1794
Anal. Chem. 1980, 52, 1794-1797
Analysis of Coal Asphaltenes by Carbon- 13 Fourier Transform Nuclear Magnetic Resonance Spectrometry Vladimh Sklend'i,
Milan HGjek,' Gustav Sebor,'
Ivo Lang,2 Miloslav S ~ c h s n e k and , ~ Zenon StarEuk
Institute of Scientific Instruments, Czechoslovak Academy of Sciences, 6 72 64 Brno, Czechoslovakia
enhancements (NOE) and spin-lattice relaxation rates of carbon-13 nuclei. Their unfavorable influences upon the results of the quantitative analysis by carbon-13 NMR may be eliminated by gated decoupling (19)or by use of relaxation reagents (20). The purpose of the present work was to investigate the influence of three different methods on the accuracy of aromaticity determination of asphaltenes from coal extracts. The results are compared on the basis of statistical evaluation by means of the variance analysis method. The influences of nuclear Overhauser enhancement (NOE) and spin-lattice relaxation time (TI)upon the results of the analysis and their values for measured samples are discussed. Some conclusions concerning the size of the molecules are presented.
The influences of three different methods on the accuracy of aromaticity determination of coal asphaitenes by carbon-13 FT NMR were investigated. The results are compared on the basis of statlstical evaluation by means of a variance analysis method. The influences of nuclear Overhauser enhancements and spln-lattice relaxatlon times upon the results of analysis and their values for measured samples are dlscussed. Some conclusions concerning the size of the particles in solution are presented.
Carbon-13 Fourier Transform Nuclear Magnetic Resonance (NMR) spectrometry is widely used for qualitative and quantitative analysis of coal and its products. The qualitative features and structural composition are deduced mainly on the basis of chemical shift data for selected model compounds ( 1 , 2 ) . From the quantitative point of view, carbon-13 NMR offers a direct means for obtaining values of a very important parameter, the carbon aromaticity fa (fa = aromatic carbons/total carbons). NMR measurement may be performed in both liquid and solid state. New methods of high resolution carbon-13 NMR in solids seem to be very promising for routine analysis of coal for the future ( 3 7 9 , but for experimental reasons the main attention is still focused on NMR of liquids, which is much easier to perform. Different types of coal extracts and coal liquefaction products were studied ( 2 , 6-11). The main problem associated with these measurements is the fact that work on a soluble material utilizes but a small part of the coal and application of these data for the whole structure analysis is therefore restricted. But regardless of this drawback, carbon-13 NMR measurements can give very useful information, especially about the chemical structure of the products of coal separation. As in the case of other fossil fuels, the determination of aliphatic and aromatic carbon contents in coal asphaltenes is one of the main problems in their analysis. Carbon-13 Fourier transform NMR spectrometry offers direct determination of these contents, utilizing integrated intensities of the measured signals. Apparent simplicity of these measurements is, however, complicated by a number of factors, which usually alter the simple relationship between integrated intensities of lines and numbers of nuclei a t resonance. Some of them have been discussed by others (9,12-18). If one does not take into account uncommon instrumental imperfections such as the nonlinear frequency response of the receiver, the effect of limited pulse strength, nonideal response of filters, and inadequate digitization of the frequency domain, there are two factors with dominant effects: the nuclear Overhauser
EXPERIMENTAL Five coal asphaltenes were used for the measurement. Dried coal samples were hydrogenated (2' = 673 K, P H p= 20 MPa, t = 3 h) and extracted by benzene. Residues after pentane extraction of benzene solution of hydrogenated coal from the Koh-i-noor mine, sample C 1 (North Bohemian region); Silvestr mine, C 4 (Northwest Bohemian region); Paskov mine, C 2; Hlubina mine, C 3; and CSA mine, C 5 (North Moravian region) were studied. All data were obtained by using a Bruker WP 80 DS spectrometer working in the Fourier transform mode with deuterium lock system and heteronuclear proton decoupler. The spectra were acquired with the following parameters: 8K data points; spectral width, 7500 Hz; measured offset, 220 ppm (from tetramethylsilane); decoupled offset, 5 ppm (from tetramethylsilane); pulse width 3.0 ws (35' flip angle); acquisition time, 0.546 s; number of scans, 3000; and temperature, 35 " C . Samples were measured in 10-mm Wilmad tubes. All solutions of asphaltenes were 10% (wt) in CDC13 (Merck) and tetramethylsilane was used as the internal standard. The Fe(acac)a(K + K) relaxation reagent was put directly into the sample tubes (C = 0.4 M). The solutions were prepared 24 h before measurement. The stability of samples was checked by IH and 13C NMR spectrometry. The intensities, chemical shifts ranges, and integral intensities were constant during a long time period (14 days). The carbon-13 spin-lattice relaxation times were measured by the fast inversion recovery method (21). The values of relaxation times were calculated by using the three-parameter nonlinear regression analysis. The content of metals was determined by the flame atomic absorption spectrometry method after the previous dry mineralization of samples with p-xylenesulfonic acid. The content of organic free radicals was measured by Electron Spin Resonance (ESR) spectrometry. Diphenylpicrylhydrazil (DPPH) radical in sealed capillarys was used as the standard for the quantitative determination of free radicals. The ESR spectra were measured on an ER-9 spectrometer (Zeiss-Jena). RESULTS AND DISCUSSION A typical spectrum of measured asphaltenes is shown in Figure 1. As can be seen, the spectrum has two well separated parts. The aliphatic region covers the range of chemical shifts from 10 to 55 ppm, where methylene (21-40 ppm) and methyl (14-21 ppm) carbons may be resolved. In the aromatic part ring-junction carbons, substituted ring carbons, and half of
Laboratory of Synthetic Fuels and Department of Petroleum Technology and Petrochemistry, Institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia. 2Mining Institute of Czechoslovak Academy of Sciences, 180 42 Prague 8, Czechoslovakia. Department of Analytical Chemistry, Institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia. 0003-2700/80/0352-1794$01 .OO/O
C
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER
I
I40
I20
100
1980
1795
I
_L
80
60
20
40
O
F
Figure 1. Carbon-13 FT NMR spectrum of sample C 2
the unsubstituted ring carbons give signals in the range from 110 to 132 ppm and resonances of the other half of the unsubstituted ring carbons range from 132 to 150 ppm (22). A marked difference between these "wide-line'' spectra and spectra of individual chemical compounds, with sharp, usually well-defined, lines follows as a consequence of a large number of different types of polycondensate molecules (23),the signals of which interfere and give complex spectra with very broad lines. T h e quantitative measurements in carbon-13 F T NMR spectrometry are complicated by the fact that two techniques which must be used to increase the sensitivity and resolution, rapid pulse excitation and broadband proton decoupling, strongly perturb the equilibrium populations of carbon-13 nuclei and, consequently, the results of the quantitative analysis. This is the reason for knowing at least approximately the values of spin-lattice relaxation times and nuclear Overhauser enhancement factors, when the quantitative carbon-13 measurements are to be made without a waste of time. If the NOE factors are considered to be equal for all carbons in the sample, the relation for the relative error of the quantitative measurement as a function of T 1and measuring parameters may be evaluated. Starting from the phenomenological Bloch equations, it is possible to compute for a steady-state condition dependence of a signal intensity ( S ) of a single line upon the total delay time (TD) to spin-lattice relaxation time ( T I )ratio: k.Mo (1 - E J 4 n a S = (1) 1 - El cos a where k is the instrumental constant, M o the magnetization in equilibrium, cy the flip angle, and E , = exp(-TD/T,), assuming that effective spin-spin relaxation time satisfies the condition: > T 1 (usually TD 1 5T,) Equation 1 is simplified and determines the signal amplitude without distortion by rapid pulsed excitation S,,,:
S,,,
= k.Mo.sin
CY
(2)
Using Equations 1 and 2, the systematic relative error ASre, of a quantitative analysis due to the shortening of TDis given as:
Dependence of this relative error on TD/T, ratio and flip angle
?
-
10'
2 - 20' 3 - 30' 4 - 10'
5 6
- 50' - 60'
7
- 70'
a-
80' 9 - 90'
" 0
1
2
3
DELAY TIME / SPIN - LAT-ICE
5
i
RELAXATION
TIME
Figure 2. Relative error of quantitative analysis as a function of delay time to spin-lattice relaxation time ratio and flip angle (10'-90')
is graphically presented in Figure 2. From the practical point of view for many-line spectra with different T 1values, the condition TD2 5T1must be satisfied for all carbons in order to get accurate results of the quantitative determination. In all samples, the spin-lattice relaxation times were measured by using the fast inversion recovery method. The character of the signals in the spectrum enables only determination of TI in the aromatic region amounting to 0.3-0.5 s in the range from 110 to 132 ppm and 0.74.9 s in the range from 132 to 150 ppm. These values are relatively short compared with individual chemical compounds and in the order of values obtained for heavy crude oil residues (12). Two effects can be responsible for this fast spin-lattice relaxation: (a) paramagnetic metal ions and free organic radicals presented in samples and (b) strong dipole-dipole interaction in the large size molecules with rather restricted molecular motion. The contents of paramagnetic meral ions were determined by AAS measurements and concentrations were by about one order of magnitude lower than the values in heavy crude oil residues (12). The contents are summarized in Table I to-
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
Table I. Molecular Weights of Measured Asphaltenes Determined by the Vapor Pressure Osmometric Method and the Contents of Iron (ppm) Obtained by AAS and Concentrations of Organic Free Radicals (OFR) Obtained by ESR Spectrometry mol. wt.b
sample=
c1 c 2 c3
780 600 600 510
c 4 c 5
OFR
Fed
1.68 x 1 0 ' 9 7.4 x 10l8 6.5 X 10l8 6.4 X
307 46 91 122 44
330
a For details, see Experimental section. Vapor pressure osmometer Knauer, benzene solution, temperature 60 "C. Spins per gram of samp1e.g-factor = 2.0029 for Contents of other metal ions were about all samples. one order of magnitude below the value of iron.
gether with contents of organic free radicals (OFR), which were measured by ESR spectrometry. The contribution of electron-nuclear dipole-dipole relaxation from paramagnetic particles was studied on acenaphtene as a model compound. Addition of D P P H radicals in concentrations of about 1018spin/g doesn't change the spin-lattice relaxation times of protonated carbons in acenaphthene = 5.9 s, = 2.9 s for 2.3 M solution of acenaphthene in CDCl,, undegassed sample). Also addition of Fe(acac), in a concentration corresponding to the contents of Fe in coal asphaltenes doesn't influence T1of protonated carbons. These results indicate that the most probable relaxation mechanism for protonated carbons in measured asphaltenes is a dipoledipole I3C-IH interaction. Assuming pure dipole-dipole proton-carbon relaxation mechanism, isotropic molecular motion and fulfilled extreme narrowing limit ( 2 4 ) , the correlation time for the protonated aromatic carbons with one attached proton can be calculated:
(c"
cHZ
Provided, that the molecular motion is isotropic, the diameter of molecules can be evaluated by using Einstein-Debyestokes equation:
11: F = 3.55,
T, = 1 - 21 s (critical F ( I , 24) = 4.26)
111: F = 0.61, T D = 1 - 10 s (critical F (1, 9) = 5.12) Standard deviations for all methods are of the order of lo-'. Similar results were obtained for all samples. Results for each delay time were compared by using a T test. It is not possible to say for which delay times values of aromaticities are comparable, but the difference between the smallest and the greatest value of aromaticity (from all measured integral intensities) for all delay times in each method is about 6% relative, Mean values of aromaticities obtained by using three independent methods (1-111) are given in Table 111. Calculated maximal differences in fa between methods I, 11, and 111, supposing all delay times for each method and all integral
(5) As the viscosity of the solution was measured (0.011 P, 303 K), the diameter was found to lie in the range of about 0.8-1.2 nm. This value is in a good accord with diameters determined by Yen (23). The magnitude of the NOE factor depends on the dipoledipole contribution to relaxation:
NOE = 1.988 * T1/TFD
Theoretically in the case of pure dipole-dipole proton-carbon interaction, NOE is maximum, independent of C-H distance and equal for all carbons in the molecule, including quaternary carbons ( 2 4 ) . In practice complications might arise, if the effective correlation times differ within the molecule and if there are other mechanisms contributing to relaxation. Three methods for the determination of the aromaticity by the carbon-13 F T NMR method were studied: broadband decoupling (I), gated decoupling (II),decoupler on only during acquisition time (0.546 s), and gated decoupling with relaxation reagent (111). Delay times between pulses were set to 1.1, 4.1, 7.1, 10.1, 13.1, 16.1, and 21.1 s. For each measurement, six independent determinations of integral intensities were carried out. Our previous results (12) indicate that methods of measurements (I-111), delay times, and integration affect the precision of the measurements of the aromaticity. In order to know which of these ones is more important, the analysis of variance (25,26)was performed for all measured samples C 1-C 5. For the comparable conditions of measurement, three contributions to the precision of the determination of the aromaticity were taken into a c c o u n t s e e Table I1 -which follow from method of measurement (M), delay time differences ( T ) and integration (I). Results of the analysis of variance are presented in Table I1 and these data give the ability to calculate the values of the F criterion. T h e F test indicates that the contribution of the uncertainty of integral determination to the scatter of the data is not significant, but changes of delay times and methods of measurements affect the precision significantly and this effect cannot be explained by random errors. The aromaticities obtained for each method were compared by using the two-parameter variance analysis with one replicate measurement-one parameter was the delay time, the other one the integration technique. These tests show, that the precision of integration as a function of delay times lies in the range of random errors (the F criterion for interaction TI is smaller than the critical value of F (27)). This fact may be demonstrated in the case of sample C 2. The calculated interaction F tests for three methods of measurement are: I: F = 0.43, T D = 1 - 21 s (critical F ( I , 29) = 4.18)
(6)
Table 11. Results of the Analysis of Variance'
c1
sample
effects M T I MT TI MI
y
2 2
5 4
c 2 a
2.06 X 4.78 x 1.82 X 3.49 x 8.35 x 2.79 x 1.67 X
lo-* 10-3
Y 2 1 5
c 3 a
6.50 X 8.60 x 10-3 5.60 X 1.17 x 10-3 5.18 x 1.11x 10-3 8.20 x
Y
2 2 5
2.75 4.34 2.86 3.62 1.56
c5
c4 a
x 10-3 x 10-3 x 10-4 x 10-3
a
Y
Y
2
1
8.12 x 10-3 4.00 x
5
2.40
5
2
X
2
a
2.20 x 6.80 x 10-3 1.70 x 6.90 X
2 1.10 x 4 x 5 6.60 x 10-5 io 1.80 x 1 0 . ~ io 5.50 x 1 0 - 4 i o 3.70 x 10-4 i o 1.10x 1 0 - ~ residual 20 10 20 1.16 x 10 3.80 X 20 1.10 X a y = degree of freedom, a = mean square, M = factor describing methods of measurements 1-111, T = factor describing delay time, I = factor describing integration technique.
io io
10-3 10-5 10-4
2 5
s
4
io io
Anal. Chem. 1980, 52, 1797-1803
Table 111. Mean Values of Aromaticity Calculated for Each Method From Six Independent Integral Intensity Measurements for All Delay Times Used sample I I1
I11 Ma
C1
c 2
c 3
c 4
c 5
0.706 0.760 0.769 0.745
0.733 0.755 0.769
0.754 0.763 0.771 0.763
0.699 0.733 0.755 0.729
0.727 0.746 0.791 0.755
0.752
a M = mean value of aromaticity obtained from methods 1-111.
intensities measurements, are below 10% relative. T h e relative error of measurement according to Equation 3 for measured relaxation times and the shortest delay time (1.1s) is maximally 10% and consequently its value has to fall with increasing TD. By using gated decoupling and relaxation reagent, the influence of NOE is eliminated for all carbons and thus the results of aromaticity determination are changed. Which types of carbons are influenced mostly in this manner, is very difficult to say, because of the complex character of the samples. The NOE is considered to be completely suppressed using delay times T DL 9 T,(28) in method 11;thus the mean aromaticity is still influenced with the conditions of measurements used. From the practical point of view, it is possible to accept the correct values of aromaticity in the range *6% relative from the mean value, obtained by each independent method (1-111) using delay times 1-21 s for this type of sample.
ACKNOWLEDGMENT We wish to express our sincere thanks to Dr. P. KubSEek, Department of Physical Chemistry, UJEP Brno, for the
1797
measurement of ESR spectra.
LITERATURE CITED Fischer, P.; Stadelhofer, J. W.; Zander, M. fuel 1978, 57,345. Bartle, K . D.; Martin, T. G.; Williams, D. F. fuel 1975, 54,226. VanderHart, D. L.; Retcofsky, H. L. f u e l 1976, 55, 202. Bartuska. V. J.; Maciel. G. E.; Schaefer, J.; Stejskai, E. 0.Fuel 1977, 56,354. (5) Retcofsky, H. L.; VanderHart, D. L. fuel1978, 57,421. (6) Schweighardt, F. K.; Friedei, R. A . ; Retcofsky, H. L. Appl. Spectrosc. 1976, 30, 291. (7) Schweighardt, F. K.; Retcofsky, H. L.; Friedel, R. A. fuel 1976, 55,313. (8) Cantor, D. M. Anal. Chem. 1978, 50, 1185. (9) Yokono, T.; Miyazawa, K.; Sanada, Y. Fuel 1978, 57, 555. (10) Gavalas, G. P.; Oka, M. Fuel1978, 57, 285. (11) DoGru, R.; Erbatur, G.; Gaines, A. F.: Yiirum, Y.: Icli, S.: Wirthlin, T. fuel 1978, 57, 399. (12) HBjek, M.: Sklenii, V ; Sebor, G.; Lang, I.;Weisser, 0.Anal. Chem. 1978, 50, 773. (13) Thiauit, L.; Mersseman, A. Org. Magn. Reson. 1976, 8, 28. (14) Schoolery, J. N. Progr. Nucl. Magn. Reson. 1977, 77, 79. (15) Dorn, H. C.; Wooton, D. L. Anal. Chem. 1976, 48, 2146. (16) Seshadri, K. S.; Rubedo, R. F.; Jewell, D. M.; Malone. H. P. Fuei1978, 57, 549. (17) Dereppe, J. M.; Moreaux, C.; Castex, H. Fuel 1978, 57, 435. (18) Rectcofsky, H. L.; Friedel, R. A. Fuel, 1976, 55, 363. (19) Freeman, R.; Hill, H. D. W.; Kaptein, R. J . Magn. Reson. 1972, 7,327. (20) Horrocks, W. De W. "NMR of Paramagnetic Molecules", LaMar, G. N., Ed.; Academic Press: New York, 1973; p 429. (21) Sass, M.; Ziessow, D. J . Magn. Res. 1977, 25, 263-76. (22) Knight, S. A. Chemy Ind. 1967, 1920. (23) Yen, T. F. "Chemistry of Asphaltenes in Coal Liquids". Preprint of the University of Southern California, 1977. (24) Leyrla, J. R.; Levy. G. C. "Topics in Carbon-13 NMR Spectroscopy", Levy, G. C., Ed.; Wiley-Interscience: New York, 1974; Vol. I. p 90. (25) Likes, J. "Projecting of Industrial Experiments"; SNTL: Prague, 1968; p 166. (26) System IBM 360, Scientific Subroutine Package, version 111, Russ. transi., Statistika, Moscow, 1974. (27) Tukey, J. W. Biometrics 1949, 5, 232. (28) Harris, R. K.; Newman. R. H. J . Magn. Reson. 1976, 24, 449. (1) (2) (3) (4)
RECEIVED for review February 27, 1.979. Resubmitted May 3, 1980. Accepted May 12, 1980.
Ion Mobilities and Residence Times under Chemical Ionization Conditions C. W. Polley, Jr., A. J. Illies, and G. G. Meisels" Department of Chemistry, University of Nebraska -Lincoln,
Lincoln, Nebraska 68588
A pulsed chemical ionization (CI)source employing a coaxial electron entrance-ion exit geometry has been used to obtain ion mobility data for various CI reagent ions. Ion mobilities obtained by this method are in good agreement with results obtained by drift tube measurements where comparison is possible. Ion mobility measurements performed at 125-150 O C provide information regarding the residence times of reagent ions under typical CI conditions. Residence times lie in the range 50-150 ps ( E / P = 5 V/(cm torr); T = 150 O C ; ion pathlength = 1.0 cm).
Since the first analytical application of chemical ionization mass spectrometry (CIMS) was reported by Munson and Field in 1966 (I),CIMS has developed into an extremely useful tool in analytical chemistry (2-7). CI spectra are the result of consecutive ion-molecule reactions and are usually kinetically controlled. A simple kinetic analysis (Appendix) shows that the conversion of sample 0003-2700/80/0352-1797$01 .OO/O
molecules to ions can be related to the residence times of reactant ions in the source (tL),the rate constants k , for the reactions of these ions with the sample molecules, and the concentration of sample molecules. [MI, approximated by
where ZToTSis the total intensity of the sample ions and IiR is the intensity of the reactant ion a t m / e = i . Since the CI response is related to sample concentration through the term in the parentheses, one can treat this term as the CI sensitivity for the sample. Knowledge of each component of this term can then be used to estimate sample sensitivities and detection limits for compounds of interest prior to experimentation. Reactant ion intensities can easily be obtained from the background spectrum of the reagent gas. The rate constant and reaction time, however, are not so easily obtained. Studies of exothermic ion-molecule reactions have indicated that the rate constants for these reactions increase with increasing polarizability and increasing dipole moment of the sample molecules. On the basis of these results a number of theories ?2 1980 American Chemical Society
