(CH~)~CHZNH-NO~ 2.4,6 .Trlnilroanlllnooclylsllica [TNAO] NO2
= Sl-CH2[CH2)~CHpNH+
NO2
NO2 t4.6 .Trinltroanllinopropyislllca (TNAP) e Si-CH$HpCHZNH
qJ-N02 NO2
Trlamlne ITA] E
SI-CH~CHZCH~NH-CHZCH~NH.CHZCHZNH~
This paper continues an earlier investigation of separation of hydrocarbons on alumina, s i l i ~ a - R ( N H ~ and ) ~ , a chargetransfer stationary phase, [3-(2,4-dinitroanilino)propyl]silica (DNAP) (15). DNAP was found to be slightly superior to the other two column types, but its separation was affected by both degree of alkylation of aromatic rings, particularly one and two ring compounds, and steric hinderances such as those presented by 2-substituted biphenyls. Work toward production of a charge-transfer column less affected by structural and substituent effects has led to preparation and evaluation of four additional bonded-phase materials, which are shown in Scheme I. These materials were evaluated both by gradient elution chromatography of 85 model compounds and by fingerprinting three different types of fossil fuel samples. The grouping of model compounds by both ring number and number of double bonds has been compared with the previous study. The percent monoaromatic, diaromatic, and polyaromatic polar hydrocarbons estimated from the fingerprints of two of the fossil fuel samples have been compared with gravimetric data from preparative alumina separations.
EXPERIMENTAL SECTION Apparatus. Separations were performed on a Model 8000 liquid chromatograph (Spectra-Physics, Santa Clara, CA), with a model 440 dual wavelength UV detector (Waters Associates, Milford, MA) used at 254 nm and 0.5 AUFS for fossil fuel samples or a Model 0413 UV detector (Spectro-Physics) used at 254 nm and 0.32 AUFS for model compound studies. Samples were injected with a WISP autoinjector (Waters). Synthesis of Column Packings. (Triamine)silica (TA). Ten grams of Lichrosorb Si-60 10-pm silica was dried at 100 OC for 4 h and added to 100 mL of Na-dried xylene and 3.5 g of [(trimethoxysilyl)propyl]diethylenetriamine was added with stirring under dry Ar. The mixture was refluxed under Ar for 48 h, with periodic removal and replacement of xylene for methanol removal, and washed with methylene chloride, methanol, acetone, and with methylene chloride. [3-(2,4,6-Trinitroanilino)propyl]silica(TNAP). Ten grams of y-aminopropylsilica, prepared according to the procedure of Nondek and Malik (16),was refluxed with 200 mL of toluene in a 500-mL round-bottom flask, while 3.0 g of 2,4,6-trinitrochlorobenzene was added alternately with 0.5-mL portions of saturated NaHCOa solution (5.0 mL total) over 20 min. After reflux of 1h, the packing material was filtered and washed with water, acetone, methanol, and methylene chloride. [8-(2,4-Dinitroanilino)octyl]silica(DNAO) and [8-(2,4,6Trinitroanilino)octyl]silica (TNAO). Ten grams of Lichrosorb Si-60 10-pm silica was dried at 100 O C for 4 h and added to 200 mL of Na-dried xylene in a 500-mL round-bottom flask with stirring under dry Ar. Five milliliters of (8-bromoocty1)trichlorosilane was added, and the solution refluxed under Ar for 2.5 h. The resulting (8-bromoocty1)silicain 100 mL of dry benzene was added to a stainless steel Parr bomb, the bomb was sealed,
initial conditions time, min composition ( % AB) linear gradient time, min final composition hold @ final time, min reverse gradient time, min final composition reequilibration time, min composition
6 99 1
5 99:l
5 99:l
5 1000
4 98:2
25 0100 0
25 7030 0
25 7030 0
15 5050 5
31 45:55 6
5 99:l
5 99:l
5 99 1
2
1oo:o
0.5 982
10 99:l
10 99:l
10
13
99:l
3.5 98:2
1oo:o
"A, n-hexane; B, methylene chloride. *A, n-hexane; B, methyl tert-butyl ether. cooled to 0 OC, and charged with dry ammonia while being stirred for 30 min. After addition of the ammonia, the inlet valve was closed and the bomb was heated to 100 OC and maintained at this temperature for 48 h, while the contents were stirred magnetically. After the contents cooled, the (8-aminoocty1)silicawas filtered, washed with methanol, acetone, water, acetone, and methylene chloride, and dried. The resulting product was then reacted with either dinitrofluorobenzene or 2,4,6-trinitrochlorobenzeneto form DNAO or TNAO, respectively, as described in the synthesis of TNAP. Column Packing. All materials except DNAP were dried for 1 h at 100 OC, slurried with CC4, and packed upward at 14000-15000 psig using a Haskel MDHF-300-DI pump (Haskel Engineering and Supply, Burbank, CA). The DNAP column was packed by E&S Industries (Marlton, NJ). Procedure. Model compounds were dissolved in n-hexane to produce concentrations of 10 mg/mL for monoaromatic, 1mg/mL for diaromatic, and 0.1 mg/mL for three-through six-ring aromatic hydrocarbons, Aliquots were injected onto each column and the solutes were eluted at 2 mL/min using the mobile phases given in Table I. The columns were maintained at a temperature of 25 h 0.1 OC. The importance of using a constant temperature environment for charge-transfer studies is emphasized. As temperature increases, recent work (17)shows overall ring number groupings remain similar on the DNAP column, but retention times of all compounds decrease, and not always in a linear fashion. A standard blend of benzene, naphthalene, phenanthrene, benz[alanthracene, benz[a,h]anthracene, and dibenzo[def,p]chrysene was run after every fourth model compound. The retention times were corrected for slight variations using eq 1, where
t'is the corrected retention time, t E is the measured retention time for the model compound, t, is the time of the nearest peak in the closest standard blend, t i is the average time of that same standard compound, and tois the retention time of an unretained compound. The gradients chosen were optimized for grouping of the six aromatic hydrocarbons in the standard blend by ring number, as illustrated in Figure 1. The initial and final solvent strengths were chosen to provide similar retention times for benzene and dibenzo[def,p]chrysene. The gradient was then varied linearly between these values to provide a regular change of solvent strength. Methylene chloride was used as the polar mobile phase for all packings except the triamine. Chlorinated solvents, methanol, and THF eluted W-absorbing material from the TA column, even after a 24-h wash. Methyl tert-butyl ether was used with that column as the polar mobile phase although the peak width for five- and six-ring compounds was broader than that obtained on the other four columns with methylene chloride. Sample Preparation. The SRC I1 process coal liquid, from Pittsburgh seam coal, was distilled, and the fraction boiling between 200 and 325 OC had acids and bases removed by extraction.
2438
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1984
Table 11. Corrected Retention Times for Retention Index Standard Compounds correction retention time (min), t ’ a
no. of double bonds
compound
TA
DNAO
TNAO
DNAP
TNAP
3 5 7 9 11 12
benzene naphthalene phenanthrene benz[a]anthracene benz[a,c]anthracene dibenzo[def,p]chrysene
0.53 2.38 11.45 19.17 26.77 29.52
0.65 1.73 5.48 14.63 24.15 29.63
0.35 1.50 5.07 13.92 22.97 26.87
0.84 3.91 13.86 21.99 29.29 32.91
0.81 3.30 12.09 16.80 21.44 24.87
OElution time for unretained peak TA, 2.02 min; DNAO, 1.72 min; TNAO, 2.10 min; DNAP, 1.80 min; TNAP, 1.71 min. See eq 1. Table 111. Average Retention Indexes for Aromatic Compound Classes
7,
no. of aromatic rings
TA
DNAO
TNAO
DNAP
TNAP
1 2 3 4 5 6
0.94 f 0.09 1.95 f 0.11 2.79 f 0.35 3.57 f 0.36 4.14 f 0.57 6.06 f 0.91
0.87 f 0.21 1.95 f 0.15 2.93 f 0.17 3.61 f 0.37 4.42 f 0.63 6.22 f 0.67
0.93 f 0.21 2.01 f 0.22 2.99 f 0.22 3.56 f 0.37 4.33 f 0.55 5.61 f 0.41
1.05 f 0.17 1.89 f 0.31 2.85 f 0.36 3.72 f 0.35 4.90 f 0.15 6.35 f 0.54
1.03 f 0.16 2.00 f 0.19 2.83 f 0.49 3.57 f 0.44 4.61 f 0.27 5.81 f 0.45
0.40
0.37
0.33
0.31
0.33
av std dev
A TOSCO process shale oil >200 OC distillate from the Western Research Institute (formerlyLaramie Energy Technology Center), and a Wilmington, CA, crude oil 370-535 “C distillate from the API Project 60 had acids and bases removed by ion exchange chromatography according to Green et al. (18). The SRC I1 and Wilmington hydrocarbons were separated into aromatic ring number compound classes using preparative chromatography described elsewhere (6,1941). The coal liquid contained about 58%, the shale oil 68%, and the petroleum 82% hydrocarbons in the three distillates. Surface Coverage of Packing. Carbon and nitrogen were determined at Huffman Laboratories (Wheatridge, CO), except for DNAP which was analyzed at Galbraith Laboratories (Knoxville, TN). The surface coverage was calculated by using a value of 435 m2/g for Lichrosorb Si-60 silica. Surface areas were determined by Micrometrics (Norcross, GA), using a modified, single-point BET method using nitrogen as the adsorbate.
1
TA
A DNAO A TNAO 0 DNAP
TNAP
t W
z
I-
Z
0 I-
z W c W
a n W
c
RESULTS AND DISCUSSION
W 0
Each column has been quantitatively evaluated for the ability to group aromatic hydrocarbons by ring number through a comparison of retention indexes (I,)within each ring number group and between columns. The concept of a retention index is one which allows comparison of data obtained with both gradient and isocratic elution chromatography. In order for I,(gradient) to be equivalent to I,(isocratic), the solvent strength under gradient conditions is adjusted to elute a series of customarily chosen unsubstituted aromatic hydrocarbons in a linear manner. Then, the relationships may be expressed as shown in eq 2 and 3.
a
a
0 0
I
2
3
AROMATIC RINGS
4
5
6
+
Figure 1. Dependence of retention time (t’) on the number of aromatic rings.
and
(3)
INvalues of 10,100,1000,10 000,100 000, and 1000 000, respectively. In both eq 2 and 3, t,’ is the corrected retention time of each standard compound, tN’ is the corrected time of
Equation 2 was presented in an earlier study (15),and eq 3 is similar to one used by Pop1 et al. (22),with one difference. I,, rather than log I,, is used on the left side of eq 3 to allow direct comparison of gradient and isocratic units. The retention index standards used and their value of I N in eq 2 are as follows: benzene, 1; naphthalene, 2; phenanthrene, 3; benz[a]anthracene, 4;benz[a,c]anthracene, 5; and dibenzo[def,p]chrysene, 6. In eq 3, these same compounds would have
each standard with the same number of aromatic rings as the sample, and tN+l’is the corrected retention time of the standard with the next higher ring number. A list of corrected retention times used for the standards is given in Table 11. Summary of Comparison of Packing Materials Using Model Compounds. An overall assessment of the importance of structural and substituent effects on grouping of compounds by ring number is possible from the data in Table 111. This table presents the average retention index and its standard
I, = log I N
log t,’ - log ” t
+ log tN+1’
- log
t”
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2437
Table IV. Retention Indexes for Unsubstituted Aromatic Hydrocarbons
no. of aromatic rings
TA
DNAO
1. TNAO
DNAP
TNAP
benzene
1
1.00
1.00
1.00
1.00
1.00
naphthalene indene biphenyl
2 2 2
2.00 1.84 2.05
2.00 1.90 1.99
2.00 1.87 1.97
2.00 1.53 2.01
2.00 1.84 1.92
1.96 f 0.11
1.96 f 0.06
1.92 f 0.07
1.85 f 0.28
1.92 f 0.08
3.00 2.98 2.34 2.46 2.95 2.40
3.00 2.99
3.00 2.98 2.74 2.82 2.89 2.72
3.00 2.98 2.43 2.54 2.85 2.19
3.00 3.01 2.58
2.85 f 0.12
2.66 f 0.33
2.58 f 0.44
4.00 3.34
4.00 3.52 3.94 3.52 3.96 3.30 4.01 2.35
compound
av for 2-ring phenanthrene anthracene fluorene acenaphthalene p-terphenyl 1-phenylnaphthalene
3 3 3 3 3 3
2.69 f 0.32
av for 3-ring benz [a ]anthracene pyrene chrysene fluoranthene naphthacene benzo[blfluorene triphenylene 9-phenylanthracene
4 4 4 4 4
4 4 4
av for 4-ring benz[a,c]anthracene benz[a,h]anthracene benzo[a]pyrene benzo[e]pyrene perylene
5 5 5 5 5
av for 5-ring dibenzo[def,p]chrysene coronene benzo[ghi]perylene av for 6-ring overall av of std dev
6 6 6
2.74 2.77
2.90 2.65 2.84
f 0.14
2.22 2.74
1.90
4.00 3.35 4.06 3.60 3.93 3.31 4.09 3.04
4.00 3.54 4.02 3.40 3.95 3.39 4.14 2.88
3.47 3.96 3.38 3.83 2.92
4.00 3.70 4.07 3.63 3.94 3.49 4.20 2.79
3.67 k 0.40
3.66 f 0.44
3.61 f 0.40
3.72 f 0.44
3.58 f 0.56
5.00 4.34 3.57 3.72 4.09
5.00 4.44 3.34 4.59
5.00 4.57 4.08 3.55 4.43
5.00 4.75 4.73 4.93 5.07
5.00 4.67 4.56 4.25 4.56
4.14 f 0.57
4.42 f 0.64
4.32 f 0.54
4.90 f 0.15
4.61 f 0.27
6.00 7.00 5.18
6.00 6.97 5.69
6.00 5.67 5.17
6.00 6.97 6.07
6.00
6.06 f 0.91
6.22 f 0.67
5.61 f 0.41
6.35 f 0.54
5.81 f 0.45
0.46
0.39
0.31
0.35
0.36
deviation for each ring number group. The standard deviations for all classes are averaged for each column. The resulta-TA, 0.40; DNAO, 0.37; TNAO, 0.33; DNAP, 0.31; and TNAP, 0.33-are similar in magnitude, with DNAP the lowest and TA the highest. The DNAO and TNAO columns have a higher average standard deviation for one-ring compounds. The individual data show this is mainly due to a decrease in retention with increase in added alkyl chain length. This particular effect, although initially apparent on the other three columns, diminishes in magnitude after three carbons on the side chain. All four nitroaryl-bonded columns show enhanced retention of monoaromatics as methyl groups are added around the ring. Overall, the TA column resolves the one- and two-ring compound classes most effectively. Grouping of three-ring compound classes is quantitatively better on DNAO and TNAO because of less difference in retention between six- and seven-double-bond structures, such as phenanthrene and acenaphthalene, and less effect from sterically hindered structures. Four-ring compound classes have similar retention index standard deviations on all five columns, and five- and six-ring compounds group together best on DNAP and TNAP. Separation of Unsubstituted Aromatic Hydrocarbons. All five columns group unsubstituted hydrocarbons well, as illustrated by the average standard deviations in Table IV. There are some individual differences, however. DNAP and TNAP appear to be less affected by five-ring structural isomerism than the other three columns. This may be an
4.71
4.00
6.14 5.30
artifact of the model compounds used, however, since it is not apparent in four-ring compounds. Coronene is retained longer than dibenzo[def,p]chrysene on all columns except TNAO, although its retention on TNAP is less than expected. Its relatively rapid elution on TNAO is surprising, considering the size of the *-electron cloud on coronene, and illustrates the difficulty of predicting retention based on charge-transfer effects alone. Effects of Alkyl Chain Length a n d Number of Substituents. An interesting effect is noted for the one-ring alkyl homologues on both DNAO and TNAO in Table V. There is a general decrease in retention with increasing chain length on the DNAO column. This trend is evident to a lesser extent on the TNAO column. Neither DNAP or TNAP shows a significant change with an increase in solute alkyl chain length. This effect is probably steric in nature. The octyl chains on DNAO and TNAO are probably arranged along the surface of the silica in a brush structure and allow more back and forth movement to the nitroaryl group than on DNAP and TNAP. An extensive evaluation of surface adsorption effects on a [4-(2,4-dinitroanilino)butyl]silica was made by Hammers et al. (231, who concluded that in nonpolar solvents such as hexane, charge transfer attachment took place along the top of a bonded layer of nitroaryl groups. In a methylene chloridehexane mobile phase, they propose penetration of solutes into the inner layer of bonded phases. The initial methylene chloride concentration used with the DNAO and TNAO columns was 1% and 2 % , respectively. If alkylated benzenes are increasingly sterically hindered in this penetration as the
2438
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Table V. Effect of Alkyl Chain Length on Retention Indexes for Aromatic Hydrocarbons
compound benzene toluene ethylbenzene n-propylbenzene n-butylbenzene n-amylbenzene n-hexylbenzene n-decylbenzene n-pentadecylbenzene n-nonadecylbenzene
chain length 0 1 2
3 4
5 6 10 15 19
av for 1-ring naphthalene 2-methylnaphthalene 2-ethylnaphthalene 2-n-butylnaphthalene
0 1 2 4
av for 2-ring pyrene 1-methylpyrene 1-ethylpyrene 1-n-butylpyrene av for 4-ring av std dev
0 1 2 4
4 TA
DNAO
TNAO
DNAP
TNAP
1.00 0.92 0.95 0.87 0.92 0.89 0.84 0.86 0.85 0.84
1.00 1.02 0.86 0.77 0.75 0.75 0.70 0.66 0.63 0.61
1.00 1.02 0.92 0.88 0.79 0.84 0.72 0.69 0.69 0.69
1.00 1.04 1.00 0.97 0.93 0.96 0.93 0.97 0.97 0.96
1.00
1.02 0.97 0.95 0.95 0.95 0.95 0.95 0.93 0.93
0.89 f 0.05
0.78 f 0.14
0.82 f 0.13
0.97 f 0.03
0.96 f 0.03
2.00 1.99 1.95 1.91
2.00 2.01 1.91 1.83
2.00 2.05 1.93 1.83
2.00 2.04 1.97 1.81
2.00 2.10 2.03 1.95
1.96 f 0.04
1.94 f 0.08
1.95 f 0.10
1.96 f 0.10
2.02 f 0.06
3.35 3.40 3.34 3.18
3.54 3.71 3.43 3.18
3.34 3.65 3.40 3.22
3.70 3.91 3.68 3.52
3.52 3.76 3.52 3.34
3.31 f 0.10 0.06
3.45 f 0.22 0.15
3.40 f 0.18
3.70 f 0.16 0.10
3.54 f 0.17 0.09
length of alkyl substituents increases, such a decrease of I, should be observed. The TA column shows less variation in retention index with increasing alkyl chain length than DNAO and TNAO, but appears to be slightly more affected than either the DNAP or TNAP column. A frequent assertion in bonded-phase amine or diamine column studies is that separations by ring number are not affected by the presence of alkyl side chains. Our experience has been that addition of methyl groups has little or no effect on retention, but, particularly in higher ring systems, there is a statistically significant reduction in retention of n-butylaryl or higher compounds for both diamine (15) and triamine columns. Chmielowiec and George (24) report a log retention index of 3.433 for pyrene, 3.524 for 3-methylpyrene, and 3.018 for 3-n-decylpyrene on diamine columns. Our results on the TA column, for pyrene, 3.35, 1-methylpyrene, 3.40, and 1-n-butylpyrene, 3.18, show the same trend. Although not presented in tabular form, the effect of the number of alkyl substituents on retention of benzene, naphthalene, fluorene, anthracene, and benz[a]anthracene was studied. All five columns show little retention difference with the addition of up to four -CH3 groups per benzene ring. Addition of the fifth and sixth methyl groups, however, causes a noticeable increase in retention for all phases except TA. Addition of one or two -CH3 groups to naphthalene, fluorene, anthracene, and benz[a]anthracene causes little change in retention on all five column types. The deviation for the retention of 12-methylbenz[a]anthracene is negative, rather than positive as one would expect. This may be because methyl substitution at the 12-position produces a steric effect, which hinders the charge-transfer interaction. Effects of Steric Hindrance. These effects are shown in Table VI. In a previous study (15) it was noted that aand @-chainbranching of alkyl groups causes decreased retention on the DNAP column. This effect is apparently balanced on the TNAP column by increased strength of the ?r-complex formed. The other columns show little effect of alkyl side chain branching. Two steric effects which decrease retention time and which have been noted in the previous study are alkyl substitution
0.14
Table VI. Effect of Steric Hindrance on Retention Index
compound
TA DNAO TNAO DNAP TNAP
benzene isopropylbenzene p-diisopropylbenzene
1.00 0.92 0.83 1,3,5-triisopropylbenzene 0.75
1.00 0.80 0.61 0.52
1.00 0.81 0.69 0.69
1.00 0.96 0.86 0.81
1.00 0.93 0.85 0.80
naphthalene 2-methylnaphthalene 2-isopropylnaphthalene 2-n-butylnaphthalene 2-isobutylnaphthalene 1-phenylnaphthalene
2.00 1.99 1.89 1.91 1.90 2.40
2.00 2.01 1.82 1.83 1.79 2.65
2.00 2.05 1.90 1.83 1.77
2.00 2.04 1.25 1.81 1.61 2.19
2.00 2.10 1.94 1.95 1.88 1.90
biphenyl 2-methylbiphenyl 2-isopropylbiphenyl 4-methylbiphenyl
2.05 1.87 1.82 2.04
1.99 1.82
1.97 1.88 1.74 2.03
2.01 2.05
1.92 1.77 1.72 1.98
benz [a]anthracene 9-phenylanthracene
4.00 3.04
4.00
4.00 2.92
4.00 2.79
4.00 2.35
1.72
1.96 2.88
2.72
1.42 1.24
at positions adjacent to a nonfused ring junction (as with 2-substituted biphenyls), and phenyl substitution adjacent to a condensed ring junction. Both of these conditions result in aromatic systems with nonplanar aromatic nuclei. The DNAP column is more susceptible to this effect, which produces less-than-expected retention times, than the other four columns, especiallywith regard to the 2-substituted biphenyls. Retention of 1-phenylnaphthalene is greatly diminished on the DNAP and TNAP columns relative to the other three, and, to a lesser extent, the same effect is noted with retention of 9-phenylanthracene. With the T A column, planarity is apparently not a requirement for bonding. DNAO and TNAO show little steric hinderance with 1-phenylnaphthalene; this may be evidence of either fewer silica surface effects with a “brush” structure or possible interaction with two separate nitroaryl groups. Effects of Naphthenic Substitution. Retention indexes of five monoaromatic, eight diaromatic, and one triaromatic hydrocarbons were tabulated, and the results are summarized as follows: Naphthenic substitution appears to cause a slightly
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
enhanced retention on all columns, but the effect is not a regular one. I t apparently is a combination of r-energy increase and steric hinderance. 5,12-Dihydrotetracene separates as a 3.5-ring compound on all columns, with DNAO and DNAP showing less enhanced retention than the others. The addition of -CH3 groups to the 2, 4, 5, and 7-positions of 9,10-dihydro-2,4,5,7-tetramethylphenanthrene should, in theory, cause a slight 7electron energy increase and increased retention. In fact, the compound elutes faster than 9,lO-dihydrophenanthreneon all columns. For all column systems, steric effects appear more important than inductive effects. Separation by Number of Double Bonds. Although the gradients were optimized for grouping of hydrocarbons by “aromatic ring number”, it is possible that some other measure of relative unsaturation of hydrocarbons would have been a better choice. One alternative is the method of Bodyszynski et al. (3,which describes hydrocarbons in termsof the number of double bonds appearing in their classical structures. Retention indexes depend on use of a ring number standard in the calculations and are not suitable for this evaluation. One measure of the effect of structural and substituent effects on grouping tendencies by number of double bonds would be the correlation coefficient of a plot of the number of double bonds vs. corrected retention time. For comparative purposes, this correlation was also made for ring number vs. corrected retention time. These results, with the correlation coefficient for double bonds shown first and for ring numbers second (in parentheses), are as follows: TA, 0.931 (0.951); DNAO, 0.882 (0.907); TNAO, 0.891 (0.917); DNAP, 0.937 (0.968); and TNAP, 0.931 (0.927). One of the columns, TNAP, does have a higher correlation with the number of double bonds. The difference is not enough, however, to present a significant improvement with the gradients used. Effects of Surface Coverage and Geometry. The effects of surface coverage need to be considered. The same silica was used as starting material for all five column packings. This silica, 10-hm Lichrosorb-60, was reported by the manufacturer to have a surface area of 550 m2/g. However, a sample analyzed by BET measurement, after drying a t 200 “C for 40 min, had a surface area of 435 m2/g. The latter value was used to calculate bonded phase loading according to eq 4,where %E
I
1
Ikj l ; C , \ 3
,
3
,
1
I
J
TNAO
t
-.cIw
m a
v) 0
m
a
1
2
3
DNAP
I
,
I
TNAP
I
(4)
3 0
is the percent carbon or nitrogen (w/w) from elemental analysis, NEis the total number of carbon or nitrogen atoms in the bonded phase, and SBET is the specific surface area of the underivitized silica according to the BET nitrogen adsorption procedure. Results for the five columns, with surface areas in pmol/m2 in parentheses, are as follows: TA % C, 10.45 (2.86), % N, 3.98 (2.18); DNAO % C, 12.71 (1.74), % N, 1.69 (0.93); TNAO % C, 8.14 (1.111, % N, 1.89 (0.78); DNAP % C, 8.24 (1.751, % N, 3.01 (1.67); TNAP % C, 9.58 (2.04), % N, 4.05 (1.66). These results show excellent correlation between % C and % N values for surface coverage of the DNAP column. Since the other columns show a higher surface coverage based on percent carbon than percent nitrogen results, the DNAO, TNAO, and TNAP columns have incomplete reactions for the nitroarylation step in bonded phase preparation. The number of bonded-phase groups, based on % N resulta alone, is highest for the T A column, similar for the DNAP and TNAP columns, and lowest for the DNAO and TNAO columns. The overall average I, standard deviation comparison between DNAO and DNAP and TNAO and TNAP, respectively, is informative. Since the nitroaryl groups are “bulky” com-
TA
I I
2439
5
IO 15 20 T I M E , minutes
I
I
25
30
Figure 2. Comparative separation of an SRC I1 process coal liquid 200-325 OC hydrocarbon sample accordlng to aromatic ring number.
pared to reverse-phase alkyl groups, it was felt that steric hinderances noted in a previous work (15) may have been partly due to this problem. Displacement of the nitroaryl groups further from the surface appears to yield no real advantage in overall hydrocarbon separation. It does appear to confer individual differences in selectivities for some difficult-to-separate PAH pairs. Examples are benz[a]pyrene, I, = 4.44, and benz[e]pyrene, 3.34, on DNAO, compared with 4.73 and 4.93, respectively, on DNAP. Although the purpose of this work was to group hydrocarbons by ring number, rather than emphasize separation within groups, these octyl-substituted columns might be of interest for special separations. They are, like the propyl-substituted phases, stable, reequilibrate rapidly after each run, and seem less subject to deactivation with long-term use than the amine phases. Application to Liquid Fossil Fuel Samples. An example of the separation of liquid fossil fuel samples possible with each column is shown in Figures 2-4. Figure 2 illustrates
2440
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 TA
:I
TA
Idl3, ,-
I
TNAO
TNAO
TNAP
il
k 1
0
5
IO
15
20
25
30
T I M E , minutes
Figure 3. Comparative separation of a Tosco process shale oil hydrocarbon sample according to aromatic ring number.
fingerprinting an SRC I1 process coal liquid distillate boiling between 200 and 324 "C. This sample is less complex relative to most petroleum-derived analogues, being composed of essentially one-through three-ring compounds. Figure 3 represents a Tosco process shale oil boiling above 200 "C,and Figure 4 represents the 370-535 "C distillate from a Wilmington, CA, crude oil which was extensively characterized in the American Petroleum Institute project 60. Proposed cut points were established by the elution times of 2methylbiphenyl, acenaphthalene, n-butylpyrene, and benz[ e ]pyrene. In order to get an approximate measurement of the area assigned to each compound class for each column, the cut points were drawn on each trace and the areas were cut out and weighed. Since the different compound classes have different ultraviolet responses, the absorbtivity, a (cm2/g), of the three classes was measured for the coal liquid and the petroleum using a UV spectrophotometer a t 254 nm. The
0
2
5
TNAP
3 IO
15
20
25
30
T I M E , minutes
Figure 4. Comparative separation of a Wilmlngton, CA, 370-535 hydrocarbon sample according to aromatic ring number.
OC
methods used in separation of these two samples into monoaromatic, diaromatic, and polyaromatic polar hydrocarbons by preparative chromatography have been previously described (6, 18, 20, 23). The following absorbtivities were obtained SRC I1 200-325 OC monoaromatics, 2540, diaromatics, 22 800, and polyaromatics, 105 000 cm2/g; Wilmington 370-535 OC monoaromatics, 3410, diaromatics, 12 900, and polyaromatics, 67 700 cm2/g. Compound class separations have not been performed on the shale oil, so its yield was approximated using the SRC I1 absorbtivities. The weight percent of each compound class was calculated for each column and these results are shown in Table VII, where they are compared with gravimetric data. Although calculation of the yield of each compound class by using UV absorbtivities is obviously a less desirable method than using gravimetric results, these comparisons allow useful conclusions to be drawn.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Table VII. Percent Monoaromatic, Diaromatic, and Polyaromatic Polar Hydrocarbons in Fossil Fuel ( % saturate-free
fuel
SRC I1 200-325
column O C
distillate
shale oil >200 "C distillate
basis) MA. DA PAP
TA DNAO TNAO DNAP TNAP gravimetric method"
46 63 29 27 24 25
49 34 70 63 69 64
1 11
TA DNAO TNAO DNAP TNAP gravimetric method
63
17 13
20 17
52 54 56
27 22
21
24
21
43 57 38 34 36 30 36
25
32 32
Wilmington 370-535 O C TA distillate DNAO TNAO DNAP TNAP gravimetric methodb gravimetric method'
70
11 22 24
30 21 27
5 4 1
10
24
40
42 34 49 37
Preparative alumina column (18). Preparative DNAP column (6). 'Preparative open column method (20). The average retention indexes obtained from model compound studies indicate that all five columns are similar in their ability to group hydrocarbons by ring number. The results from actual fossil fuel fingerprints, however, show large monoaromatic hydrocarbon yields from the TA and DNAO columns with all three fuels. The SRC I1 distillate had larger diaromatic and smaller polyaromatic hydrocarbon yields on both trinitroaryl columns. The DNAP column results from this fuel sample are closest to those obtained by preparative gravimetric techniques. This is not surprising, since the preparative SRC I1 separation into compound classes was performed on alumina and work in this laboratory has shown that gravimetric yields for ring number groupings on both preparative alumina and DNAP columns are in good agreement. The shale oil sample shows larger monoaromatic and smaller diaromatic hydrocarbon yields using the TA and DNAO columns. However, the Wilmington sample exhibited this behavior only on the DNAO column. Patterns in the data from the various columns are not as clear-cut with this sample, because of its greater complexity and higher boiling range. One explanation for monoaromatic hydrocarbon yield differences that show up on fingerprints of the three fossil fuel samples, but were not predicted by model compound studies, could be the presence of a compound type that was not included in the model compound study, such ag diphenylalkane homologues. This area is under further investigation by this laboratory.
CONCLUSIONS Model compound studies predict that the five chargetransfer columns studied should show little difference in ability to group aromatic hydrocarbons by ring number. Fingerprinting of actual fossil fuel samples, however, indicates that of the charge-transfer types studied, DNAP-silica produces results closer to those obtained from traditional alumina separations used in the petroleum industry. Further work is needed to identify the compound types involved in differences
2441
in monoaromatic-diaromatic separations between the amine type column and the nitroaryl columns. No advantage is conferred on nitroaryl charge-transfer ring number groupings when the charge-transfer group is elevated further from the surface of the silica. In fact, the DNAO column has the largest deviation in monoaromatic hydrocarbon yields of the columns examined with fossil fuel samples. The DNAO and TNAO columns, however, may be of interest for use in specialized separation techniques such as the separation of isomeric PAH.
ACKNOWLEDGMENT The authors wish to thank Patrick L. Grizzle and John B. Green for helpful advice, Steven A. Holmes for the TOSCO shale oil sample, and Christopher P. Renaudo for assistance in data reduction. Registry No. Benzene, 71-43-2; naphthalene, 91-20-3;indene, 95-13-6; biphenyl, 92-52-4; phenanthrene, 85-01-8; anthracene, 120-12-7;fluorene, 86-73-7;acenaphthalene, 208-96-8; p-terphenyl, 92-94-4; 1-phenylnaphthalene, 605-02-7; benz[a]anthracene, 5655-3; pyrene, 129-00-0;chrysene, 218-01-9; fluoranthene, 206-44-0; naphthacene, 92-24-0;benzo[blfluorene, 30777-19-6;triphenylene, 217-59-4; 9-phenylanthracene, 602-55-1;benzo[a]pyrene, 50-32-8; benzo[e]pyrene, 192-97-2; perylene, 198-55-0; dibenzo[def,p]chrysene, 191-30-0; coronene, 191-07-1; benzo[ghi]perylene, 191-24-2; toluene, 108-88-3; ethylbenzene, 100-41-4; n-propylbenzene, 103-65-1; n-butylbenzene, 104-51-8; n-amylbenzene, 538-68-1; n-hexylbenzene, 1077-16-3; n-decylbenzene, 104-72-3; n-pentadecylbenzene, 2131-18-2; n-nonadecylbenzene,29136-19-4; 2-methylnaphthalene, 91-57-6; 2-ethylnaphthalene, 939-27-5; 2-n-butylnaphthalene, 1134-62-9; 1-methylpyrene, 2381-21-7; 1-ethylpyrene, 17088-22-1; 1-n-butylpyrene, 35980-18-8; isopropylbenzene, 98-82-8; p-diisopropylbenzene, 100-18-5; 1,3,5triisopropylbenzene, 717-74-8;2-isopropylnaphthalene, 2027-17-0; 2-isobutylnaphthalene, 26490-07-3;2-methylbiphenyl, 643-58-3; 2-isopropylbiphenyl, 19486-60-3;4-methylbiphenyl, 644-08-6.
LITERATURE CITED (1) McKay, J. F.; Amend, P. J.; Harnsberger, P. M.; Cogswell, T. E.; Lathan, D. R. Fuel 1981, 6 0 , 14-16. (2) Bartles, K. D.; Zander, M. Erdoel Kohle. Erdaas. Petrochem 1983. 36. 15-22. Chmlelolec, J.; Beshal, J. E.; George, A. E. Fuel 1980, 59, 838-844. Abbott, S. R. J . Chromatogr. Sei. 1980, 18, 540-550. Later, D. W.; Lee, M. L.; Battle, K. D.; Kong, R. C.; Varsllaros, D. L. Anal. Chem. 1981, 5 3 , 1612-1620. Grlzzle, P. L.; Green, J. 6.; Sanchez, V.; Murgla, E.; Lubkowitz, J. Prepr. Div. Pet. Chem., Am. Chem. Soc. 1981, 2 6 , 838-850. Boduszynskl, M. M.;Hurtublse, R. J.; Allen, T. W.; Silner, H. F. Anal. Chem. 1983, 55, 225-241. Karlesky, 0.;Shelly, D. C.; Warner, I. M. J . L i q . Chromatogr. 1983, 6 , 471-495. Matsunaga, A. Anal. Chem. 1983, 5 5 , 1375-1379. Lochmuller, C. H.; Ryall, R. R.; Arnoss, C. W. J . Chromafogr. 1979, 178, 298-301. Matlln, S. A.; Lough, W. J.; Bryon, D. G. HRC CC, J . High Res. Chromatogr. Chromatogr. Commun. 1980, 3 , 33-34. Hemetsberger, H.; Klar, H.; Ricken, H. Chromatographk 1980, 13, 277-286. Mouerey, T. H.; Slggla, S.; Uden, P. C.; Crosley, R. J. Anal. Chem. 1980, 52, 885-89 1. Eppert. 0.; Schinke, I. J . Chromafogr. 1983, 260, 305-327. Grlzzle. P. L.; Thomson, J. S. Anal. Chem. 1982, 5 4 , 1071-1078. Nondek. L.; Malek, J. J . Chromatogr. 1978, 155, 187-190. Thomson, J. S., unpubllshed Data. Green, J. 6.; Hoff, R. J. J . Chromatogr. 1981, 209, 231-250. Vogh, J. 6.; Thomson, J. S. Anal. Chem. 1981, 5 3 , 1345-1350. Dooley, J. E.; Hirsch, D. E.; Thompson, C. J.; Vogh, J. W.; Ward, C. C. Hydrocarbon Process. 1974, 5 3 , 141-146. Linville, B., Ed. DOE/BETC/IC-80/3, Annual Report 41, 1980;p K-2/ 12. Pope, M.; Dolonsky, V.; Mostecky, J. J . Chromatogr. 1974, 9 1 , 649-658. Hammers, W. E.; Theeuwes, A. G. M.; Brederode, W. K.; De Ligny, C. L. J . Chromatogr. 1882, 234, 321-336. Chmlelowiec, J. George, A. E. Anal. Chem. 1980, 5 2 , 1154-1157.
RECEIVED for review March 2,1984. Accepted June 18,1984.
