1775
Anal. Chem. 1987, 59, 1775-1779
Quantitation of Carbon Types Using DEPT/QUAT NMR Pulse Sequences: Application to Fossil-Fuel-Derived Oils Daniel A. Netzel
Western Research Institute, P.O. Box 3395, University Station, Laramie, Wyoming 82071
A practical procedure Is descrlbed whereby the fraction of carbon types (13CH,, n = 0-3) In fossil-fueklerlved dls under Ideal and nonldeal experlmental condltlons can be quantltatlvely determlned by using the pulse sequences DEPT (dlstortionless enhancement by polarization transfer) and QUAT (quaternary-only carbon spectra). Because these two technlques differ In the mechanlsm for signal enhancement, the NMR data are not dlrectly related and, thus, a methodology was developed to relate the NMR data from the two techniques. The procedure was tested by using a pure compound, o -ethyltoluene, and a model hydrocarbon mixture Containing 10 components of known concentratlon. I t Is shown that a previously described method In the literature used to compensate for the mlssettlng or field lnhomogenelty of the 'H pulse and to provlde qualltatlvdy good edited spectra can also be used to obtain quantitative carbon-type analysis. The methodology descrlbed Is shown to be useful not only for pure compounds and simple mixtures but also for the analysis of complex mixtures such as fossll-fuel-derived products. The DEPT/QUAT technlque Is preferred over most quantitative spln-echo experlments because the time necessary for data acqulsftlon and reduction Is much less.
A number of NMR multiple pulse techniques have been developed recently whereby the carbon types with a complex molecule or mixture can be readily identified through spectral editing. These new pulse sequences can be classified into spin-echo and polarization transfer techniques. Most noteworthy of the spin-echo techniques are the attached proton test (APT) (I), the gated spin-echo (GASPE) (2, 3), the part-coupled spin-echo (PSCE) (4,5),and the quaternary-only carbon spectrum (QUAT) (6). Important polarization techniques include insensitive nucleus enhancement by polarization transfer (INEPT) (9, subspectral editing using a multiple quantum trap (SEMUT) (8),and distortionless enhancement by polarization transfer (DEPT) (9). Although the spin-echo and polarization transfer techniques give comparable information in regards to carbon-type identification, they differ not only in theory but also in complexity of operations for quantitative determination of carbon types. The theory and advantages of these and other multiple pulse techniques are discussed in the reviews by Turner (10) and by Benn and Gunther (11). Of the many multiple pulse techniques for spectral editing, only the APT, GASPE, PCSE, and DEPT methods have been used to obtain either full or partial quantitative information on the carbon types present in complex mixtures. Shoolery (12) used the APT method to obtain the ratio of the tertiary aromatic carbons to quaternary aromatic carbons for a gas-oil sample. Quantitation of all carbon types in a complex mixture using APT has not been reported. Although it has been reported that the modified APT (double spin-echo technique) (1) gives intensities comparable to a normal carbon-13 spectrum.
In a series of publications, Cookson and Smith (2,3,13-15) have demonstrated the use of the GASPE method to obtain the concentration of the carbon types in coal-derived liquids and petroleum fractions. Snape and others (16-20) used the PSCE technique to identify and quantify the carbon types in coal liquefaction products. Dereppe and Moreaux (21) compared the quantitative results obtained for the carbon types in fossil fuel materials by using the DEPT method with the results from the integration of the normal carbon-13 spectra obtained under quantitative conditions. They studied only the aliphatic region of the spectra and assumed the absence of quaternary carbons. Their results indicated a one to one correspondence between the analytical results of the two techniques. Bardet et al. (22) used the DEPT technique in the structural analysis of lignins and concluded that quantitative data might be obtained from the edited spectra. However, Barron et al. (23) in their studies on coal-derived oils, concluded that the use of signal intensities in DEPT subspectra to determine quantitatively the amounts of CH, groups present in oil mixtures suffers in accuracy due to variable polarization and relaxation rates. Doddrell and Pegg (24) performed an extensive study of the variables associated with obtaining good DEPT and quaternary-only carbon spectra. However, they did not extend their study to obtaining quantitative data for all four carbon types because of the different mechanisms affecting quantitative measurements associated with the DEPT (polarization transfer) and quaternary-only carbon spectral (spin-echo) techniques. Thus, it has been demonstrated in the literature that quantitative information on carbon types in complex mixtures can be reliably obtained by using the GASPE and PCSE techniques but that obtaining reliable quantitative data about the carbon types by using the DEPT method in conjunction with a multiple sequence for quaternary-only carbon is still subject to further studies. It is the purpose of this paper to demonstrate that the atom percent of the carbon types in complex fuel mixtures can be readily obtained quantitatively from the DEPT and quaternary-only carbon spectra.
EXPERIMENTAL SECTION Sample Preparation. A 1:l (v/v) solution of o-ethyltoluene in CDC1, was prepared by using a stock solution of CDC13 containing 0.1 M chromium(II1) acetylacetonate [Cr(AcAc),], the relaxation reagent, and about 1%tetramethylsilane (Me&), the chemical shift reference. A hydrocarbon mixture was prepared containing 10 components (toluene, 2,2,4-trimethylpentane, oethyltoluene, acenaphthene, 2,3-dimethylnaphthalene, 1methylnaphthalene, tetralin, cyclohexane, heptane, and tetradecane) each 0.1 M in CDC1,. The solution contained 0.04 M Cr(AcAc)3and about 1%Me,Si. Both samples were frozen in liquid nitrogen and sealed in 5-mm NMR tubes. The fossilfuel-derived oils were prepared in a similar manner but were not sealed. NMR Measurements. A JEOL FX-270 NMR spectrometer with a C/H dual 5-mm probe was used for all experiments. The observed frequencies for 'H and I3C were 269.65 and 67.88 MHz, respectively. Conventional 'H spectra were obtained by using
0 1987 American Chemical Society 0003-2700/87/0359-1775$01.50/0
1776
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
a pulse angle of 42", a pulse delay of 20 s, and a total of seven transients, which included two dummy pulses. Carbon-13spectra were obtained by using a pulse angle of 75O, 96 real transients, 8 dummy transients, a pulse delay of 20 s, and gated lH decoupling to ensure quantitative results. DEPT and QUAT experiments were performed on the sealed samples by using the pulse sequences and methodology described by Bendall and Pegg (24) and Bendall et al. (6), respectively. For both the DEPT and QUAT spectra, the carbon-hydrogen spin coupling constant, J, was set equal to 140 Hz.The 'H pulse width was set equal to 45', 90°,and 135' to obtain the three initial DEPT spectra. The 90' 'H pulse width of 45 k s was obtained at the point where the intensity of the CH, resonance of oethyltoluene containing Cr(AcA& was zero. In both the DEPT and QUGT experiments, the 13C90' and 180' pulse widths used were 8 and 16 gs, respectively. A long pulse delay of 20 s was used in the DEFT and QUAT experiments to reduce any errors caused by carbon types with long spin-lattice relaxation times. For the hydrocarbon mixture 96 transients and 8 dummy pulses (to achieve thermal equilibrium) were collected for the 045, OIs5, and the quaternary-only carbon spectra. Twice as many transients were taken to obtain the DEPT Ow spectrum to eliminate a factor of 2 in the addition and subtraction of the initial DEPT spectra (24). All experiments were carried out by using 16K data points over a spectral width of 15 kHz and at a temperature of 21.0 O C . The experimental conditions for the four oil samples were nearly the same as those above except that the 13C 90' and 180' pulse widths used were 6 and 12 ks, respectively (a new probe), and pulse delays of 10 s for the DEPT experiments and 20 s for the QUAT and I3C normal spectra were used. The number of scans for all oils was 700 for each spectrum except for 8% which was 1400 scans.
METHODOLOGY The carbon type subspectra were obtained from the addition and subtraction of the initial DEPT spectra according to eq 1, 2, and 3 (24), where 6, equals the edited spectrum OCH
=
O90
OCHz OCH3
- x(045 + YO135)
=
O45
- YO135
= O45 + YO135 - z090
(1) (2) (3)
obtained when the 'H pulse width was set to rotate the 'H spins 4 5 O , 90°, and 135'. The coefficients x , y, and z were determined experimentally (x = 0.20, y = 1.17, and z = 0.711), which vary only slightly from the theoretical values (x = 0.00, y = 1.00, and z = 0.707). Because of missetting or field inhomogeneity of the 'H pulse, it was necessary to correct the initial 90° DEPT spectrum of the hydrocarbon mixture for residual methylene carbons by subtracting approximately 9% of the methylene subspectrum (OCHJ from Ow before calculating the subspectra of the methine and methyl carbons. The corrected linear combinations of subspectra for the methine and methyl carbons are given in eq 4 and 5 , reO'CH
=
(090
o'CHs
=
(045
- UOcH,) -
+ y8135)
x'(o45
+ yo135)
- z'(890 - a o C H , )
(4) (5)
spectively. It was also found necessary to correct the quaternary-only carbon spectrum for about 20% residual methylene carbons (24) by subtracting the DEPT OCH2 spectrum from the quaternary-only carbon spectrum, 8 4 " (eq ~ ~ 6). For O'QUAT = OQUAT
- ~OCH,
(6)
the model hydrocarbon mixture, the coefficients x'and z'were found not to differ significantly from the values initially determined for x and z , which were obtained without the methylene carbon correction. However, the coefficients a, b, y, x', and z'for a large number of fossil-fuel-derived oils were found to be 0.11 f 0.05, 0.03 f 0.01, 1.47 f 0.08,0.24 i 0.02, and 0.85 f 0.05, respectively. Although the DEPT method provides quantitative data for methyl, methylene, and methine carbons, it does not provide
ll
w 900
180
IMP
1 ' 1 l.0
Figure 1. Normal and
,
ilP
~
'
IW
~
DEPT I3C NMR
'
,
IO
'
,
60
'
"
,
40
~
,, I
20
~'
,
8
,
~
0
pp-
spectra of o-ethyltoluene.
any quantitative data on the quaternary carbons. The integrated edited DEPT spectra for the three carbon types cannot be compared to the integrated quaternary-only carbon spectrum because the methods to obtain the spectra differ in signal enhancement efficiencies. I t is possible, however, to relate quantitatively through eq 7 the integrated spectra obtained
by the two techniques to obtain a complete quantitative carbon-type analysis, where CIOcH, is the sum of the integrated values of the carbon types obtained from the DEPT ~ Tthe integrated value from the subspectra and 8~ - ~ Q T J is spectra obtained from the computer subtraction of the quaternary-only carbon spectra from the I3Cnormal spectrum. All integration values were referenced to the integration value of the normal I3C spectrum taking into account the differences in gain settings between spectra. Equation 7 is solved for the proportionality constant k. Once h has been determined, the fraction of carbon types, f,", in a sample can be determined by using eq 8-10 where O:n is
the integrated value from the specific carbon-type spectrum.
RESULTS AND DISCUSSION Figure 1shows the normal 13CNMR spectrum (ON) and the three initial DEPT spectra (O&, Ow, and 6135)of o-ethyltoluene. The initial DEPT spectra were combined linearly to give subspectra containing only methine, methylene, and methyl carbon types. These edited spectra as well as the quaternary-only carbon spectrum are shown in Figure 2. The signal enhancement effect of the DEPT method relative to the normal 13C and the quaternary-only carbon methods can clearly be seen in Figures 1 and 2, respectively. It should also be noted that the solvent (CDCl,) 13Cresonances are absent in the DEPT spectra because polarization transfer does not exist for the C-D bond under the conditions used in these experiments. The normal 13C spectrum and the three initial DEPT spectra for the hydrocarbon mixture are shown in Figure 3. Figure 4 shows the corrected quaternary-only carbon spectrum and the corrected DEPT subspectra for the hydrocarbon mixture. The corrected quaternary and methine carbon subspectra still show some evidence of residual aliphatic resonances but these are out-of-phase with the single quaternary and methine carbons a t 30.1 and 24.7 ppm, respectively, for 2,2,4-trimethylpentane. In this simple mixture, the
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
ii
eouAr
Table I. Fraction of Carbon Types of o-Ethyltoluene and a Hydrocarbon Mixture from ' C DEPT Spectral Data Only
Y
Ill
8CH
1777
hydrocarbon mixture exptl theor
o-ethyltoluene exptl theor
carbon type CH
aromatic aliphatic CH2 CHn
0.565
0.571
0.143 0.291
0.143 0.286
0.410 0.012 0.396 0.182
0.405 0.013 0.392 0.190
OCH, ~~
'
-,Io
#do
140
'
>do
' A '
'
d-20
b
. . I
~
Table 11. Fraction of Carbon Types from DEPTand QUAT Spectral Data for o-Ethyltoluene and a Hydrocarbon Mixture
Flgure 2. Quaternary-only and edited DEPT 13C NMR spectra of o-
ethyltoluene. carbon type
o-ethyltoluene exptl theor
hydrocarbon mixture exptl theor
0.219
0.161
C
aromatic aliphatic
0.222
0.008
0.167 0.010
0.341 0.010 0.329 0.151
0.333 0.010 0.322 0.156
CH
%I 1
I
aromatic aliphatic
I
CH2 CH,
0.442
0.444
0.112 0.228
0.111 0.222
Table 111. Hydrogen and Carbon Aromaticities for o-Ethyltoluene and a Hydrocarbon Mixture
o-ethylT
I 200
,
,
I
,
,
110
, , ,
160
,
p
" 1.0
110
100
(0
F 60
20
I
20
I
b *~1
Figwe 3. Normal and DEPT 13C NM spectra of a hydrocarbon mlxture.
9'C" I
A 7 ~~" Flgure 4. Quaternary-only and edited DEPT I3C NMR spectra of a hydrocarbon mixture. 100
110
140
140
\,e
IO0
,o
20
1.
residual resonances presented no problems with the identification and integration of the quaternary and methine carbons. The atom fractions of the carbon types calculated from the integration of the edited DEPT spectra for o-ethyltoluene and the hydrocarbon mixture are given in Table I. It appears from the data given in Table I that the DEPT method can provide accurate relative-carbon-type analysis for methyl, methylene, and methine carbons in a mixture. Barron e t al. (23) also found that the DEFT sequence is able to accurately generate CH, subspectra of complex oil mixtures. The fractions of carbon types obtained from the combined DEPT and QUAT spectra data for o-ethyltoluene and the hydrocarbon mixture are given in Table 11. The agreement
hydrogen aromaticity NMR ('H direct) carbon aromaticity NMR (13C direct) NMR (13C DEPT) carbon aromaticity NMR (I3C direct - QUATlb NMR (13C DEPT)
toluene exptl theor
hydrocarbon mixture exptl theor
0.333
0.333
0.231
0.229
0.656 0.661
0.667
0.501 0.502
0.500
0.569 0.565
0.571
0.386 0.410
0.405
Carbon aromaticity without quaternary carbons. Normal spectrum minus the 13C quaternary-only carbon spectrum.
13C
between the experimental and theoretical values is good for the two samples and substantiates the fact that residual methylene carbons resulting from missetting or field inhomogeneity of the 'H pulse can be adequately compensated for to give quantitative data. Table 111 gives the hydrogen and carbon aromaticities (fraction of aromatic hydrogen or carbon) calculated from the integration of the 'H and 13C normal spectra for o-ethyltoluene and the hydrocarbon mixture. Also listed in Table I11 are the carbon aromaticity values calculated from the combined DEPT and QUAT spectral data. The agreement between the experimental data and the corresponding theoretical values is excellent in most cases and shows that using an average J value for the aromatic and aliphatic C-H bond can give quantitative data for aromatic carbons. The DEPT/QUAT NMR data for determining the fraction of carbon types can be validated by calculating the fraction relative to the fraction of the tertiary aromatic carbons of the total aromatic carbons in the samples (see eq 11). This
utc)
',f
= ~cH'/(~cH' + fc")
(11)
value is compared with the value for the fraction of the tertiary aromatic carbons calculated from the weight percents of hydrogen and carbon and the hydrogen and carbon aromaticities obtained from the normal 'H and I3C NMR spectra (25). In
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
Table IV. Fraction of Tertiary Aromatic Carbons and Weight Percent Hydrogen from DEPT and QUAT Spectral Data for o-Ethyltoluene and a Hydrocarbon Mixture o-ethyltoluene exptl theor tertiary aromatic carbons hydrogen ( % )
0.653 (0.672)" 10.2
0.667 10.1
hydrocarbon mixture exptl theor 0.679 (0.672)" 10.9
tween the data from the two methods and between the experimental data and theoretical values. Table IV also lists the weight percent hydrogen calculated from the carbon type distribution for o-ethyltoluene and the hydrocarbon mixture. The weight percent hydrogen was calculated according to eq 12, where CT = 1and HT is the sum
0.667 10.9
Determined from calculated weight percents of hydrogen and carbon and the hydrogen and carbon aromaticities from 'H and 13C suectra (ref 25).
eq 11f C H a and fca are the fractions of the carbon type obtained from the DEPT/QUAT method. Table IV lists the values for the fraction of tertiary aromatic carbons calculated from the DEPT and QUAT spectra and from elemental and aromaticity data for o-ethyltoluene and the hydrocarbon mixture. Good agreement is observed be-
of the hydrogen-type distribution calculated from the carbon-type distribution. Equation 12 is valid only for hydrocarbons. Calculating the weight percent of hydrogen from the atom fraction of the carbon types by the DEPT/QUAT method and comparing this value with the weight percent hydrogen from a microcombustion technique also provide a check on the validity of the fraction of carbon types by NMR (15) for hydrocarbons systems only. The excellent results given in Tables I-IV are somewhat surprising in view of the many discussions in the literature about the factors affecting the quantitation of the normal,
Table V. Carbon- and Hydrogen-Type Distributions for Oils from Tar Sands and Oil Shales
carbon type
tar sand" tar sandb distillate pyrolytic oil western shale oil' eastern shale oild (ambient-316 "C) (400 "C, 120 min) (MBFA) (tube reactor) C (atom % ) H (atom % ) C (atom 70) H (atom %) C (atom %) H (atom % ) C (atom %) H (atom %)
C aromatic olefinic aliphatic CH aromatic olefinic aliphatic CH2 olefinic aliphatic CH, aromatic and aliphatic
8.2 0.0 1.9
10.5 0.3 1.1
14.8 0.0 0.3
22.7 0.0 0.0
8.6 0.0 17.9
4.8 0.0 10.0
8.0 0.5 15.0
4.6 0.3 8.7
7.6 1.0 2.5
4.4 0.6 1.4
18.0 0.2 8.1
12.4 0.1 5.6
0.0 38.5
0.0 43.2
0.9 43.7
1.0 50.5
1.6 57.9
1.9 66.7
0.4 34.0
0.5 46.9
25.0
42.0
20.1
34.9
14.5
25.1
16.6
34.4
" From the distillation of Northwest Asphalt Ridge steamflood oil. From the isothermal pyrolysis of Asphalt Ridge tar sand. From the material balance Fischer assay (MBFA) of Green River oil shale. From the tube reactor pyrolysis of New Albany oil shale. Table VI. Average Structural Parameters for Oils from Tar Sands and Oil Shales
structural parameter atomic H/C combustion NMR (13C DEPT) hydrogen aromatic, % NMR ('H direct) NMR (13C DEPT) hydrogen olefinic, % NMR ('H direct) NMR (13C DEPT) hydrogen aliphatic, % NMR ('H direct) NMR (13C DEPT) carbon aromatic, %' NMR (13C direct) NMR (13C DEPT) carbon olefinic, %f NMR (13C direct) NMR (I3C DEPT) carbon aliphatic, % NMR (I3C direct) NMR (13C DEPT)
tar sand" distillate (ambient-316 "C)
tar sandb pyrolytic oil (400 "C, 120 min)
1.75 1.78
1.80 1.73
1.76 1.73
4.5 4.8
4.4 4.6
4.3 4.4
12.2 12.4
0.0 0.0
2.0 1.3
2.9 2.4
0.9 0.7
95.5 95.2
93.6 94.1
92.8 93.2
86.9 87.0
17.6 16.8
20.5 20.2
23.6 24.9
43.7 41.2
0.0 0.0
1.7
2.6 2.6
0.6
82.4 83.3
79.5 79.9
76.4 75.2
56.3 58.7
a
western shale oil' (MBFA)
eastern shale oild (tube reactor) 1.54 1.45
" From the distillation of Northwest Asphalt Ridge steamflood oil. *From the isothermal pyrolysis of Asphalt Ridge tar sand. 'From the material balance Fischer assay (MBFA) of Green River oil shale. dFrom the tube reactor pyrolysis of New Albany oil shale. eIncludes olefinic carbons. fSum of CH and CH, olefinic carbons from 1-alkenes.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
Ill
1779
the various sources of errors possible and judgment calls in the manipulation of the 13CDEPT and QUAT data to obtain the carbon-type distribution. Also in support of the quantitation of the 13C DEPT and QUAT technique for the determination of the carbon-type distribution in fossil-fuel-derived products is the close agreement of the atomic H/C ratios for the oils calculated from elemental analyses by a microcombustion technique and by NMR carbon-type distribution.
I
CONCLUSIONS It was found that a relatively simple procedure for relating
/I
DEPT and QUAT spectra can provide quantitative carbontype analysis. The use of Cr(AcAc), for reducing the spinlattice relaxation time for the quantitation of normal carbon-13 spectrum, a single value for the C-H scalar coupling constant (140 Hz), and the missetting or field inhomogeneity of the 'H pulse did not adversely affect the quantitation of carbon types in a single compound or for complex mixtures of aromatic and aliphatic hydrocarbons such as found in fossil-fuel-derivedoih. Registry No. C, 7440-44-0; o-ethyltoluene,611-14-3.
LITERATURE CITED
* ppn
Figure 5. Normal and edited DEPT I3C NMR spectra of a pyrolytic oil from Asphalt Ridge tar sand (400 O C , 120 mln).
spin-echo, and polarization transfer carbon-13 NMR spectra. The three major factors contributing to the quantitation of the various carbon-13 spectra are (1) the spin-lattice and spin-spin relaxation times, (2) the dipolar scalar coupling for the aliphatic and aromatic carbon-hydrogen bond, and (3) the missetting or field inhomogeneity of the lH pulse. A complete discussion of the errors associated with each factor is given in the article by Bendall and Pegg (24). Figure 5 shows the normal 13C NMR spectrum and the corrected edited spectra for the various carbon types for an oil obtained from the isothermal pyrolysis of a tar sand. These spectra are typical of a large number of fossil-fuel-derived oils. Table V gives the carbon- and hydrogen-type distributions calculated from the 13C DEPT and QUAT NMR spectra for a tar sand distillate, a tar sand pyrolytic oil, a western shale oil, and an eastern shale oil. The hydrogen-type distribution was calculated from the carbon-type distribution and normalized to 100%. Table VI lists several average structural parameters and the atomic H/C ratios for the four oils. The structural parameters listed in Table VI were calculated from the I3C DEPT and QUAT NMR data and from the spectral data obtained directly from conventional 'H and 13C NMR spectra. The agreement of the data obtained from two methods of spectral analyses is extremely good considering
Patt, S. L.; Shooiery, J. N. J. Magn. Reson. 1082, 46, 535. Cookson, D. J.; Smith, B. E. Fuel 1083, 6 2 , 34. Cookson, D. J.; Smith, B. E. Fuel 1083, 6.2, 987. Le Cocq, C.; Lallemand, J-Y. J. Chem. Sac., Chem. Commun. 1081, 150. Brown, D. W.; Nakashima, T. T.; Rabenstein, D. L. J. MaQn. Reson. 1081, 45, 302. Bendall, M. R.; Pegg, D. T.; Doddrell, D. M.; Johns, S. R.; Wllling, R. 1. J. Chem. SOC.,Chem. Commun. 1082. 1138. Doddrell, D. M.; Pegg, D. T. J. Am. Chem. SOC. 1080, 102, 6388. Bildsoe, H.; Donstrup, S.; Jakobsen, H. J.; Sorensen, 0. W. J. Magn. Reson. 1083, 53, 154. Doddrell, D. M.; Pegg, D. T.; Bendall, M. R. J. Magn. Reson. 1082, 48, 323. Turner, C. J. Prog. Nucl. Magn. Reson. Spectrosc. 1084, 16, 311. Benn. R.; Gunther, H. Agnew. Chem., Int. Ed. Engl. 1083, 2 2 , 350. Shoolery, J. N. J. Natl. Prod. 1084, 4 7 , 226. Cookson, D. J.; Smith, B. E. Org. Magn. Reson. 1081, 16, 111. Cookson, D. J.; Smith, B. E. Fuel 1082, 6 1 , 1007. Cookson, D. J.; Smith, B. E. Anal. Chem. 1085, 5 7 , 864. Snape, C. E. Fuel 1082, 6 1 , 775. Snape, C. E. Fuel 1082, 6 1 , 1165. Snape, C. E. Fuel 1083, 6 2 , 621. Snape, C. E. Fuel 1083, 6 2 , 989. Snape, C. E.; Marsh, M. K. Prepr. Pap .-Am. Chem. SOC.,Div. Pet. Chem. 1085, 30, 247. Dereppe, J. M.; Moreaux, C. Fuel 1085, 64, 1174. Bardet, M.; Foray, M. F.; Robert, D. Makromol. Chem. 1085, 186, 1495. Barron. P. F.; Bendall, M. R.; Armstrong, L. G.; Atkins, A. R. Fuel 1084, 6 3 , 1276. Bendall, M. R.; Pegg, D. T. J. Magn. Reson. 1083, 5 3 , 272. Netzel, D. A.; Thompson, L. Fuel 1088, 65, 597.
RECEIVED for review November 4,1986. Resubmitted March 13, 1987. Accepted March 31, 1987. The author expresses his thanks and appreciation to the United States Department of Energy for funding of this work under Cooperative Agreement Number DE-FC21-83FE60177.
