Gas Chromatographic Determination of Combustible Carbon in an Inert Matrix Dale L. Fanter and Clarence J. Wolf McDonnell Douglas Research Laboratories, McDonnell Douglas Corp., St. Louis, Mo. 63166 COMBUSTIBLE CARBON contained within the matrix of a hightemperature inert material, such as steel, is generally measured by direct combustion in a flowing oxygen stream followed by a determination of the product carbon dioxide (1). An ASTM standard method (2) permits a wide latitude of Combustion and detection conditions. Induction heating has largely replaced tube furnaces for combustion, and the original technique of gravimetric COZdetermination has been improved by chromatographic (3, 4), conductometric (9,or infrared (6) methods. A low pressure combustion method is described ( I , 7), but has apparently been little used in recent years because of the comparatively long analysis time and the availability of commercial equipment. The various methods described in the literature, as well as commercial instruments, are intended to be used with samples weighing more than 500 mg. Our interest in relatively rare carbonaceous chondrites required a carbon determination procedure consuming 50 mg or less. Therefore, a small closed “bomb,” low-pressure combustion system followed by a cryogenic trap, a calibrated gas buret, and chromatographic separation was developed for total carbon analysis of 50-mg meteorite samples.
Oxygen i-lMvlanometer/ 1
n 1I
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Figure 1. Schematic diagram of the low-pressure carbon combustion system To G.C.
Sample i o o j $ i Vacuum
EXPERIMENTAL
Apparatus. The combustion train and gas analysis systems are schematically illustrated in Figure 1 and 2, respectively. The reactor, fabricated of quartz and connected to the borosilicate glass vacuum line via a quartz-borosilicate glass graded seal, is similar to that described by Krouse and Modzeleski (8). Metal bellows valves (Hoke 200-A) were used to eliminate contamination from stopcock grease within the reactor system. Procedure. Oxidizable carbon was removed from the reactor prior to admitting the sample by the following sequence: the system was evacuated to Torr; filled with 380 Torr pure oxygen; and heated for 24 hours at 1100 “C. After the conditioning period, the reactor was carefully opened at the top and a weighed meteorite charge was dropped into the quartz finger. A 1- x 2-cm section of silver mesh was inserted into the side arm of the reactor to adsorb halogens evolved during subsequent heating of the sample. The reactor was resealed, evacuated, flushed with oxygen several times, filled to 380 Torr oxygen and sealed at the constriction, forming a closed “bomb.” The oxygen used to ( I ) Walter G. Boyle, Jr., in “Techniques of Metals Research,” Vol. 111. Part I, R. F. Bunshah, Ed., Interscience, New York, N. Y . ,
1970, pp 335-41. (2) Apparatus for microdetermination of carbon and hydrogen in organic and organometallic compounds, ASTM E191-64, American Society for Testing and Materials, Philadelphia, Pa. (3) W. K . Stuckey and J. M. Walker, AXAL.CHEM., 35,2015 (1963). ( 4 ) J. M. Walker and C. W. Kuo, ibid., p 2017. ( 5 ) E. L. Bennet, J. H. Harley, and R. M. Fowler, ibid., 22, 445
(1950). (6) G. A. Tipler. A i a l y s i (Loiidori),88, 272 (1963). (7) R. M.Fowler, W. G. Guldner, T. C. Brvson. J. L. Hague. and H. J. Schmitt, ANAL.CHEM.. 22,486 (1950;. (8) H. R . Krouse and V. E. Modzeleski, Geocliim. Cosrnochim. Actu. I
34,459 ( 1 970).
.
Sample
2
Figure 2. Schematic diagram of product gas separation train and calibrated gas buret flush and fill the reactor was scrubbed to remove traces of water and carbon dioxide by passing it through a bed of anhydrous C a S 0 4(Drierite) and NaOH (Ascarite). The sample was heated at 1100 “C for 24 hours, cooled to room temperature, and connected to the gas analysis system at the break-seal (Figure 2). Valve V I was closed; the first trapping loop was cooled with liquid nitrogen; and the iron slug was used to break the seal, thus permitting the contents of the reactor to pass into the trap. Valve VI was opened slightly, and the unused oxygen was removed. When the Torr, the second trap pressure in the system decreased to was cooled with a dry ice-isopropyl alcohol bath to -78 “C and the Toeppler pump was activated to transfer the gases into the calibrated gas buret. The -78 “ C trap retained H 2 0 and the oxides of sulfur. After the carbon dioxide was transferred into the calibrated flask, the mercury level was raised to compress the carbon dioxide into the calibrated volume. Carbon dioxide was thus determined on an absolute basis. Valve Vz was opened and the mercury level raised, displacing the CO, into the sample loop. Contents of the loop were injected into the gas chromatograph (Figure 3) by means of a 4-way valve. Carbon dioxide was separated from air with a 15-ft (4.6-m) Porapak Q column [2-mm i.d. ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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Injector
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15 f t Porapak Q
12 f t Porapak Q
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Figure 3. Diagram of gas chromatographic separation system
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Figure 5. Chromatograms of the combustion products from a 47-mg sample of Pueblito de Allende meteorite well Model 19 1-mV). The areas of all peaks were determined with a digital integrator (Infotronics Model CRS 100). Injection
/-
+TC
4
2
I
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Time (min)
Figure 4. Chromatogram of calibration mixture. The lower trace shows the separation of C 0 2 from “air” in the 100 OC Porapak column. The chromatogram following the separation of “air” in the -78 “C Porapak column is shown in the upper trace Teflon (Du Pont) tubing packed with 8OjlOO mesh Porapak Q (Waters Associates)] at 100 “C while oxygen, nitrogen, and carbon monoxide were separated with a 12-ft(3.7-m) Porapak Q column held at -78 “C. This chromatographic system is similar t o that described by Obermiller and Charlier (9). Two thermistor detectors (Carle Model 101) were used and the chromatograms were recorded on a dual pen recorder (Honey(9) E. L. Obermiller and G. 0. Charlier, J. Gus Chromarogr., 6,446 (1968). 566
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RESULTS AND DISCUSSION
The chromatogram from the injection of a typical calibration mixture containing C 0 2 with known concentrations of 02, NP,and CO is shown in Figure 4. The lower trace shows the response from Dectector-1 following the separation of COz from “air” while the upper trace indicates that the components of air, i.e., Oz, N?, and CO are resolved on the -78 “C Porapak column. The chromatogram of the products following the combustion of a 47-mg sample of a carbonaceous chondrite, Pueblito de Allende, is shown in Figure 5 . The initial base-line offset results from rotation of the 4-way injection valve. The presence of O2in the product gas indicates incomplete removal of this gas during the initial trapping step. Nz together with O2 suggest a leaking vacuum system, while CO indicates incomplete combustion of the sample. The area of the air peak in Figure 5 corresponds to a n impurity of 0.4%, indicating 99.6% pure CO?. The reproducibility of the method was determined by analyzing several NBS steel samples for carbon. Steel samples weighing between 23 and 5 5 mg were used for these studies. The results are summarized in Table I where the NBS value together with measured carbon content is listed. The error listed for the measurements corresponds to twice the standard
Table 11. Combustible Carbon in Meteorites sample Carbon content, Meteorite size, mg Measured Reported (Ref.) Pueblito de Allende 48.49 0.28 86.71 0.29 0.3 (10) 43.75 0.31 Murchison 9.56 2.3 2.0 (11) Mokoia 28.70 0.97 0.57-0.84 (12, 13) Murray 14.34 2.6 2.78 (14) 8.45 3.2 Karoonda 18.82 0.10 0.10 (12)
Table I. Analysis of Calibration Standards Carbon content, Samplea NBS Measured 10 g 0.240 0.234 1 0 . 0 0 8 12h 0.407 0.428 i 0.008 1 . 0 7 z k 0.008 16 e 1.09 a Numbers correspond to NBS nomenclature.
deviation. The precision within a given series corresponds to rt 3 % and the mean error is also within 3 % of the NBS accepted value. The maximum relative error, corresponding t o sample 12h, i? within 5 % of the accepted value. Results of the determination of the combustible carbon content of 5 carbonaceous chondrites are summarized in Table IT. Sample sizes ranged between 8 and 87 mg, and the measured values are in good agreement with the values reported by other investigators. Since carbonaceous chondrites are nonhomogeneous rare samples, too valuable t o fragment large pieces to obtain a truly representative sample, the agreement between the values shown in Table I1 and those reported previously is good. Measurements o n selected samples of Allende indicate that the repeatability of the method for meteorite analysis is within +4% of the average value. One of the major disadvantages of the method proposed here is the relatively long time required for an individual analysis. When two separate furnaces are used, one to condition a new reactor and a second to perform the combustion, it is possible to analyze one sample every 24 hours. However,
the accuracy of these techniques for the analysis of small samples justifies the use of a low pressure combustion bomb followed by a chromatographic determination of the product gases.
RECEIVED for review August 28, 1972. Accepted November 6, 1972. This research was conducted under the McDonnell Douglas Independent Research and Development Program. (10) E. A. King, Jr., E. Schonfeld. D. A. Richardson, and J. S. Eldridge, Science. 163,928 (1969). ( 1 1) K. Kvenvolden, J. Lawless, K. Pering, E. Peterson, J. Flores, C. Ponnamperuma, I. Kaplan, and C. Moore, Nature, 228, 923 (1 970). (12) C. B. Moore and C. Lewis, Science, 149, 317 (1965). (13) G. Boato, Geocliim. Cosmocliim Acta, 6, 209 (1954). (14) H. B. Wilk, ibid.,9,279 (1956).
Py roIys is -Gas Chromatography of Perf Iuoro-n-pentane Raymond R. Rogers, Gordon S. Born, Wayne V. Kessler, and John E. Christian Department of Bionucleonics, School of Pharmacy and Pharmacal Sciences, Purdue Unioersity, West Lafayette, Ind. 47907 RECENTSTUDIES in our laboratories required a rapid and efficient method for the separation and identification of the 358980 "C pyrolysis products of perfluoro-n-pentane (n-C5FI?). It is the purpose of this paper to report the pyrolysis of n-C5F,? using a Curie point pyrolyzer and the subsequent separation and identification of the pyrolysis products by gas-solid and gas-liquid chromatography. A survey of the literature revealed little information regarding the pyrolysis of n-C5FI2. The pyrolysis of n-C5FI?was reported in 1951 by Rogers and Cady ( I ) . The n-C5F12was pyrolyzed in a chamber constructed of a 1-liter borosilicate glass flask with a platinum wire coil at the center. The products of n-C5Flppyrolysis were separated and identified by making an analytical fractional distillation and measuring the vapor densities and boiling ranges of the different cuts. Unsaturated product was determined by chlorination of the C3 cut. Temperatures of the platinum wire ranged from 840 to 1325 "C; however, pyrolysis products of the n-C5Flpwere not detected below 900 "C. When n-C5FI?was subjected to higher temperatures, the pyrolysis products identified by
Rogers and Cady ( I ) were CF4, C2F6, C3F8, C3F6, carbon, and one o r more forms of C4F8. The authors also indicated the presence of a high polymer on the walls of the pyrolysis flask. In 1952, Steunenberg and Cady ( 2 ) reported a study of the pyrolysis of fluorocarbons, including n-CSFI2. The pyrolysis vessel used for this study was constructed of gold plated copper tubing and a platinum filament. The methods used for separating and identifying the n-CjFip pyrolysis products were similar to those described by Rogers and Cady ( I ) . Steunenberg and Cady ( 2 ) identified C2F6, C3F8, C3F6, C4F8, carbon, and a -CF- polymer as pyrolysis products of n-C6F12. Campbell and Gudzinowicz (3) reported the separation of fluorocarbon and sulfur-fluoride compounds by gas-liquid , chromatography in 1961. The separaton of CF4, C Z F ~CsF6, cyclic-C4F8,and i-C4F8was achieved on a column consisting of 33 No. 3 Kel-F polymer oil. Greene and Wachi ( 4 ) investigated the separation of some C1-C4 perfluorocarbons on a gas-solid column of 45/60 mesh ( 2 ) R. K. Steunenberg and G. H. Cady, ibid.,74,4165 (1952). (3) R. H. Campbell and B. J. Gudzinowicz, ANAL.CHEM.,33,
( 1 ) G . C. Rogers and G. H. Cady. J . Amer. Cliem. Soc.. 73, 3523 ( 1951 ).
842 (1961). (4) S . A. Greene and F. M. Wachi; ibid.,35,928 (1963). ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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