1920
Anal. Chem. 1988, 60, 1920-1924
improvement in the separation efficiency, reducing uv2,will result in an improved detection limit. This will require a concurrent decrease in the flow cell volume to limit band broadening at detection. Our flow cell volume was just small enough to limit broadening in the microbore HPLC of polymers, since the flow cell volume was about a factor of 10 less than detected solute peak volumes. Since the RIG signal is quite sensitive to the eluting solute peak width, further investigations have been directed toward exploiting this dependence to more sensitively probe changes in polymer samples, as may occur in processing conditions. The novel device is extremely simple to construct and maintain, and thus, may be quite useful for process analysis applications as well as routine laboratory work. ACKNOWLEDGMENT D.O.H. and R.E.S. thank L. Burgess and M. Schurr for helpful discussions concerning the interferometric mechanism. Registry No. Polystyrene, 9003-53-6.
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31)
LITERATURE CITED (1) Snyder, L. R.; Kirkland, J. J. Introduction to Modern Liquid Chromatography, 2nd ed.; Wley-Interscience: New York. 1979. (2) Yeung, E. S.; Synovec, R. E. Anal. Chem. 1986, 58, 1237A-1256A. (3) Yeung, E. S. In chemlcel Analysis, Detectors for Liquid chromatoga phy, Vol. 89; Elving, P. J., Winefordner, J. D., Eds.; Wiley-Interscience: New York, 1986. (4) Munk, M. N. I n Liquld ChromatographyDetectors; Vickrey, T. M.. Ed.; Dekker: New York, 1983. (5) Scott, R. P. W. Liquid Chromatography Detectors, Elsevier: Amsterdam, 1977. (6) Stolyhwo, A.; Colin, H.; Guiochon, G. J . Chromatogr. 1983, 265, 1-18. (7) Stolyhwo, A,; Colin, H.; Guiochon, G. Anal. Chem. 1985, 5 7 , 1342-1354. (8) Berry, V. V. J . Chromatogr. 1980, 199, 219-238.
-
(32) (33) (34) (35) (36) (37) (38)
Berry, V. V. J . Chromatogr. 1982, 236, 279-296. VanDerWal, S.; Snyder, L. R. J . Chromatogr. 1983, 255, 463-474. Small, H.; Miller, T. E. Anal. Chem. 1982, 5 4 , 462-469. Bobbin, D. R.; Yeung, E. S. Anal. Chem. 1984, 56, 1577-1581. Mho, S. I.; Yeung, E. S. Anal. Chem. 1085, 57, 2253-2256. Synovec, R. E.; Yeung, E. S. Anal. Chem. 1983, 55, 1599-1603. Synovec, R. E.: Yeung, E. S. J . Chromatogr. 1984, 283, 183-190. Synovec, R. E.; Yeung, E. S. Anal. Chem. 1984, 56, 1452-1457. Synovec, R. E.; Yeung, E. S. J . Chromatogr. Sci. 1984, 23, 214-221. Bornhop, D. J.; Dovichi, N. J. Anal. Chem. 1986, 58, 504-505. Bornhop, D. J.; Nolan, T. G.; Dovichi, N. J. J . Chromatogr. 1987, 384, 181-187. Bornhop, D. J.; Dovichi, N. J. Anal. Chem. 1087, 59, 1632-1636. Synovec, R. E. Anal. Chem. 1087, 59, 2877-2884. Wilson, S. A,; Yeung, E. S. Anal. Chem. 1085, 5 7 , 2611-2614. Woodruff, S. D.: Yeung, E. S. Anal. Chem. 1982, 5 4 , 2124-2125. Woodruff, S. D.; Yeung, E. S. J . Chromatogr. 1983, 260, 363-369. Pawliszyn, J. Anal. Chem. 1086, 58, 243-246. Pawliszyn, J. Anal. Chem. 1988, 58, 3207-3215. Dandridge, A.; Miles, R. 0.; Giallorenzi, T. G. Electron. Lett. 1980, 16, 948-949. Dandridge, A. Appl. Opt. 1981, 2 0 , 2336-2337. Dandridge, A,; Tveten, A. B. Appl. Opt. 1081, 2 0 , 2337-2339. Kachei, V.; Menke, E. I n Flow Cytometty and Sorting, Hydrodynamic Properties of Flow Cytomefric Instruments;Melamed, M. R., Mullaney, P. F., Mendelson, M. L., Eds.; Wiley: New York, 1979; Chapter 3. Lighthill, J. Waves in fluids; Cambridge University Press: Cambridge, England, 1978; pp 286-298. Renn, C. N.; Synovec, R. E. Anal. Chem. 1988, 6 0 , 200-204. Hecht, E.; Zajac, A. Optics; Addison-Wesley: Reading, MA, 1979; Chapter 9, pp 301-307. Pawliszyn, J. J . Liq. Chromatogr. 1087, 10, 3377-3392. Synovec, R. E.; Yeung, E. S. Anal. Chem. 1985, 57, 2162-2167. Synovec. R. E.; Yeung, E. S. Anal. Chem. 1988, 58. 2093-2095. Merck Index, 10th ed.; Merck: Rahway, NJ, 1983; No. 8732. Renn, C. N.; Synovec. R. E. Anal. Chem. 1988, 6 0 , 1188-1193.
RECEIVED for review March 8,1988. Accepted May 16,1988. D.O.H. and R.E.S. thank the NSF Center for Process Analytical Chemistry for support of this work (Project Number 86-2).
Determination of Carboxylic Acids by Isotope Dilution Gas Chromatography/Fourier Transform Infrared Spectroscopy Edwin S. Olson,* John W. Diehl, and Michael L. Froehlich
University of North Dakota Energy and Mineral Research Center, Box 8213, University Station, Grand Forks, North Dakota 58202 A method has been developed for determlnlng carboxylic acids by Isotope dllutlon with ‘*O-enrIched carboxylic aclds and measuremsnt of the Isotope ratlo In methyl esters of the anaiyte and standard mlxture by caplllary gas chromatography/Fourler transform Infrared (GC/FTIR) spectroscopy. The dlfference In the carbonyl absorptlon maxima for the analyte and standard esters allowed separate absorbance chromatograms to be reconstructed by lntegratlon over two narrow frequency ranges in each analytehtandard GC/FTIR spectrum. Area ratios obtained from the absorbance reconstructed chromatograms were plotted versus concentration ratios to give a nonlinear cailbratlon plot, which was expressed as a thlrd-order polynomial by least-squares polynomlal fitting. The method was more accurate than a GUMS method developed wlth the same standards, analytes, and range of concentrations. The Isotope dilutlon GC/FTIR method was appiled to the quantitative analysis of aqueous mixtures of coal oxidation products.
The quantitative analysis of mixtures of carboxylic acids resulting from the oxidation of coals presents a challenge to
the analytical chemist. These products consist mainly of aliphatic and aromatic di- and polycarboxylic acids, which are highly soluble in the aqueous reaction medium. Inorganic compounds are often present and may react with derivatizing reagents. Determinations of carboxylic acids and other highly polar organic compounds usually require addition of isotope enriched internal standards to obtain accurate results, because even highly polar homologous standards are neither extracted from aqueous solutions nor derivatized reproducibly or to the same degree as the analytes. A deuterium isotope gas chromatography/mass spectrometry (GC/MS) method was used for analysis of the products of coal oxidation with ruthenium tetraoxide (1,2). This method was accurate when the standard possessed four or five deuterium atoms per molecule (3)but less accurate than desired in determinations with standards that possessed only two deuterium atoms per molecule. However, the lack of possible sites for deuterium substitution in aromatic polycarboxylic acids and the unfortunate ease with which hydrogens are exchanged in carboxylic acids with acidic hydrogen on carbon (e.g., malonic acid) make deuterium isotope dilution GC/MS difficult to apply to these compounds. Although 13C-enriched aliphatic dicarboxylic acids are not difficult to prepare, they are relatively expensive, and 13C-
0003-2700/88/0360-1920$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
enriched aromatic polycarboxylic acids are not so easily synthesized. Carboxylic acid standards with 180-enrichment are relatively easy to synthesize, and because of isotope shifts in the intense carbonyl absorptions of carboxylate derivatives, a new possibility for determining isotope ratios by gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy was evident. GC/FTIR spectroscopy has been very useful in qualitative analyses of volatile mixtures, but there are very few reports describing quantitative analyses. Sparks et al. ( 4 ) used a packed column GC/FTIR system to determine pentyl propionate (with acetophenone as the internal standard). Quantitative capillary GC/FTIR methods required the development of more sensitive instrumentation, and recently Gurka and Pyle studied the determination of halobenzenes with a spectrometer equipped with an improved detector and light pipe (5). The development of an isotope dilution GC/FTIR method was pursued in this research by preparing several aliphatic and aromatic l*O-enriched carboxylic acid internal standards and examining the methyl esters of the analytes and standards by GC/FTIR to determine isotope ratios. The carbonyl absorption maxima of the analyte and "0-enriched standard esters differ by approximately 33 cm-l, and absorbance chromatograms specific to the analyte and standard could be reconstructed by integration of the stored GC/FTIR spectra over narrow frequency ranges. Calibrations based on integration of the respective carbonyl absorbance chromatograms of the methyl esters of the standards and their respective analytes are described in this paper. The isotope dilution GC/FTIR method was then applied to the analysis of complex aqueous mixtures of carboxylic acids from coal oxidations. These data provided information about the structural features of the coals that are important in understanding the chemistry of coal processing (2). EXPERIMENTAL SECTION Synthesis of Carboxylic Acid Standards. The 180-enriched carboxylic acid standards were synthesized by modification of the procedure of Chen et al. (6) for the preparation of adipic acid-1804by hydrolysis of adiponitrile with water-180containing hydrogen bromide. The syntheses of propanedioic (malonic) acid-180, from malononitrile, octanoic acid-I8O2from octanenitrile, octanedioic (suberic) acid-180, from l,g-dicyanohexane, and 1,2,4,5-benzenetetracarboxylicacid-1808from 1,2,4,54etracyanobenzene gave products in 93,86, 97, and 44% yield, respectively. Since hexacyanobenzene hydrolyzes to pentacyanophenol (7), benzenehexacarboxylic (mellitic) acid-18012was prepared from mellitimide previously synthesized by the procedure of Wohler (8)and Schwartz (9). 1,2,4,5-Tetracyanobenzenewas prepared according to the procedure of Thurman (IO) and purified by elution with tetrahydrofuran from a column of neutral alumina. Anhydrous hydrogen bromide was generated from tetralin and bromine (11). All other reagents were purchased from Aldrich except water-180 (98+ 9'0 isotopic purity, Cambridge Isotope Laboratories) and were used as received. Reaction mixtures were frozen (liquid nitrogen) and sealed inside dry Cryule ampules (Wheaton), which were then heated at 85 "C for 1-4 days in a Parr bomb containing water to equalize pressure. The ampules were periodically agitated in a laboratory ultrasonic bath to break up the solid mass which formed during the syntheses of the l80-enriched 1,2,4,5-benzenetetracarboxylic,benzenehexacarboxylic, and octanedioic acids. Complete descriptions of the syntheses may be obtained from the authors on request. Small samples of the 180-enriched carboxylic acids were converted to the methyl esters with an ether solution of diazomethane (prepared by using Diazald from Aldrich) and determined from the mass spectra of the methyl esters. As usual in electron impact mass spectra of polycarboxylate esters, the molecular ions of the several isotopic species were not observed or were not very intense; hence, the relative intensities of the series of ions resulting from loss of labeled methoxy radicals (lsOCH3)from the several isotopic
1921
$1 a
m
30
2050
1670
1290
910
7
530
Figure 1. GC/FTIR spectrum of dimethyl octanedioate-'*0, and dimethyl octanedioate.
molecular ions were used to determine the isotopic purity. All standards were 98+ % isotopically pure except benzenehexacarboxylic acid-18012which was 86% isotopically pure, because the I 6 0 present in the precursor diluted the oxygen pool. Instrumentation. Gas Chromatography. The gas chromatograph was a Hewlett-Packard Model 5890A equipped with an on-column capillary injector and a J & W 60 m X 0.32 mm column with a 1.0 Nm film DB5 phase and was oven temperature programmed from 40 to 50 "C at 30 "C/min and 50 to 350 "C at 5 "C/min. The carrier gas was hydrogen at a linear velocity of 21 cm/s measured at 350 "C. Fourier Transform Infrared Spectroscopy. A Nicolet 2OSXB FTIR spectrometer was equipped with a mercury cadmium telluride (MCT-A) detector, a Nicolet 1280 computer with a fast Fourier transform coprocessor, and a gold-coated 15 cm X 1.5 mm i.d. light pipe heated at 250 "C (12). Because of the low dead volume of the light pipe and the chromatographic conditions used, no make-up gas was necessary to achieve good resolution and peak shape. Sets of eight interferograms were collected at 8-cm-l resolution and coadded by the coprocessor every 1.5 s. The coprocessor transformed each coadded interferogram, then phase-corrected and ratioed the transform against the carrier gas background to produce one data point, which was stored on the hard disk. The stored spectra were integrated over the desired frequency ranges to generate the absorbance reconstructed chromatograms. Calibrations. Standard solutions in dry dimethylformamide were prepared such that the concentration of each standard acid (Ci) remained constant while the concentration of each analyte acid (C,) was varied over the concentration ratio (Ca/Ci) range of 0.2 to 6.1. Aliquots of these solutions were methylated with diazomethane in ether, diluted with dichloromethane, and examined with the GC/FTIR system. Areas were obtained for each calibration point by integration of the respective analyte and standard absorbance reconstructed chromatographic peaks by using the "SMD" command with base line correction on. In each absorbance reconstructed chromatogram the beginning and ending integration points of the peak were selected by the on-screen cursor. Base line correction references the integration to the chromatogram base line, and if this feature had been disabled, the integration would have been referenced to zero. This latter mode of integration produced significant errors. The area data obtained were used to produce a ten-point plot of area ratios (Aa/A,) versus concentration ratios (Ca/Ci). Third-order polynomials were derived from the data by least-squares fitting with the ASYST (Macmillan Publishing Co.) program for the IBM PC. RESULTS AND DISCUSSION The infrared absorption maxima of the carbonyl groups of the methyl esters of the 180-enriched standards were approximately 33 cm-' lower than the maxima of the esters of the unenriched (l60)analytes. Figure 1 shows an FTIR spectrum obtained by coadding the several spectra comprising
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
Table I. Infrared Reconstitution Frequency Ranges frequency range, cm-'
frequency
analyte (methyl ester)
range, cm-'
dimethyl malonate methyl octanoate dimethyl octanedioate tetramethyl 1,2,4,5-benzenetetracarboxylate hexamethyl benzenehexacarboxylate
1772-1764 1763-1755 1763-1755 1758-1750 1764-1756
GC/FTIR spectrum of hexamethyl benzenehexacarboxylate-'8012 and hexamethyl benzenehexacarboxylate.
Figure 2.
standard (methyl ester)
dimethyl malonate-1804 1738-1730 methyl octanoate-180, 1730-1722 dimethyl octanedioate-1804 1730-1722 tetramethyl 1,2,4,5-ben~enetetracarboxylate-~~O~1726-1718 hexamethyl benzenehexacarboxylate-180,2 1732-1724
Figure 4. Reconstructed chromatogram of dimethyl malonate-180, (1743-1720 cm-').
Figure 5. GC/FTIR calibration plots for octanedioic acid: (A) sec-
ond-order curve, ( 6 )third-order curve.
Flgure 3. Reconstructed chromatogram of dimethyl malonate (1782-1759 cm-I).
the GC/FTIR peak of a 1:l mixture of dimethyl octanedioate and dimethyl octanedioate-1804. A spectrum of the methyl esters of benzenehexacarboxylic acid and the corresponding standard is shown in Figure 2. Because of the broadness of the infrared spectral peaks, there was always some overlap of the carbonyl absorbances of the standard and analyte esters. The spectra obtained during the elution of each of the analyte and standard esters were integrated over a range or window of 8 cm-' nAm= f4 cm-l) corresponding to the carbonyl stretching bands of the analyte and the standard esters to generate the respective integrated absorbance reconstructed chromatograms. These frequency ranges are given in Table I. Reconstructed absorbance chromatograms over the selected ranges are shown in Figures 3 and 4 for the 1:l mixture of dimethyl malonate and dimethyl malonate-1804as examples of chromatographic peaks that were integrated in the subsequent quantitation. Since the average peak eluted over
about 24 s, each integration was performed on approximately 16 data points. This was an adequate number for precise integration, as demonstrated by the calibration tests discussed below. The areas obtained by integration of the separate reconstructed chromatograms were then used to produce the calibration curves. A calibration plot for octanedioic (suberic) acid is shown in Figure 5. The nonlinearity is believed to result from the absorbance overlap mentioned above. This same problem arises in isotope dilution mass spectrometry when the analyte and standard have ions with overlapping m/z values such that a hyperbolic relationship exists between the isotope ratio and the mole ratio (13). Initially, the area ratio data for several of the acids in our study were linearized by using a function similar to the Colby-McCaman formulation (13) for treating mass spectrometer isotope ratio data. This approach did indeed produce a linear curve from the infrared data; however, the method was extremely sensitive in that small deviations from the points defining the curve were magnified with consequent poor precision and accuracy in calibration tests. Rather than linearizing the data by normalization methods, the alternative approach to calibration is to describe the relation between area ratios and concentration ratios with a curvilinear equation. A polynomial expression (shown in eq 1) was fitted to the peak area data by the method of least Y = a,, alX azX2 ... anXn (1)
+
+
+
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
Table 11. Results of Calibration Tests compound malonic acid octanoic acid octanedioic acid
% accuracy
1,2,4,5-benzenetetracarboxylicacid
0.1 1.2 0.6 0.9
benzenehexacarboxylicacid
5.0
re1 std dev, % 1.0 0.9 0.2 1.2 1.0
squares. Y is the peak area ratio of analyte to standard, and X is the concentration ratio of analyte to standard. This equation was limited to a third degree polynomial to avoid oscillation of the calibration curve (14,15). The polynomial analysis is not affected by chromatographic separation of the analyte and labeled standard, whereas, as pointed out by Jonckheere et al. (14),the chromatographic effect “destroys the validity of the basic IDMS equation and subsequent calculation procedures based on this formula.” Sparks was able to use second-order polynomials for fitting the GramSchmidt integrated intensities of the two time-resolved peaks of the pentyl propionate analyte and the acetophenone standard (4). Initially five-point calibrations were performed with each carboxylic acid over the concentration ratio range of 0.20-5.0, and either a second- or a third-order polynomial could be fit equally well to the data. In ten-point calibrations, a better fit was obtained with a third-order polynomial. Curves representing the second- and third-order polynomials (eq 2 and 3) for the octanedioic acid data are illustrated in Figure 5. The goodness of fit for the third-order equation was 1% versus 7% for the second-order equation. Y = 0.2666 0.5592X - 0.0471X2
Y = 0.1711
+ + 0.7514X - 0.1267X2 + 0.0085X3
(3)
A set of six calibration tests was performed with each carboxylic acid a t concentrations of 0.2 mg/mL. The determined analyte concentrations (Table 11)had relative standard deviations from 0.2 to 1.2%. This indicates that both the absorbance reconstructions (integrations of the absorbances over the ranges corresponding to the carbonyl frequencies of the standard and analyte esters) as well as the integrations of the respective reconstructed chromatograms were highly reproducible. Determinations of malonic acid, octanoic acid, acid gave octanedioic acid, and 1,2,4,5-benzenetetracarboxylic percentage accuracies of approximately 1% for each analyte (Table 11). The accuracy in these determinations may be attributed to the high degree of precision obtained in the integration data and the adequacy of the polynomial fit. Somewhat larger percentage accuracies (5.0%) were found for benzenehexacarboxylic acid (see discussion below). The calibration was remarkably stable with respect to time. Periodic calibration tests performed over several weeks following a calibration showed no change in percentage accuracies. An alternative to integration of the spectra over the 8-cm-I window was attempted in hopes of obtaining linear calibration curves. Reconstructed chromatograms based on peak absorbance maxima were generated for 1,2,4,5-benzenetetracarboxylic acid. However, area ratios from the chromatograms reconstructed from the peak maxima data still did not give linear calibration curves. A calibration test using this method showed percentage accuracies similar to those obtained with absorbance reconstructions over 8-cm-’ windows. The relative standard deviations observed in these tests (Table 11)were less than half of those found in isotope dilution GC/MS (Finnigan-MAT 800ITD) determinations with the same calibration test mixtures of analytes and 180-enriched standards (3).The GC/MS method gave relative standard deviations for the determined analyte concentrations ranging
1923
from 2.4 to 4.5%. The percentage accuracies obtained with the GC/FTIR method (Table 11)were also better than those obtained with GC/MS (1.5-5.4%), even though the GC/MS calibration curves were linear. The relative standard deviations and percentage accuracies observed for this 180-isotope dilution GC/FTIR method were comparable with those found with deuterium isotope dilution GC/MS determinations of other polycarboxylic acids at similar concentrations. With standards possessing four or five deuterium atoms per molecule of carboxylic acid, isotope dilution GC/MS with both ion trap and quadrupole instruments gave relative standard deviations of 0.8-3.6% and percentage accuracies of 0.&3.9% (1, 3). The poorest percentage accuracies were observed for benzenehexacarboxylic acid in both GC/FTIR and GC/MS determinations and were believed to result from exchange of oxygen in the standard with the aqueous solution. Because of the solubility and polarity of the benzenehexacarboxylic acid, other GC methods for determining this compound are highly inaccurate. Loss of enrichment in the acid standards due to exchange of the l8O atoms with l60in the water was monitored by recovering the standards from an aqueous solution (pH 3), converting them to methyl esters, and measuring their isotope ratio by both GC/FTIR and GC/MS. Exchange rates were found to be negligible over a one-day period for most of the acids; however, benzenehexacarboxylic acid did exchange noticeably, and the exchange was significant over a longer period. Consequently, the internal standard and calibration solutions were prepared in dry dimethylformamide to prevent loss of enrichment during storage. Exchange was also investigated in a surrogate system for coal oxidation by addition of standards and analytes to the solution obtained after oxidation of benzene with ruthenium tetraoxide. After the solution was stirred for 10 min, the acids were determined. The percentage accuracy in the determination of benzenehexacarboxylic acid was 4% and that of benzenetetracarboxylicacid was 1%. Oxygen exchange in the benzenehexacarboxylic acid standard was demonstrated by the shifting of envelope of isotopic ions in the mass spectrum. As a result of these findings, a short contact time of standards with the aqueous solution was used in the determinations of carboxylic acids from the ruthenium tetraoxide oxidation of coal. Alternative methods for work-up of the acid products are being investigated to avoid the exchange problem. Two applications of the isotope dilution GC/FTIR method are described here. Oxidation of a North Dakota lignite (Beulah-Zap seam) with neutral hydrogen peroxide resulted in products consisting of oxidized coal particulates and an aqueous solution of carboxylic acids. After addition of the 180-enrichedstandards, the slurry was fiitered, and the filtrate was methylated with diazomethane in ether. The ether layer was separated, dried, and concentrated, and the isotope ratio was determined by GC/FTIR. The reconstructed chromatogram is shown in Figure 6. The weight percent yields of carboxylic acid products from the hydrogen peroxide oxidation are reported in Table 111. The 1,2,4,5-benzenetetracarboxylicacidJ808 standard was used to determine not only the amount of 1,2,4,5-benzenetetracarboxylic acid but also the amounts of the other two isomers. 1,2,3,4-Benzenetetracarboxylicacid and 1,2,3,5-benzenetetracarboxylic acid were assumed to be extracted identically with the 1,2,4,5-isomer and have the same calibration curves. The amounts of the benzenepolycarboxylic acids were large relative to other aromatic mono- and dicarboxylic acids found among lignite oxidation products, indicating that there should be a significant proportion of polynuclear aromatic rings in the coal structural precursors to these polycarboxylic acid
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
2
II
4
6
3
7
L
Flgure 6. Reconstructed Gram-Schmidt chromatogram of methyl esters of the products from the hydrogen peroxide oxidation of Beulah lignite: 1, dimethyl malonate idimethyl malonate-’80,; 2, dimethyl succinate; 3, trimethyl 1,1,2-ethanetrlcarboxylate;4, trimethyl 1,2,3propanetricarboxylate; 5, tetramethyl bentenetetracarboxylte (three isomers) itetramethyl benzenetetracarboxylatatal80,;6, pentamethyl benzenepentacarboxyhte; 7, hexamethyl benzenehexacarboxylate + hexamethyl benzenehexacarboxylate-”0 ,*. Table 111. Yields of Oxidation Products from Beulah Lignite Determined by Isotope Dilution GC/FTIR coal (ma0 H202 RuO~ oxidation oxidation wt % of
compound malonic acid benzenetetracarboxylicacids (sumof 3 isomers) benzenehexacarboxylic acid
4.2
0.23
0 2.08
0.33
1.08
products. Large amounts of malonic acid were found; this may have important implications concerning the nature of the bridging aliphatic groups and is currently being investigated more thoroughly. The amounts of octanoic and octanedioic acid in this sample were below the GC/FTlR detection limits. The carboxylic acid products from the ruthenium tetraoxide oxidation of the same lignite were determined by addition of
the 180-enriched standards, conversion to the methyl esters by methods previously described ( I ) , and isotope dilution ratio measurement with the GC/FTIR. The weight percent yields of oxidation products determined by this method are reported in Table 111. Malonic acid was not found, since it is not stable in the ruthenium tetraoxide system. Octanoic and octanedioic acid were below detection limits. Yields determined by GC/FTIR for benzenehexacarboxylic acid and benzenetetracarboxylic acids (1.08% and 2.08%) were comparable to those determined by GC/MS methods (1.13% and 2.21%) (3). Registry No. Dimethyl malonate, 108-59-8;dimethyl malonate-1804,115436-83-4; methyl octanoate, 111-11-5;methyl octanoate-ls02, 115420-79-6; dimethyl octanedioate, 1732-09-8; dimethyl octanedioate-1804,115420-80-9; tetramethyl 1,2,4,5benzenetetracarboxylate, 635-10-9; tetramethyl 1,2,4,5-benzenetetracarboxylate-1808, 115420-81-0; hexamethyl benzenehexacarboxylate, 6237-59-8; hexamethyl benzenehexacarboxylate-18012, 115436-82-3. LITERATURE CITED Olson, E. S.;Diehl, J. W.; Froehlich, M. L. Fuel 1987, 66,968-972. Olson, E. S.; Dlehl, J. W.; Froehllch, M. L. Fuel Process. Technol. 1987, 75, 319-326. Olson, E. S.; Froehlich, M. L.; Diehl, J. W., unpublished results. Sparks, D. T.; Lam, R. E.; Isenhour, T. L. Anal. Chem. 1982, 5 4 , 1922- 1926. Gurka, D. F.; Fyle, S. M. €nviron. Scl. Techno/., In press. Chen, T. S.;Stephens, J. C.; Leitch, L. C. J . Labelled Compd. 1970, 6 , 174-178. Friedrich, K.; Oeckl, S. Chem. Ber. 1973, 706, 2361-2365. Wohler, F. Justus Lieb&s’ Ann. Chem. 1841, 37, 263-284. Schwartz, H. Justus Lleblgs’ Ann. Chem. 1848, 66, 46-54. Thurman, J. C. Chem. I d . (London) 1964, 752. HadbWk of Preparatlve Inorganic Chemistry. 2nd ed.; Brauer, G., Ed.; Riley, R. F., Transl. Ed.; Academic: New York, 1963; Vol. 1, pp 282-283. Olson, E. S.; Diehl, J. W. Anal. Chem. 1987, 59, 443-448. Colby, R. N.; McCaman, M. W. Elomed. Mass Spectrom. 1978, 6 , 225-230. Jonckheere, J. A.; De Leenheer, A. P.; Steyaert, H. L. Anal. Chem. 1983, 55, 153-155. Schoeller, D. A. J . Clln. Pharmacol. 1988, 26, 396-399.
RECEIVEDfor review November 23,1987. Accepted May 27, 1988. This research was supported by Contract No. DOEFC21-86MC10637 from the U.S. Department of Energy. Reference herein to any specific commercial product by trade name or manufacturer does not necessarily costitute or imply ita endorsement, recommendation, or favoring by the United States Government or any agency thereof.
