Analyses of Complex Mixtures of Hydrocarbons by Time-of-Flight Mass Spectrometry-Open Tube Chromatography Martin H. Studier Chemistry Division, Argonne National Laboratory, Argonne, Ill. 60439
Ryoichi Hayatsu Enrico Fermi Institute, University of Chicago, Chicago, 111. 60637 THE ANALYTICAL POTENTIAL of the combination of time-offlight (TOF) mass spectrometry and gas-liquid chromatography was first demonstrated by Gohlke ( I ) in 1959. MCFadden et al. (2) and Banner et al. (3) have used the (TOF) instrument in combination with capillary columns. A combination of the three following factors have significantly increased the power of the TOF-capillary column combination : (1) Continuous ion source with a limit of sensitivity of torr (4). (2) A “blanking generator” (5) which permits elimination of superfluous parts of the mass spectrum and a choice of the mass range to be used for detection. (3) A system for loading an open tubular column by vacuum distillation which permits more sample to be loaded without loss of resolution than the conventional method with the sample dissolved in a solvent. Components in complex chromatograms representing less than 10-12 moles can be readily detected and characterized by significant mass spectra. EXPERIMENTAL APPARATUS
Sample Handling System. The chromatographic separator (obtained from Perkin-Elmer Corporation) was a stainless steel open tube column 91.5 meters long and 0.25 mm inside diameter coated with Apiezon L. The entire column was enclosed in a small laboratory oven containing a circulating fan. The inlet part of the column was bent in the form of a “U” to serve as a cold trap for loading samples. An auxiliary vacuum system for preparation and handling of samples inch copper is illustrated in Figure 1. It was made from tubing and metal valves. High vacuum was obtained with an oil diffusion pump, isolated from the system with a liquid nitrogen trap. The inlets to the column and to the spectrometer were wrapped with heating tape and insulated with aluminum foil. The interior of the spectrometer source was heated with tantalum filaments (4). Sample tubes were made of glass, quartz, or copper with graded seals and glass metal seals when appropriate. Mass Spectrometry. The mass spectrometer was a nonmagnetic time-of-flight instrument (Bendix Corporation Model 12 with 170 cm flight tube without source baffle) modified for continuous ion generation ( 4 ) and equipped with a blanking generator (5). The multiplier had three gates feeding two analog outputs and one oscilloscope. In normal operation, the scope gate has a negative bias of 150 volts so that all peaks not removed by the two preceding gates reach the scope anode and are displayed on the screen. A switch was installed so that this voltage could be applied to the middle gate. When this was done the entire spectrum (1) R. S. Gohlke, ANAL.CHEM., 31, 535 (1959). (2) W. H. McFadden, R. Teranishi, D. R. Black, and J. C. Day, J . Food Sci., 28, 478 (1963). (3) A. E. Banner, R. M. Elliott, and W. Kelly, “Gas Chromatog-
raphy,” Fifth International Symposium, Academic Press, New York, 1964. (4) M. H. Studier, Rev. Sci. Instr., 34, 1367 (163). ( 5 ) J. R. Haumann and M. H. Studier, Rev. Sci. Znstr., in press.
MASS SPECTROMETER
I I
V
T I
Figure 1. Schematic diagram of sample handling system disappeared from the scope and the current was integrated and amplified by one of the analog outputs and recorded on a strip chart. The strip chart served as the chromatogram. The first gate on the multiplier was used for fast scans of the mass spectrum recorded on an oscillographic recorder. During scans, peaks are removed from the integrating analog and sharp depressions are produced in the chromatogram thus marking the point at which the scan was made. When appropriate, ’ the spectrum can be displayed on the scope and photographed. EXPERIMENTAL PROCEDURE
Numerous analyses were made during a study comparing the organic compounds found in meteorites with those formed in a Fischer-Tropsch type of synthesis (6). The experimental procedure can be illustrated by an example with reference to Figure 1. A gaseous mixture of carbon monoxide and deuterium in a quartz tube containing an iron catalyst is placed at SI. As the tube is heated, progress of the reaction is followed by taking small aliquots in the volume bounded by valves VI, V2, and Vs. The aliquot is examined in the mass spectrometer by opening the valve slightly at inlet 11 or 12. Liquid nitrogen is placed on UI and the noncondensable gases are pumped away. During this time, the gases are examined for highly volatile products such as methane, ethane, ethylene, etc. These can frequently be observed after the bulk of the carbon mcjaoxide and hydrogen has been pumped away. The trapped residue is distilled from U1 to SZwhich is then placed in position S3 for a preliminary mass analysis. Dewar flasks with liquid nitrogen are placed on Ss and U3. The dewar is removed from Ss and as the sample warms slowly, it distills into U 3 past inlet, I*. Periodically small samples are admitted to the spectrometer. Usually a great deal of qualitative information about a sample can be obtained by this simple procedure. Relatively pure components can often be (6) M. H. Studier, R. Hayatsu, and E. Anders, Geochim. Cosmochim. Acta, 32, 151 (1968). VOL 40 NO. 6, MAY 1968
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Figure 2.
Chromatogram of synthetic product from carbon monoxide and deuterium Temperature programmed from 45” C at a rate of 0.3”/min
separated from complex mixtures and identified by this simple procedure. If more quantitative information is desired, the sample is distilled back into Ss and moved to position S4. The entire column and inlet are evacuated. An aliquot at a temperature chosen to give an appropriate size sample is taken between valves V4 and Vs. Liquid Nzis put on U4, and the aliquot is flushed on the column by filling the inlet with helium, admitting the aliquoted sample and heating the inlet. After the sample has been transferred, the inlet is flushed out with batches of He to remove traces of sample not transferred to U4. With helium flowing, the dewar on U4 is lowered for a few minutes in the heated oven to put the sample in a sharp band on the column. The dewar is removed and the oven is temperature programmed. The mass spectrum due to air is blanked out. The integrated spectrum above mass 50 is recorded on a strip chart and constitutes the chromatogram. When a chromatographic peak appears several five second scans are taken across the peak to determine its composition. Figure 2 is a chromatogram of a sample of a synthetic product produced from a mixture of deuterium and carbon monoxide (D&O = 1) in the presence of a catalyst consisting of an iron meteorite with alumina added (6). The sample was taken after the temperature had been slowly raised from room temperature to 300’ C over a period of three days. Deuterium instead of hydrogen was used so that contam1012
ANALYTICAL CHEMISTRY
inants could be detected easily. Fragmentation patterns of deuterated compounds show a preponderance of even masses and are readily distinguished from hydrogen compounds which have prominent peaks at odd masses. The initial temperature of the separating column was 45” C and was increased at a rate of 0.3” C/min for the entire run. Resolution and sensitivity of this single run were adequate for separation and mass spectral characterization of a hundred perdeutero aliphatic and aromatic hydrocarbons with from five to fourteen carbon atoms. Two small contaminant peaks were detected. Identification of individual compounds was aided by reference to vapor pressure data, to compilation of mass spectra (7-10) and to numerous original publications of mass spectral and chromatographic information (e.g. 11-15). (7) Index of Mass Spectral Data, American Society for Testing and Materials, No. 356, 1963. (8) Mass Spectral Data, American Petroleum Institute, Project 44. (9) F. W. McLafferty, “Mass Spectrometry of Organic Ions,” Academic Press, New York, 1963. (IO) H. Budzikiewicz, C. Djerassi, and D. H. Williams, “Mass Spectrometry of Organic Compounds,” Holden-Day, Inc., San Francisco, 1967. (11) F. Baumann and S.M. Csicsery, J. Chromatog., 26,262 (1967). 35, 1930(1963). (12) D. L. MartinandJ. C. Winters, ANAL.CHEM., (13) J. Q. Walker and D. L. Ahlberg, ibid., 35, 2022 (1963). (14) T. L. Chang and C. Karr, Anal. Chem. Acta, 26, 410 (1962). 38, 1047 (1966). (15) D. J. McEwen, ANAL.CHEM.
Table I. Perdeutero Hydrocarbons in Chromatogram of Figure 2 No. in Figure 2 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
Compound Pentanes and pentenes 2-Methylpentane 3-Methylpentane n-Hexane and hexene 2-Methylhexane Benzene 3-Methylhexane Heptene Branched heptene Branched heptene n-Heptane 1-Heptene Contaminant-hydrogen compound Contaminant-hydrogen compound 2-Methylheptane Branched octane 3-Methylheptane Toluene Branched octene Branched octene Branched octene n-Octane I-Octene 2,3-Dimethylheptane 2-Methyloctane 3-Methyloctane Ethylbenzene Branched nonene p-Xylene m-Xylene Branched olefin n-Nonane I-Nonene 0-Xylene Isopropylbenzene 3,CDimethyloctane 2,3-Dimethyloctane n-Propylbenzene 2-Methylnonane 3-Methylnonane p-Methylethylbenzene rn-M ethylethylbenzene Branched decene Branched decene 0-Methylethylbenzene n-Decane 1-Decene 1,2,4Trimethylbenzene 3,CDimethylnonane 2,3-Dimethylnonane m-Diethylbenzene 2-Methyldecane
Peaks that were identified with some degree of certainty were numbered and listed in Table I. Numerous resolutions were made by mass analyses even though chromatographic separations were not obvious. Chromatographic separation of others were made even though not resolvable by mass analysis alone (e.g., the isomeric xylenes). It is probable that there are some errors in the assignments of Table I. Detailed information of the chromatographic and mass spectral properties of branched paraffins and of olefins is sometimes lacking. Mass spectra of some isomeric aromatic compounds are very similar. This accounts in part for some of the vague descriptions in Table I. The specificity of our description is a measure of our degree of confidence in each
No. in Figure 2 53 54 55 56 57 58 59 60 61 62 63 64 65 66
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102
Compound m- and p-Methyl-n-propylbenzene
3-Methyldecane o-Methyl-n-propylbenzene
Branched undecane Branched undecane Branched undecane n-Undecane Dimethylethylbenzene Branched alkane 4,s-Dimethyldecane 3,CDimethyldecane 2,3-Dimethyldecane 2-Methylundecane Pentane-1-phenyl branched pentane phenyl 3-Methylundecane Dimethylethylbenzene Dimethylindane butane-2-phenyl2-methyl (?) Branched olefin Branched olefin n-Dodecane Branched alkane Branched alkane Branched hexane-phenyl Branched alkane Branched alkane Branched alkane Branched alkane 4,5-Dimethylundecane Naphthalene 3,4-Dimethylundecane 2,3-Dimethylundecane 2-Methyldodecane Hexane-2-phenyl 3-Methyldodecane Hexane-1-phenyl Branched olefin Branched olefin Branched olefin n-Tridecane Branched alkane Branched tetradecane 4,5-Dimethyldodecane 3,CDimethyldodecane 2,3-Dimethyldodecane 2-Methylnaphthalene 2-Methyltridecane 3-Methyltridecane Heptane-I-phenyl 1-Methylnaphthalene olefin n-Tetradecane
+
+
+
assignment. Our object in presenting the data of Figure 2 and Table I in such detail is t o give a practical idea of the value of the analytical method. ACKNOWLEDGMENT
We are grateful t o Leon P. Moore and to Emily B. White for technical assistance. RECEIVED for review December 6, 1967. Accepted February 26, 1968. Work performed under auspices of U. S. Atomic Energy Commission. This work was supported in part by NASA (Grant NsG-366). VOL 40, NO. 6, MAY 1968
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