1564
Anal. Chem. 1982, 5 4 , 1564-1570
Gas Chromatographic Retention Characteristics of Phenols with Superox-:! 0M Curt M. White"' and Norman C. LI Department of Chemlstty, Duquesne lJnlversl&, Plttsburgh, Pennsylvania
152 19
The gas chromatographlc retentlon characterlstlcs of a varlety of underlvatlzed phenols have been studled by uslng Superox-2OM coated on fused slllca. The relatlve retentlon tlmes of these compounds were measured at 160, 150, and 140 O C In order to determlne the effect of operatlng temperature on relatlve retentlon. Thls lnformatlon Is used to predlct relatlve retentlon tlmes of phenols for whlch we had no standards. The llnear temperature-programmed retentlon Indexes of the solutes were measured. The retentlon of phenols on thls phase Is a function of the compounds vapor pressure, its ablllty to hydrogen bond wlth the statlonary phase, and the strengths of those hydrogen bonds. These propertles are In turn governed by sterlc, Inductive, and resonance effects of the substltuents. Llnear free-energy relatlons between the logarlthm of the ratio of the actlvlty coefflclents of phenol to substltuted phenol (calculated from relatlve retentlon data) and the chromatographic substltuent constant, a,, have been determlned for some phenols on Superox-2OM. Lastly, It was shown by nuclear magnetlc resonance experlments that Superox-20M Is apparently a poly(ethy1ene glycol) (also called a polyoxlran or poly(ethy1ene oxide)).
reporting the use of didecyl phthalate (modified with H3P04) list different orders of elution for 4-ethylphenol and 2,3-dimethylphenol. Although all the above phases show pronounced selectivities, their thermal instabilities, with the exception of the newly reintroduced Pluronic L 64, render their use somewhat limited. In addition to the reports cited above describing single stationary phases, a number of investigators have studied mixed phases. Thus, didecylphthalate with SPAN 80 (15), as well as didecyl phthalate with Atpet 80 (16),have been reported as being capable of resolving all the methylphenols, dimethylphenols, ethylphenols, and phenol. In spite of widespread interest in the gas chromatographic separation of phenols, only one group has carried out a comprehensive investigation of the retention characteristics of underivatized phenolic compounds. Hrivnak, Macak, Buryan, and their co-workers have developed gas chromatographic methods for the assay of phenols (1, 2, 17-21), and have applied the techniques to the analysis of phenols in brown coal, coal carbonizationliquids, and coal gasification products. In addition, they have described the retention characteristics of a few alkylated phenols with didecyl phthalate, tricresyl phosphate, and di(3,3,5-trimethylcyclohexyl)o-phthalate stationary phases in capillary columns. They have published the results of a comprehensive investigation of the retention characteristics of underivatized alkyl phenols with capillaries coated with tri-2,4-xylenyl phosphate and with packed columns coated with polyphenyl ether with six rings. Tri-2,4xylenyl phosphate displays pronounced selectivity toward phenols but lacks thermal stability, while packed columns coated with polyphenyl ether with six rings possess good thermal stability but lack sufficient efficiency and/or selectivity tQ separate even the simplest case of phenolic isomerism, the methylphenols. An additional problem in the analysis of underivatized phenols is that of column activity. Thus, the phases mentioned above usually are deposited together with several percent phosphoric acid to deactivate adsorptive sites on the support material in order to minimize peak tailing which must of course be minimized if reliable quantitative data are to be acquired. However, even when mixtures containing as much as 10% phosphoric acid are used, tailing is still evident in some published chromatograms of underivatized phenols. In the present investigation, the retention characteristics of a variety of underivatized phenols eluted from fused-silica capillary columns coated with Superox-20M (22,23)have been studied. The relative retention times of these compounds have been determined at 160,150, and 140 "C, and the effects of operatingtemperature have been evaluated within these limits. This information is used to predict relative retention times of phenols for which we had no standards. The linear temperature-programmed retention indexes of the solutes were determined, The mechanisms of phenol separation with this phase have been related to the inductive, steric, and resonance effects of the substituents. Linear free-energy relationships were then formulated for some phenols with Superox-20M. Lastly, the structure of the stationary phase has been investigated.
The gas chromatographic separation of underivatized phenols has been the subject of intense investigation since the late 1950s. Nevertheless, only a few stationary phases have been identified that are capable of separating even the simplest cases of phenolic isomerism,the mono- and dimethylphenols. Isomeric alkyl phenols often have similar chemical and physical proerties, making their separation with packed columns very difficult. The separation and quantitative analysis of complex mixtures of phenols thus require the use of high-resolution capillary gas chromatography combined with stationary phases that display pronounced selectivity. Further, these phases must be coated on completely inert supports in order to prevent peak tailing. The most selective gas-liquid chromatographic stationary phases available for the analysis of phenols that are capable of separating underivatized mono- and dimethylphenols include (A) didecyl phthalate (1-4) [135 "C] (41, (B) di(3,3,5trimethylcyclohexyl) o-phthalate (2, 5-8) [125 " C ] (7),(C) tricresyl phosphate (2) [130 "C] (9),(D)tri(2,4-xylenyl)phosphate (2, 7 , I O ) [135 "C] (2),and (E) Pluronic L 64 (11) [240 "C] (12). The temperatures in brackets are the approximate practical upper temperature limit of these phases. Janak and Komers (13)as well as Ono (14) have used sugars as stationary phases to separate all the dimethylphenol isomers. None of the sugars gave separation of 3- and 4methylphenol. Only di(3,3,5-trimethylcyclohexyl)o-phthalate (8)and didecyl phthalate ( 3 , 4 )have been reported to separate mixtures of all the ethylphenols, methylphenols, dimethylphenols, and phenol. Unfortunately, the two manuscripts Author to whom correspondence should be addressed Analytical Chemistry Division, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, PA 15236. 0003-2700/82/0354-1564$01.25/0
0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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Table I. Chromatographic Conditions column used for re1 retention time expts column length column inside diameter Superox-SOMfilm thickness injection port temperature detector temperature He flow ratea He linear velocity sample injection sizeC split ratio oven temgerature
column used for linear temp programmed retention indexes expts
29 m
30 m 0.20 mm 0.10 pm 275 "C 275 "C 1.75 mL/minb 55 cm/s -2.0 p L 300:l 75-220 "Cat 2 "C/min
0.30 mm
0.25 pm 275 "C 275 "C 0.85 mL/min 20 cm/s -2.0 pL 280:l isothermal
a He carrier gas was purified by passing it through an oxygen absorption trap. an HP7671A automatic liquid sampler.
EXPERIMENTAL SECTION A Hewlett-Packard Model,5840A gas chromatographequipped with a flame-ionizationdetector and chromatographicdata system was used. Retention times were measured to f'/looth of a minute by the chromatographicdata system. The purity of all standards was checked by gas chromatographyand the standards were used as received. The structures of two chemicals whose origins were unknown were corroborated by infrared (IR) spectrophotometry and by proton magnetic resonance ('H NMR). Not all the compounds investigated are phenolic; 1-indanol, 2-indanol, and 1,2,3,4-tetrahydro-l-naphthol which are alcohols (although for simplicity they will be referred to as phenols for the remainder of this paper) were included because of their possible importance in coal chemistry. Two Superox-20Mcolumns were prepared. Uncoated fusedsilica was obtained from Hewlett-Packard and coated statically with methylene chloride solutions of Superox-20M. No pretreatment was given to the fused-silica other than rinsing with distilled water at 80 "C anld drying with Nz at 150 "C for 12 h. Chromatographic conditioris used to measure relative retention times at 160, 150, and 140 ['C appear in Table I. Mixtures of the standards were prepared in methylene chloride and were chromatographed five times at each temperature investigated. Relative retention times were calculated relative to phenol, using adjusted retention times, t k. Linear temperature-programmed phenolic retention indexes were calculated by using measured retention times generated by chromatographinga methylene chloride mixture of the standard phenolic compounds four tirnes. The chromatographicconditions used to measure these retention indexes appear in Table I. Since relative retentions change as a function of temperature, column positioning in the (commercial air oven; f l - 2 "C) gas chromatographwas of concern. Therefore, all measurements were made when the column was positioned in the approximate center of the oven, and its position was not changed during the course of the experiments. The re4ention times thus were very reproducible. However, it must be pointed out that the relative retentions using other instruments with different temperature gradients may lead to different relative retention times. The degree to which the relative retentions differ will be a function of how closely the conditions and parameters compare with those used in this investigation. The situation described above creates a dilemma in the reporting of relative retention time information. When experiments are conducted as described, the relative retention times are reproducible to the fourth si,gnificant figure. In the present investigation,the actual experimentallydetermined,average relative retention times, are reported to three, and in most cases, to four significant figures, with the understanding that values reported to three significant figures should be rounded to two, and values reported to four significant figures should be rounded to three significant figures for those attempting to reproduce them. The proton magnetic resonance ('H NMR) spectra of Superox-2OM and Carbowax-2OM were determined by using a Varian XL-100-15 NMR spectrometer equipped with a deuterium lock. Solid Superox-2OM and Carbiowax-2OM were dissolved in CD2C12 (99.9% D) containing a trace of tetramethylsilane (Me&) and placed in 5 mL 0.d. sample tubes. The lH NMR spectra were
At 75 "C.
All samples were injected by
acquired at a temperature of 30 "C using a spectrum width of 1200 Hz and an accumulation time of 3.4 s following a 90" pulse. Twenty-four transients were accumulated with a total delay of 8.4 s between pulses. The free induction decay was exponentially weighted, employing a time constant of minus one second before transformation.
RESULTS AND DISCUSSION The relative retention times of the compounds investigated were measured isothermally at 160, 150, and 140 "C in order to determine the effect of operating temperature on retention. These data are presented in Table 11. The relative retention times were quite reproducible. The relative standard deviations of the measurements ranged from 0.000 to 0.27% while the grand average standard deviation of all measurements was 0.006. The logarithm of the relative retention times of the compounds investigated varies linearly as a function of operating temperature as shown in Figure 1. The order of elution of various phenols changes as a function of operating temperature, perhaps due to differences in polarity of the stationary phase a t different temperatures (24). As an example, at 160 "C, 2-sec-butylphenol is not completely resolved from the 3-ethylphenol and 2,3,5,6-tetramethylphenolpeak, while at 150 "C and 140 "C, 2-sec-butylphenol is completely resolved from these compounds. This may be due to slightly increased selectivity a t lower temperatures (25). One of the more useful applications of determing the retention characteristics of homologous series of compounds isothermally on well-defined stationary phases is that the retention of compounds in that homologous series for which no standards are available can be predicted. It is well-known that the logarithm of the relative retention time (or Kovats retention index) is a linear function of carbon number for a variety of compound types (26). There are exceptions, and accurate predictions are not always possible (19). Nevertheless, if it is assumed that the relationship between the logarithm of relative retention time and carbon number is linear (from C7 to Clo) for homologous phenols with Superox-2OM (the data presented here show that it is linear from C7 to C9),then the relative retention time of n-butylphenols substituted in the 2 and 4 positions can be predicted. This is shown graphically in Figure 2. The linear least-squares regression line for each homologous series in Figure 2 and its 95 % confidence interval were determined and extrapolated to ten carbon atoms. Given ten carbon atoms, the equation describing the straight line was solved for log a at 160 "C for 4-n-butylphenol and converted to a, which had a value of 3.913 f 0.054. The 95% confidence interval bounds were also extrapolated to ten carbon atoms. The predicted a values for 4-n-butylphenol at 150 and 140 "C were 4.192 f 0.56 and 4.494 f 0.065, respectively. The predicted a values for 2-n-butylphenol were calculated in a similar way and had values of 2.295 f 0.038, 2.411 f 0.012, and 2.518 f 0.019 at 160, 150, and 140
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST I982
0.8
0.6
-
..
$ 0.4 K
-
0 0
0.2
0.0
I 160
150
7
I40
I
I
I
9
IO
I1
#of C atoms
'C
I
I
S
Figure 2. Line 1 Is a plot of the logarithm of the 160 O C relative retention times of 4methylphenoi, 4-ethylpheno1,and 4-n-propylphenol as a function of the number of carbon atoms. Line 2 Is a plot of the logarithm of the 160 'C relatlve retention times of 2-methylphenol, 2-ethylpheno1, and 2-n-propylphenol as a function of the number of carbon atoms. Numbers refer to compounds in Table 11. The values of the correlatlon coefficlents of the linear least-squares lines were
I
P 4 1
0.99.
Since isothermal gas chromatography leads to long analysis times for complex mixtures of phenols, the retention characteristics of the phenolic solutes were also investigated under conditions of linear temperature programming. The respective retention indexes were then Calculated with the approximately linear relationship of Van Den Do01 and Kratz (27)
I60
I
I-
I
I
150
140
1
.14
L-
1
8-
7
0
-'0-
e
0 6 3 7-
-2-
-
-1
-
where I is the retention index, tR(subshce) is the measured retention time of the substance for which the retention index is to be determined, tR(n)and tR(n+l)are the measured retention times of the bracketing standards that elute just before and after the compound of interest, and n is the bracketing interval which can have values of zero, one, two, or three (28). The retention indexes for polycyclic aromatic hydrocarbons (PAH) (29),and nitrogen- and sulfur-containgaromatic compounds have similarly been investigated. Other phenols were used as bracketing standards since the reproducibility of retention index data increases with the similarity of the sample to the chosen standard reference compounds,and since under temperature-programming conditions retention data are dependent not only on temperature but also on the rate of temperature programming (27, 30). Thus, phenol, 4-tertamylphenol, and 1-naphthol were selected as the bracketing standards to which retention indexes o f 100.00, 200.00, and 300.00, respectively, were assigned. The value of n for compounds that eluted before phenol was set to zero, n = 1 for those that eluted between phenol and 4-tert-amylphenol, n = 2 for compounds that eluted between 4-tert-amylphenoland 1-naphthol, and n = 3 for those eluting after 1-naphthol. Retention indexes of compounds that eluted before phenol were calculated from an extension of the phenol/4-tertamylphenol interval, and those compounds eluting after 1naphthol were calculated from an extension of the 4-tertamylphenol/l-naphthol interval. All bracketing standards eluted during the temperature program period. The linear temperature-programmed phenolic retention indexes, I, obtained using a 30 m X 0.20 mm fused silica capillary column coated with a 0.10 pm film of Superox-2OM are presented in Table 11,while a chromatographicprofile of all the compounds studied is illustrated in Figure 3. Superox-20M separates the
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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Table 11. Relative R e t e k o n Times (a)of Underivatized Phenols Determined at 160, 150, and 140 "C and Linear Temperature Programmed Phenolic Retention Indexes aa
n0.g
compound
160 "C
150 "C
140 "C corr coeffC
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
2,6-dimethylphenol ( l ) d 2,6-di-tert-butylphenol (2) phenol (3) 2-methylphenol ( 4 ) 2,4,6-trimethylpbenol (5) 2-tert-butyl-6-methylphenol (6) 2,3,6-trimethylpheno1(7) 2,4,6-tri-tert-amylphenol (8) 1-indanol(8) 2-ethylphenlol (9) 4-methylphanol (10) 2,5-dimethylphenol (10) 2,4-dimethyl]phenol (11) 3-methylphehnol(1 2) 2-isopropylghenol (14) 2-indanol (131) 2,3-dimethyljphenol (15) 2-n-propylplienol (16) 4-ethylphenol (17) 3,5-dimethylphenol (17) 3-ethylphenol (18) 2,3,5,6-tetramethylphenol(l8) 2-see-butylphenol (20) 2-tert-butylghenol(21) 2-isopropyl-!5-methylphenol (thymol) (21) 1,2,3,4-tetrahydro-l-naphthol (19) 3,4-dimethylphenol (22) 4-isopropylphenol (23) 2-methyl-5-iis~opropylphenol (23) 2,3,5-trimethylphenol (24) 3-isopropylphenol(25) 2-tert-butyl-&methylphenol (26) 4-n-propylphenol (27) 4-tert-butylghenol (28) 3-tert-butylphenol(29) 4-sec-butylplnenol (30) 4-chromanol ( 31 ) 3,4,5-trimethylphenol (33) 4-indanol (3:2) 3,5-diisoprogylphenol (34) 4-tert-amylphenol(35) 2,3,4,5-tetramethylphenol(36) 5-indanol (3'7) 6-methyl-4-indanol (38) 7-methyl-4-indanol (39) 5,6,7,8-tetrahydro-l-naphth01(40) 7-methyl-5-iiidanol (41) 2-phenylpheinol (42) 5,6,7,8-tetralhydro-2-naphthol(43) 2-cyclohexylphenol (44) sesamol (45) 2-meth yl-1-napht hol (46) 1-naphthol (47) 2-naphthol (48) 3-phenylphenol (49) 4-phenylphenol (50)
0.710 0.783 1.000 1.000 1.020 1.042 1.190 1.271 1.355 1.311 1.345 1.345 1.359 1.388 1.559 1.639 1.742 1.742 1.932 1.932 1.991 1.991 2.013 2.079 2.079 2.169 2.279 2.324 2.324 2.324 2.372 2.538 2.737 3.028 3.064 3.229 3.593 3.874 3.874 4.326 4.707 4.930 5.124 5.258 5.689 6.822 7.664 9.133 9.392 10.81 11.14 15.22 25.01 29.14
0.693 0.768 1.000 1.000 1.022 1.048 1.187 1.275 1.360 1.342 1.366 1.366 1.383 1.408 1.614 1.649 1.798 1.798 1.998 1.998 2.060 2.060 2.112 2.187 2.187 2.187 2.366 2.428 2.428 2.428 2.477 2.708 2.879 3.204 3.246 3.426 3.746 4.118 4.118 4.705 5.072 5.311 5.493 5.670 6.156 7.426 8.370 10.14 10.33 12.15 12.49 17.00 b b
0.677 0.755 1.000 1.011 1.023 1.057 1.189 1.281 1.351 1.370 1.383 1.383 1.411 1.426 1.673 1.673 1.858 1.858 2.064 2.064 2.125 2.125 2.222 2.297 2.297 2.222 2.455 2.540 2.540 2.540 2.591 2.891 3.028 3.389 3.436 3.634 3.900 4.371 4.371 5.119 5.460 5.717 5.878 6.100 6.653 8.047 9.1 24 11.30 11.30 13.63 13.99 b b b
1.000 0.997 1.000 -0.866 -0.981 -0.994 -0.500 -0.995 -0.866 -0.999 -0.997 -0.997 -0.999 -0.999 -0.999 -0.972 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.983 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0,999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999 -0.999
1e.f
78.17 86.83 100.00 101.39 102.12 106.61 110.55 117.93 117.93 120.10 121.01 121.01 121.72 122.32 133.27 130.05 137.68 139.16 143.70 143.70 145.44 145.44 149.44 151.61 151.61 147.03 153.61 156.11 156.11 156.63 156.85 161.59 166.14 172.85 173.57 176.68 180.60 187.54 186.72 197.07 200.00 202.38 203.74 206.06 211.01 221.82 229.23 240.05 241.62 250.74 251.61 271.15 300.00 309.55 354.07 359.42
5
0.03 0.05 0.00 0.00 0.00 0.03 0.02 0.04 0.04 0.02 0.05 0.05 0.03
0.06 0.04 0.04 0.02 0.03 0.03 0.03 0.03 0.03 0.02 0.04 0.04 0.07 0.01 0.02 0.02 0.02 0.04 0.06 0.02 0.01 0.04 0.03 0.04 0.03 0.03 0.03 0.00 0.03 0.03 0.03 0.03 0.07 0.03 0.04 0.01 0.05 0.03 0.05 0.00
0.10 0.06 0.09 a Average of five determinations. Eluted after 327 min, which is the maximum retention time the Hewlett-Packard chromatographic data system on a 5840 A GC can measure. Correlation coefficient of the least-squares line plotted through the logarithm of the relative retention times as a function of temperature in Figure 1. Numbers in parentheses after compound names refer t o peak numbers in Figure 3. e These linear temperature programmed retention indexes were calculated from experiments employing the 8 m X 0.20 mm fused-silica column coated with a 0.10 pm film of Superox20M. f Average of four determinations. 8 T ese numbers refer t o lines in Figures 1and 2. b b
b b
b b
R
three methylphenols froiri one another, the six dimethylphenols from one another, and the three ethylphenols from one another. However, 2,5-dimethylphenol and 4-methylphenol coelute, as do 3,5-climethylphenol and 4-ethylphenol. Due to the acidic and highly adsorptive nature of phenols, peak tailing can be a maj'or problem in their gas chromatographic analysis. As can be seen from the chromatograms presented in Figure 3 no tailing a t the approximate level of
5-40 ng per compound is observed for the phenols chromatographed with the described system, and the eluted peaks are symmetrical, due, most likely, to the slightly acidic surface of fused silica and to its low metal content (31). The linear temperature-programmed phenolic retention indexes were quite reproducible on this column. In order to determine the degree to which the retention indexes can change from one system to another, a different column and
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
1 0
IO
I 75
1
I
95
115
I
I
20
I
1
I
I
40 30 TIME, minulei
I35 I55 TEMPERATURE, 'C
I
50
I
60
I
1
175
195
I
]
1
1
70 73 215 220
Figure 3. Gas chromatographlc profile of a mixture of the phenolic solutes obtained using a 30 mm X 0.20 mm fused silica capillary column coated with a 0.10 pm film of Superox-2OM employing a He linear velocity of 55 cm/s, a split ratio of 300:1, and approprlate temperature programming. Numbered chromatographlc peaks are identified In Table 11, as numbers In parentheses after compound names. different experimentalconditionswere used, and the retention indexes were measured. The new column dimensions were 29 m X 0.30 mm with a Superox-20M film thickness of 0.25 pm. This column was temperature programmed from 75 "C to 240 "C a t 4 "C/min. He linear velocity at 75 "C was 37 cm/s. The order of elution of the compounds remained the same and more than half of the retention indexes were within 1.5 index units of those presented in Table 11,but the retention indexes of some compounds changed by as much as 3.60 index units. For example, 4-chromanol,2,6-di-tert-butylphenol, and 3,5-diisopropylphenol gave retention indexes of 184.20 f 0.07, 83.32 f 0.16, and 193.91 f 0.05, respectively. Superox-20M separates all the ortho, meta, and para alkylated phenolic isomers investigated, with the order of elution being ortho, para, meta. According to Verzele and Sandra, Superox-2OM is a poly(ethy1ene glycol) or polyoxiran (22,23), of average molecular weight 20 000, and differs from Carbowax-20M in that all traces of the polymerization catalyst have been removed. The retention characteristics of a few alkylated phenolic compounds have been investigated by others with Carbowax-20M using both packed columns and capillaries (32, 33) and their findings indicate that Carbowax-2OM does not separate meta and para alkylphenolic positional isomers. Since Carbowax-20M contains trace amounts of polymerization catalyst that is claimed to be absent from Superox-20M, and since trace impurities are known to affect retention (34)) separation of meta and para alkylphenolic isomers by Superox-20M may be due to the absence of phase contaminants. The retention of phenols with Superox-20M is a function of the vapor pressure of the compound, ita ability to hydrogen bond with the stationary phase, and the strengths of those hydrogen bonds. In the case of alkylphenols, the order of elution of positional isomers is ortho, para, meta. Alkyl substituents in the ortho position increase steric hindrance of the phenolic hydroxyl group, reducing its ability to form hydrogen bonds with the stationary phase, resulting in rapid elution of the ortho isomer. Thus, 2,6-di-tert-butylphenol elutes before phenol and 2-tert-butylphenol. The order of elution of 2-butylphenols is sec-butylphenol, tert-butylphenol, and n-butylphenol and is explainable by the size and geometry of the substituent relative to its ability to sterically hinder intermolecular hydrogen bond formation. It should be noted that the retention characteristics of sec-butylphenol and tert-butylphenol were measured, while the retention of n-butylphenol has been predicted (see Figure 2). In the case of para and meta alkyl phenolic positional isomers, their retention is governed by the ability of the alkyl substituent to the influence the electron density around the hydroxyl oxygen atom, thus affecting the strength of the
resulting intermolecular hydrogen bond between the phenolic hydroxyl group and the etheric oxygen of the stationary phase. The greater the electron density around the phenolic oxygen, the weaker will be the intermolecular hydrogen bond. For 4-butylphenols, the order of elution is 4-tert-butyl, 4-sec-butyl, and 4-n-butyl, a consequence of the inductive effects of the butyl groups. Since the tert-butyl group has a stronger electron-donatinginductive effect than the other butyl groups considered, the electron density around the phenolic oxygen atom is highest for 4-tert-butylphenol. Therefore, intermolecular hydrogen bonding of this species is expected to be relatively weaker; thus, it elutes before the other 4-butylphenols. It should be noted that the retention characteristics of 4-tert-butylphenol and 4-sec-butylphenol were measured, while the retention of 4-n-butylphenolbas been predicted (see Figure 2). When the substitutent changes from alkyl to aryl, as in the case of the phenylphenols,the order of elution becomes ortho, meta, para, due to resonance effects of the phenyl substituent. o-Phenylphenol elutes much earlier than the meta and para isomers, because of steric hindrance to hydrogen bonding. p-Phenylphenol elutes after rn-phenylphenol presumably because the strength of the intermolecular hydrogen bonds formed by p-phenylphenol are st,ronger than those formed by rn-phenylphenol. The para-phenyl group removes electron density directly from the hydroxyl oxygen atom via a resonance effect, while the meta-phenyl group cannot. Thus, the electron density at the hydroxyl oxygen atom of p-phenylphenol is lower than that of rn-phenylphenol and, consequently, hydrogen bonds formed by p-phenylphenol are stronger than those formed by m-phenylph.eno1, causing p phenylphenol to be retained longer. The following equation was used by Karger et al. (35,36):
where a is the relative retention time, ICx and K O are the partition coefficientsof compound x and o (substituted phenol and phenol), tRx and tRo are the retention times of a meta or para substituted phenol and phenol, tA is the retention time of a nonretained gas such as methane, P,O and P,O are the saturation vapor pressure of phenol and substituted phenol, and yomand yxmare the activity coefficients at infinite dilution of phenol and substituted phenols, respectively. The experimentally determined N values (relative retention times) and saturation vapor pressures (37) for the same alkyl phenols studied by Karger were substituted into eq 2 (for 160, 150, and 140 "C) allowing the equation to be solved for yom/yrm, The logarithm of yom/yx"(phenoksubstitutedphenol) was then plotted as a function of the Hammett substituent, constant, CT. The correlation coefficients of the linear least-squares regression lines at 160,150, and 140 "C were 0.298,0.383, and 0.353, respectively. The data used to construct the plots are displayed in Table 111. As can be seen from the correlation coefficients of the least-squares treatment of the data, the probability of a linear correlation is small. This may be because the substituents investigated were all alkyl and therefore have very similar CT values. As a consequence, other effects, such as small differences in the activity coefficients of nonhydrogen-bonded phenols, may be reflected in the low correlation. It should be noted that Karger's investigation included the alkyl substituents studied here as well as metaand para-&, -C1, -F, and -OCH3 substituents, which have widely different r values relative to those of alkyl substituents. If the data from the investigation of Karger and co-workers (35)for only the alkyl substituents are plotted without inclusion of the other substituents such as halogen and methoxy, there is no immediately evident correlation; the correlation
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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Table 111. Hammett Sulbstituent Constants (a), Karger's Chromatographic Substituent Constants (uc), Vapor Pressures for Components, and the Calculated Activity Coefficient Ratios Used in This Investigation vapor pressure, mmc calculated yom/yx" no. substituenta Ub 1 6 0 ° C 150°C 140°C 160°C 150°C 140°C
*
3-methyl 4-methyl 3-ethyl 4-ethyl 3-isopropyl 4-isopropyl 3-tert-butyl 4-tert-butyl H a
+0.01 -0.01 -0.05 -0.09 -0.11 -0.13 -0.17 -0.14 0
-0.07 -0.17 -0.07 -0.15 -0.07 -0.15 -0.12 -0.20 0
Same alkyl substituents examined by Karger.
/
3-k -A --0
-020
-014
-008 ~
P = l I I ot 150'C P:108at160'1:
/004
-002 :
i
~
!
3/
,( ..
210 210 135 125 98 87 60 72 310
150 140 90 84 64 57 40 46 275
Taken from ref 35.
0.940 0.918 0.867 0.779 0.7 50 0.652 0.593 0.703
105 100 62 58 45 39 27 32 205
0.768 0.690 0.674 0.610 0.576 0.503 0.472 0.536
0.730 0.675 0.643 0.584 0.569 0.483 0.453 0.5 29
Taken from ref 37.
-
010 :
f
cc
1
1 6
I
5
I
4
I
3
1
2
I
I
I
0
PPM
Figure 5. Proton magnetic resonance spectrum of Superox-2OM, see Experimental Section for conditions. Note the 13C-H side band of the proton spectrum (Inset).
spective gas chromatographic retention information obtained with it can be achieved. In order to establish that Superox20M and Carbowax-20M a poly(ethy1eneglycol) are chemically identical (other than the presence or absence of traces of polymerization catalyst), the 'H NMR spectra of the phases were measured. The lH NMR spectra of these materials were identical and are illustrated in Figure 5 for Superox-2OM. The singlet at 3.597 ppm is assigned to equivalent methylene protons, while the singlet at 2.080 ppm is attributable to a trace of water. the 13C-H satellites of both materials were identical with one another and with the 13C-H satellite spectrum of poly(ethy1eneglycol) (also called polyoxiran or poly(ethy1ene oxide)) published by Connor and McLauchlan (38). According to these workers, in those monomeric units within the chain that contain a 13Cnucleus, -13CH2-CH2-0, the 13C-H coupling causes the two pairs of methylene protons to be nonequivalent, which results in a spectrum that approximates half an AA'XX' system. Thus, it appears that SuperoxSOM is a poly(ethy1ene glycol) (also called polyoxiran or poly(ethy1ene oxide)) (39).
ACKNOWLEDGMENT We acknowledge M. Verzele who provided the stationary phase to Milton Lee and Bob Wright who prepared the capillary columns used in this work. Richard Sprecher obtained the 'H NMR spectra. Helpful discussions were provided by Robert N. Hazlett, Dennis Finseth, Richard Sprecher, Hyman Shultz, Steven Cheder, Barry Batts, Milton Lee, Ed Olson, Barry Karger, Richard Laub, Pat Sandra, and M. Verzele.
LITERATURE CITED (1) Hrlvnak, J. J . Chromatogr. Sci. 1970, 8 , 802-603. (2) Hrlvnak, J.; Macak, J. Anal. Chem. 1971, 43, 1039-1042. (3) Dletz, W. A. J . Chromatogr. Sci. 1972, 10, 423-424. (4) Brady, R. F., Jr.; Pettkt, 6. C. J . Chromatogr. 1974, 93, 375-381. (5) Sassenberg, W.; Wrabetz, K. 2.Anal. Chem. 1961, 184, 423-427. (6) Kolsek, J.; Matlclc, M. J . Chromatogr. 1963, 12, 305-313. (7) Landautt, C.; Guiochon, G. Anal. Chem. 1967, 39, 713-721.
1570
Anal. Chem. 1982, 5 4 , 1570-1572
(8) Husain, S.;Kunzelmann, P.; Schildknecht, H. J . Chromafogr. 1977, 737, 53-60. (9) Schomburg, G.;Husmann, H.;Weeke, F. J. Chromafogr. 1975, 7f2, 205-217. (IO) Brooks, V. T. Chem. Ind. (London) 1959, 1317-1318. (11) Guenther, F. R.; Parris, R. M.; Chesier, S. N.; Hiipert, L. R. J . Chromafogr. 1081, 207, 256-261. (12) Grob, K., Jr.; Grob, K. J . Chromatogr. 1977, 740, 257-259. (13) Janak, J.; Komers, R. “Gas Chromatography 1958”;Desty, D. H., Ed.; Butterworths: London, 1958;Chapter 26. (14) Ono, A. Chromatographla 1080, 73, 574-575. (15) Ettre, L. S.; Obermiller, E. I n “Encyclopedia of Industrial Chemlcai Analysis”; Interscience: New York, 1973;Vol. 17. (16) Averill, W. Perkln-Elmer Qas Chromatography Applications, Application No. GC-DS-001; Perkln-Elmer Corporation: Norwaik, CT, 1963. (17) Hrivnak, J.; Beska, E. J. Chromafogr. 1074. 8 9 , 309. (18) Macak, J.; Buryan, P.; Hrivnak, J. J. Chromafogr. 1974, 89, 309-317. (19) Buryan, P.; Macak, J.; Hrivnak, J. J . Chromafogr. 1077, 737, 425-430. (20)Buryan, P.; Macak, J. J . Chromatogr. 1977, 739, 69-75. (21)Buryan, P.; Macak, J. J . Chromafogr. 1078, 750, 246-249. (22) Verzele, M.; Sandra, P. J . Chromatogr. 1078, 758, 111-119. (23) Sandra, P.; Verzele, M.; Verstappe, M.; Verzeie, J. J . High Resoluf. Chromafogr. Chromafogr. Commun. 1979, 2 , 288-292. (24) Haken, J. K. Adv. Chromatogr. 1076, 74, 367-407. (25) Karger, B. L. Anal. Chem. 1967, 39 (E),24 A-50 A.
(26) James, A. T.; Martin, A. J. P. Blochem. J. 1952, 50, 679-690. (27) Van Den Dool, H.; Kratz, Dec. P. J . Chromatogr. 1983, 7 7 , 463-471. (28) Laub. R. J. Anal. Chem. 1080, 52, 1219-1221. (29) Lee, M. L.; Vassiiaros, D. L.; White, C. M.; Novotny. M. Anal. Chem. 1979, 57, 768-774. (30) Majlat, P.; Erdos, 2.; Tackacs, J. J . Chromatogr. 1074, 97, 89. (31) Dandeneau, R. D.; Zerenner, E. H. J . High Resolut. Chromafogr. Chromatogr. Commun. 1979, 2 , 351-356. (32) Hoshika, Y. J . Chromatogr. 1977, 744, 181-189. (33) Baker, R. A.; Malo, B. A. Environ. Sci. Techno/. 1087, 7 , 997-1007. (34) Evans, M. B.; Smith, J. F. J . Chromafogr. 1988, 36, 489-503. (35) Karger, B. L.; Elmehrik, Y.; Andrade, W. J. Chromafogr. Sci. 1989, 7 , 209-217. (36) Karger, B. L.; Elmehrik, Y.; Stern, R. L. Anal. Chem. 1988, 4 0 , 1227-1232. (37) Jordan, T. E. “Vapor Pressures of Organic Compounds”; Interscience: New York, 1954. (38) Connor, T. M.; McLauchlan, K. A. J. Phys. Chem. 1085, 6 9 , 1888-1893. (39) Verzele, M.; Sandra, P.; Verzele, J. Int. Lab., submltted.
RECEIVED for review January 27,1982. Accepted April 5,1982. N.C.L. acknowledges support by Department of Energy Contract to Duquesne University, No. DE-AC-22-80PC30252.
Determination of Phenols in a Coal Liquefaction Product by Gas Chromatography and Combined Gas ChromatographyIMass Spectrometry Curt M. White*’ and Norman C. Li Department of Chemlstty, Duquesne Unlversiw, Pittsburgh, Pennsylvania
152 19
The phenolic fraction of a SRC-I1 middle distillate was Isolated and the individual phenolic constituents further separated and Identified by using gas chromatography and combined gas chromatography/mass spectrometry. This complex mixture of phenols was separated with a high-resolution fused-silica capillary column wali-coated with Superox-2OM. Identification of 29 compounds was possible. All compounds except one were Identified by using two identification parameters: (1) cochromatography with authentic standards and, (2) matching mass spectra. Ail major and most minor constituents have been identlfled.
gasification products has been performed by Karr et al. (1-3), Pichler et al. (4-6), and Buryan and Macak (7-11). Although the phenolic materials in coal carbonization products and gasification products have been extensively characterized, little detailed information is available on the exact nature of phenols in direct coal liquefaction products. Several methods have been developed for separating phenolic fractions from coal liquefaction products and subsequent analysis of the resulting fractions by bulk characterization techniques such as nuclear magnetic resonance and/or highresolution mass spectrometry (12-16). Schabron, Hurtubise, and Silver have described analyses of phenol-rich fractions from coal liquefaction products using high-performanceliquid chromatography, chemical spot tests, ultraviolet absorption spectroscopy, and fluorescence spectroscopy (17). After considerable effort, only four compounds could be positively identified, phenol and 2-, 3-, and 4-methylphenol. Guenther and co-workers have examined a solvent-refined coal (SRC) for phenolic materials using GC/MS, and were able to positively identify seven compounds-phenol, the three methylphenols, and three dimethylphenols (18). Similarly, Zingaro et al. have examined the phenolic contents of coal liquefaction products but were able to identify only phenol and the methylphenols (19). Lastly, White and Schmidt positively identified 12 phenolic compounds in the byproduct waters from two coal liquefaction processes using gas-solid chromatography and GC/MS (20). Due to the dramatic increase in the qualitative analytical chemistry of fuels, it may be desirable, for the purposes of this paper, to define the meaning of “positive identification” of compounds. For these purposes, a positively identified compound is one which has been identified by a minimum of two
The analysis of phenols in coal and coal-derived products has received considerable attention. The phenolic products from the carbonization and gasification of coal have been extensively characterized using a wide variety of analytical methods and techniques. The literature contains a plethora of publications on the characterization of phenolic materials in these products, the most reliable characterizations being performed with gas chromatography and combined gas chromatography/mass spectrometry (GC/MS). Indeed, the best gas chromatographic methods available for the analysis of complex mixtures of phenols have been developed by chromatographers interested in separating coal-derived phenolic mixtures. Some of the most extensive work on the detailed characterizations of phenols in coal carbonization and Author to whom correspondence should be addressed Analytical Chemistry Division, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, PA 15236. 0003-2700/82/0354-1570$01.25/0
0 1982 American Chemical Society
