romatography of Hydrocarbons Dale L. Fanter, John Q. Walker, and Clarence J. Wolf Research Dioision, McDonnell Douglas Corp., St. Louis, Mo. 63166
The pyrolysis gas chromatography of 83 C6 to Clo compounds was studied. The pyrolysis product patterns for 24 alkanes, 49 straight chain, branched chain, and cyclic alkanes; and 10 alkynes are reported in terms of relative mole responses within specified retention index intervals. The patterns are repeatable and vary slightly within the temperature range 575O to 625 O G . A comparison of pyrograms with mass spectrograms for several C6HI2isomers which are difficult to characterize mass spectrometrically Is presented.
PYROLYSIS GAS CHROMATOGRAPHY (PGC) is a relatively new technique which has evolved in order to use the analytical capabilities of gas chromatography on nonvolatile solids. Levy ( I ) has carefully and in great detail reviewed the entire field of PGC. Materials such as high molecular weight polymers, drugs, and even bacteria have been thermally fragmented and the chromatogram of the pyrolyzate has been recorded. The method can also be applied to help solve one of the major problems remaining in gas chromatography today, namely, the qualitative identification of eluted compounds. Several investigators have recognized the potential of PGC in solving this problem and have reported on the vapor phase pyrolysis of organic compounds (2-6). Dhont (2, 3) and Weurman (4) utilized PGC by trappjng the eluted vapors at the outlet of a conventional G C and pyrolyzing the trapped compounds. Levy and Paul (7) described a system which consists of a conventional GC, a flow delay-line trap pyrolyzer, and a second GC. A few selected compounds from an unknown mixture could be trapped in the delay-line and subsequently pyrolyzed. The chromatogram of the pyrolyzate (hereafter called pyrogram) was reproducible and could be used for identification. Sutton and Harris (8) connected the output from a conventional G C directly to the PGC system and studied the pyrolysis of several hydrocarbons. Goforth and Harris inserted a trapping column between a conventional GC and a rapid analysis PGC unit (9). Walker and Wolf (IO) described a GC-pyrolysis-GC system for the complete analysis of chromatographic effluents. Here, the technique of interrupted elution chromatography was adapted to the separation chromatograph so that all the unknowns can be pyrolyzed and identified in a single G C analysis. Keulemans together with Berry (5) and Cramers ( 6 ) investigated the rates and mechanisms of gas phase thermal decomposition of hydrocarbons to determine the experimental parameters necessary to use PGC as an identification method. Wolf and Rosie ( I I ) (1) R. L. Levy, Chronzatogr. Reu., 8,48 (1966). (2) J. H. Dhont, Nature, 200,882 (1963). (3) J. H. Dhont, Analyst (London), 89, 71 (1964). (4) C. Weurman, Chern. Weekblad, 59, 489 (1963). (5) A. I. M. Keulemans and S. G. Perry, “Gas Chromatography, 1962,” M. Van Swaay, Ed., Butterworth, (Washington, D. C.),
1962, pp 356. (6) C. A. M. G. Cramers and A. I. M. Keulemans, J. Gas Chromatog., 5, 58 (1967). (7) E. J. Levy and D. G. Paul, J. Gas Chrornutogr., 5, 136 (1967). (8) R. Sutton and W. E. Harris, Canadian J. Chem., 45, 2913 (1967). (9) R. R. Goforth and W. E. Harris, Preprints of papers “7th International Symposium on Gas Chromatography and Its Exploitation,” C. L. A. Harbourn, Ed., paper 16. (10) J. Q. Walker and C. J. Wolf, ANAL.CHEM., 40,711 (1968). (11) T. Wolf and D. M. Rosie, ibid., 39, 725 (1967). 41 68 *
ANALYTICAL CHEMISTRY
Vapoiizer
Pyrolyzer
SeFlu
I
Solitier l o M.S. or vent
Figure 1. Block diagram of vapor phase pyrolysis gas chromatographic system studied the effect of temperature on the vapor phase pyrolysis of 20 simple organic molecules. The pyrograms reported by the various investigators are not completely reproducible and at present they cannot be used for interlaboratory comparison. This paper is an attempt to form standard methods for the reporting of PGC data in a manner analogous to that used in reporting mass spectra. The resuIts of the vapor phase pyrolysis of 83 different hydrocarbons plus several cis-trans isomers are reported. The hydrocarbons include straight and branched alkanes, alkenes, and alkynes and represent compounds which are similar, yet possess sufficient structural differences to determine the usefulness of PGC as a method of identification of chromatographic effluents. The pyrograms are significantly different so that, with the exception of cis-trans isomers, the compounds can be identified. In addition, it is shown that in several instances, compounds which are difficult to distinguish mass spectrometrically are readily identified by their pyrograms. EXPERIMENTAL
Two separate pyrolysis systems were used for the work reported here. Two different systems were used to show that the thermal fragmentation pattern was repeatable, at least in our own laboratory, and to increase the rate at which compounds could be investigated. While identical in basic design, they contain different components obtained from different manufacturers. Both systems consist of a regulated helium supply, gas valve, injection system, vaporizer, variable temperature pyrolyzer, isothermal chromatographic column, splitter, flame ionization detector, recorder, and digital integrator. A block diagram of the system is shown in Figure 1. Liquid samples to be pyrolyzed were introduced through a silicone septum with a Hamilton Syringe (Model CR 700-20).
The gas valve was used to admit known calibration gases to the system. The vaporizer simulated a conventional GC column and was maintained at 200 "C. It was constructed of stainless steel tubing (3 ft by inch 0.d.). In both systems, the pyrolyzer consisted of a 20-inch long gold tube having a 0.040-inch i.d. and wound into a lI2-inch diameter helix. The pyrolyzer is similar to that described by Cramers and Keulemans (6). The gold helix was located inside a nickel cylinder of large thermal mass which was placed inside a combustion furnace controlled with a Wheelco 407 capacitrol coupled with a 610 Pilot Amplifier and a saturable core reactor. The temperature of the pyrolyzer was controlled to within + 5 "C. The systems could be operated and controlled at any temperature in the range 200 to 900 "C. In one system the helium flow rate exiting the pyrolyzer was 50 ml/min. At the three temperatures used for pyrolysis 575, 600, and 625 "C, the corresponding residence timesLe., the time it takes the hydrocarbon to pass through the high temperature region of the furnace-were 1.9, 1.8, and
1.7 seconds, respectively. The analysis column was a 15-foot by lIs-inch 0.d. stainless stee ltube packed with 25 % D C 200 on 60/80 mesh Chromosorb P. The column was operated isothermally at 120 "C and was housed in an Aerograph Model 600-D gas chromatograph containing the flame detector (FID). The column conditions were chosen so that the pyrolysis products from Cscompounds (including the unreacted parent) were separated in less than 8 minutes. The column separated on the basis of boiling point and did not always completely separate structural isomers. In the second system, the helium flow rate leaving the pyrolyzer was 30 ml/min. The residence times in the furnace were 3.2, 3.0, and 2.8 seconds for pyrolysis temperatures of 575, 600, and 625 "C, respectively. A 30-foot by l/s-inch 0.d. DC-200 column on 60/80 Chromosorb P was used for the analysis. The column was operated isothermally at 170 "C in a Perkin-Elmer Model F-11 gas chromatograph equipped with a flame detector. The splitter delivered 10% of the gas stream to the flame detector and 90% was either vented to the atmosphere
Table I. Thermal Fragmentation Patterns Observed from 600 OC Pyrolysis of Alkanes
I
I
Normalized mole-area for a w e n retention index interval
~~
~~~~
"01.40, NO. 14, DECEMBER 1968
e
2169
Peak a b c d e
I g
Ncrrnalizei R.I.
msle.area
CI
10’2
c2
44
c3
90
C4 460
25 12 38
50! 553
16
j
€01 E43 7!0
50 15 7
I(
776
13
h i
h Figure 2. Pyrogram from the 600 “C thermal fragmentation of 2,4dimeth3il-3-ethylpentane
through a restrictor or delivered to the mass spectrometer through a molecular separator (12). The compounds studied were obtained from either Chemical Samples Co., (Cleveland, Ohio) or from James Hinton Inc., (Valparaiso, Fla.). The purity of each sample was determined by decreasing the pyrolyzer temperature to 200 “C and injecting an aliquot of the material into the system. The purity of all compounds was equal to or greater than the manufacturers’ stated value of 99 %. The sensitivity of the flame detectors was determined directly for most of the products reported. For those not directly determined the sensitivity was estimated from the response of similar compounds. The area and elution time of each peak in the program was measured and time recorded with the aid of an Infotronics digital integrator (Model CRS100). The area of each peak was converted into moles. If a particular peak is actually a composite of two similar compounds, such as 1-pentene and 2-pentene, little error is introduced in the determination since the molar response in a flame detector is essentially identical for hydrocarbon isomers. The moles were normalized with respect to the most abundant compound in the pyrogram other than the parent. The normalization technique assures that the fragmentation pattern is preserved even though different amounts of sample may be decomposed. Prior to the pyrolysis experiment, a mixture containing C1 to Clonormal alkanes was introduced. The elution time of all 10 normal alkanes was determined. It was assumed that CH4 was not retained by the column and that its elution time corresponded to the dead time of the system. The elution time of each peak in the pyrogram was converted to a retention index following the method of Kovats (13). The pyrograms (12) M. A. Grayson and C. J. Wolf, ANAL.CHEM., 39, 1438 (1967). (13) E. sz. Kovats, “Advances in Chromatography,” Vol. I, J. C . Giddings and R. Keller, Eds., Marcel Dekker, New York, 1965 pp 229.
Table 11. Thermal Fragmentation Patterns Observed from 600 “C Pyrolysis of Straight Chain Alkenes
2 170
e
ANALYTICAL CHEMISTRY
Table 111. Thermal Fragmentation Patterns Observed from 600
I
O C
Pyrolysis of Branched and Cyclic Alkenes
Normalized mole-area for a given retention index interval
are, therefore, reported in tabular form, the normalized mole areas as a function of Kovats’ Index (25 unit intervals). This procedure tends to minimize differences in analysis procedures by providing an index system which is essentially independent of column conditions. With both systems usually 2-10z of the parent was thermally fragmented. The low decomposition rate minimized secondary reactions. While the chromatograms of the pyrolytic products were not identical in both systems, the pyrogram spectra reported in terms of normalized concentration and retention index (Kovats index) are essentially identical. The chromatogram of one of the more complex pyrolyzates, 2,4-dimethyl-3-ethylpentane,is shown in Figure 2. Both real times and the Kovats’ retention index are shown along the abscissa. Eleven separate pyrolytic peaks are resolved together with the large parent peak at 870 R.I. Although peak c appears to be the largest peak, the sensitivity for CHI, peak a, is much lower and on a mole basis CHd is the largest pyrolytic product. Peaks e, f , and g are Cs compounds, h and i are Cacompounds, while j and k are C? compounds. After one of the gold reactors had been used for more than 1000 separate pyrolysis experiments, it was carefully slit lengthwise and opened. Surprisingly, there was no deposit of carbon. This is contrary to the results reported by Cramers and Keulemans (6) who stated that “cracking reactions are invariably accompanied by carbon formation; Au as reactor material has the advantage that carbon can easily be burnt
I
off with air.” It is possible that a carbon rich varnish is formed on the reactor walls, but there is definitely no carbon black formed. RESULTS The thermal fragmentation patterns observed from the 600 OC pyrolysis of 24 different saturated hydrocarbons are summarized in Table I. The name of the material pyrolyzed is given in the first column and the number of carbon atoms in the compound in the second column. The third column gives the normalized value for methane; the fourth column the value for ethylene and ethane; the fifth column the value for propylene; and the sixth column the value for 1-butene plus 2-butene plus butadiene. The rest of the columns show the normalized value for the product observed with retention index within the interval shown in the top row of the table. In a few cases two peaks are observed within the same interval. For example, in the pyrolysis of 2,2,3-trimethylpentane two products are observed whose retention index is greater than 625 but less than 649; this is indicated by the slanted line and shown as 7/2. The first elutant in the interval has a normalized value of 7 , the second a value of 2. The only saturated products observed were methane and a small amount of ethane. However, the Czpeak always contained much more ethylene than ethane. VOL. 40, NO. 14, DECEMBER 1968
2171
Table IV. Thermal Fragmentation Patterns Observed from 600 O C Pyrolysis of Alkenes (Dienes)
Table V. Thermal Fragmentation Patterns Observed from 600 "C Pyrolysis of Alkynes Normalizea mole-area for a given retention index interval
Compound
1- Hexyne
'50"15C/ 250 350 150 250 350 400 4011 453, '# (cl),(cZ) ('3) lC4' 449 499 6
1
31
I
74IlOG
2- Hexyne
6
97 100
3- Hexyne
6
100 28
4- Methyl-1-pentyne
6
28
,
525 549
550 574
6
6
10 IGO i 7 26
11
700,' 724
7251 750 749
779
7751 800,' 8 5 b >90G 799 849 899
15
j
81
15
2
8
23
3
23
4
!
4
4
1-Nonyne
9
2-Nonyne
9
40 100 5 3 / 38 32 43 100 23 19
3-Nonyne
9
94 100 36
57
4-Nonyne 9 44 100 ___ 65 24 _______ 7-Methyl-3-octyne 9 100 50 91 88
7 '
14
14 9
6
2
52
47
6
15
9
10
j 17
19
45
14
9
22
16 -~ 31
The fragmentation patterns from the 600 "C pyrolysis of 19 straight chain alkenes and 18 branched chain and cyclic alkenes are summarized in Tables I1 and 111, respectively. The largest peak in the spectra of all straight chain alkenes except 3-hexene and 3-heptene is ethylene. The thermal fragmentation pattern from 12 alkenes containing two double bonds (dienes) is shown in Table IV. ANALYTICAL CHEMISTRY
6251 650' 675/ 649 674 699 48
13
2 19
6
0
600j
524
15
4.Methyl-Z-pentyne
21 72
575 599
52
6'100, io
,
1 500 524
39
6
6
6 62
8
8
10
10
6
6
8
11
10
8
8
23
19
13
18
17
18
In none of the examples cited is ethylene the largest peak. In only one case (2,3-dimethylbutadiene) is methane the largest and only with 1,5-hexadiene is C3 the largest. The large peaks observed with retention indices of 500-650 indicate that considerable carbon skeleton rearrangement occurs without C-C bond cleavage. The fragmentation patterns from 9 alkynes is shown in
-
Table VI. Repeatability of the Thermal Fragmentation Patterns from 600 OC Pyrolysis of 1-Hexene
Normalized mole-area for a given retention index interval Run No.
1 2 3 4 5 6 7 8 Avg U
50,' a 150/ 150 250
38.2 38.0 38.2 38.2 38.0 37.9 37.8 40.0 38.2 0.7
100 100 100 100 100 100 100 100 100
-
250/'
3 5 d d 475/e
350
400
90.6 83.7 98.2 90.1 89.5 90.4 88.6 86.5 88.8 2.3
67.9 67.5 68.0 68.1 68.2 68.4
67.8 65.6 67.6 0.9
500/ '
499
524
23.4 23.9 23.8 23.3 23.8 24.2 23.8 21.0 23.3 0.9
3.4 4.1 3.8 3.5 3.9 4.4 4.4 5.4 4.1 0.6
Table V. In the three 1-alkynes studied, a large amount of propyne was observed. With the chromatographic conditions used in our experiments, propyne is resolved from propylene and the ratio of these compounds is indicated in the C3column as propylene/propyne. The fragmentation patterns observed from the pyrolysis of 1-hexene at 600 "C from 8 separate determinations is shown in Table VI. These data were obtained over a period of 2 weeks. The repeatability of the normalized molar areas is excellent; in fact, it is necessary to use 3 significant figures to show the deviation. The average value of the 8 determinations is shown in the second last row and the standard deviation (G-) is shown in the last row. The per cent deviation from
the average value for C1, C3, C4, 1-pentene (peak with retention index 480), and pentadiene (peak with retention index 520) is 1.8, 2.6, 1.3, 3.9, and 15%, respectively. All of the 83 hydrocarbons shown in Tables I through V can be readily identified from their thermal fragmentation patterns. The only class of compounds which cannot be identified from their pyrograms are cis-trans isomers. This is indicated in Table VI1 where the fragmentation patterns from cis- and trans-2-hexene; cis- and trans-3-hexene; and trans-2-trans-4, cis-2-trans-4-, and cis-2-cis-4-hexadiene are shown. The pyrograms from the cis-trans isomers are virtually identical and are not useful for qualitative identification. However, these results do give a good indication of the repeatability of the thermal fragmentation technique. There is, however, a small difference in the amount of parent which decomposes at a given set of experimental conditions. The trans isomer is usually slightly more thermally stable than its corresponding cis form. The effect of pyrolysis temperature on the thermal fragmentation pattern of 4-methyl-2-pentene and 1-hexene is summarized in Table VIII. When the temperature in the pyrolytic reactor changes, the residence time of the hydrocarbon in the high temperature region also changes, so that two changes are produced simultaneously. Over the limited temperature range studied, the fragmentation pattern varies only slightly indicating that small variation in either the pyrolysis temperature or the residence time need not be considered. It is important to note that the per cent of parent converted into products increases with both temperature and residence time, but the normalized fragmentation pattern remains the same. Sutton and Harris (8) reported that air had the effect of increasing the amount of hydrocarbon decomposition. When air was present, they observed that the amount of decomposition was equivalent to that observed at a temperature 10-20°C greater than in the absence of air. In several experiments dry nitrogen was bubbled through liquid 1-hexene to remove all the dissolved air. A small sample (1 pl) was then carefully injected so that air was not present during the pyrolysis. The fragmentation pattern is identical in the presence and ab-
Table VII. Comparison of Thermal Fragmentation Patterns from 600 OC Pyrolysis of cis and trans Alkene Isomers
Cis-2.trans-4 hexadiene
31
10
3
7
13
36
100
Cis-2-C IS-4 hexad iene
31
10
1
4
14
39
100
VOL. 40, NO. 14, DECEMBER 1968
e
2173
3 IMelhyl.2.perlene
2 Melhyl.2.penlene
r
I
.,.
I
3.3 Divrlhyl+bileie
I,.
82
3.3 0ime:hyi.l.Mene
i
r 62
2:u 2C
c
130 130
205 205
930 930
403 403
533 533
6CC 6CC
730 730 Re1enl;cn Index
Figure 4. Pyrograms of six different CeHlz isomers spectrometry to describe a spectrum can also be used to describe a thermal fragmentation spectrum. As in mass spectrometry where the peaks with large mass-to-charge ratios are of particular importance for identification, the peaks with large retention indices are also of interest. That is, in hydrocarbon analysis mass-to-charge peaks occurring at 15, 27, and 43 are of little use for identification because most hydrocarbon spectra show large values for these 3 peaks. Similarly, in hydrocarbon pyrolysis CI, CZ,and CSare usually large. However, the absence (or small value) of any one of these three is important to note. The relative ratio of the C1,Cz,C8,and C4products can give a good indication of the degree of branching in the parent compound. For example, the pyrolysis of cyclohexene produces essentially no CH4 (see
Figure 3. Mass spectra of six dif€erent C~HIB isomers sence of air and, contrary to the findings of Sutton and Harris find no enhancement of methane in the presence of air.
(a), we
DISCUSSION
The spectra tabulated in Tables I through V are sufficiently different so that, with the exception of cis-trans isomers, the compounds are uniquely characterized. The identification technique is basically similar to that used in mass spectrometry although the physical processes in the two methods are entirely different. Hence, much of the terminology used in mass
Table VIII. Effect of Pyrolysis Temperature on Thermal Fragmentation Pattern
c
I
Normalized mole-area for a g i v e n r e t e n t i o n index i n t e r v a l
350:'
400 (C,)
33
2-pentene
31 34
73
-
401/449
-* 4501499
-
70
25
66 -
21
500/524
525/549
550/574
3
I
5 ~~
174
o
ANALYTICAL CHEMISTRY
~
Product
c1 cs c 4
cs C6
+ cz
Table IX. Comparison of Pyrograms from Three Independent Laboratories +Hexane Theocis-2-Hexene 1-Heptene This worka Ref (6)b Ref (7)c reticald This worka Ref (Qh This work5 Ref (7)” 100 100 100 100 100 100 100 100 33 30 40 22 12 12 44 52 19 15 21 17 47 32 48 45 6 4 5 6 46 27 12 10 13 3
1-Octene This worka Ref (7)c
c 7 5
E
100
100
43 31 23
54 24 28
8
4
3
3
Helium carrier gas, pyrolysis temperature 600 “C in Au tube, 4-5 decomposition. Nitrogen carrier gas, pyrolysis temperature 577 “C in Au tube, 30-60z decomposition. Helium carrier gas, pyrolysis temperature 650 “C in quartz tube, approximately 25 decomposition. Calculated from the Kossiakoff-Rice theory (16).
Table 111) while the highly branched alkane 2,2-dimethylbutane forms only C1, Ca,and C, products (see Table I). In many cases, it is extremely difficult to differentiate between structural isomers with a mass spectrometer. The mass spectra of six CbHIZisomers (2-, 3-, and 4-methyl-2-pentene; 2-ethyl-1-butene; 3,3-dimethyl-1-butene ; and 2,3-dimethyl1-butene) are shown in Figure 3 in bar graph form where the relative intensity (normalized with respect to the largest peak in the spectrum) is given on the ordinate and the mass-tocharge ratio (?Hie) is plotted along the abscissa. The data for this figure were taken directly from Cornu and Massot (14). The spectra are all quite similar with large peaks occurring at mje values of 41, 69, 84, 39, 27, and 42 and it would be difficult to positively identify which one of the compounds gave rise to an unknown spectrum. Although the large peaks at 53, 55, and 56 probably would distinguish 3-methyl2-pentene and 2-ethyl-1-butene from the other compounds, these two compounds could not be distinguished from each other. The pyrograms can be compared in an analogous manner, that is, the relative peak intensity is plotted as a function of retention index. The bar graph pyrograms of the same six compounds are shown in Figure 4. The pyrograms of all six compounds are entirely different so that positive identification from the thermal fragmentation pattern is straightforward. It is interesting to compare the results reported in this work with other similar studies. Cramers and Keulemans (6) and Levy and Paul (7) investigated the vapor phase pyrolysis of organic compounds while in a carrier gas and flowing through a reaction chamber. No attempt was made to obtain interlaboratory reproducibility and although the basic experiments are the same, many differences, such as reaction temperature, carrier gas, and total % decomposed, are to be noted. A comparison of the 3 studies is shown in Table IX. The data reported by Levy and Paul are in terms of chromatographic peak areas. This data was recalculated in terms of a mole response using the sensitivity factors given by Dietz (15). The qualitative agreement among the three laboratories is quite good and suggests that when standard procedures are adopted, thermal fragmentation spectra between independent investigators may be as reproducible as are mass spectra. The theoretical product distribution calculated for the decomposition of n-hexane is shown in the fifth column. The technique developed by Kossiakoff and Rice (16) for (14) A. Cornu and R. Massot, ‘Compilation of Mass Spectral Data,” Heyden and Son Ltd., London, 1966. (15) W. H. Dietz, J . Gas Chronzatogr., 5, 68 (1967). (16) A. Kossiakoff and F. 0. Rice, J. Ameu. Chem. SOC.,65, 590 (1943).
thermal decomposition assumes that a chain reaction is initiated by the breaking of the parent molecule into two free radicals. The free radicals then abstract a hydrogen atom from another molecule of the parent forming a new large free radical. The large radical decomposes rapidly and unimolecularly in certain definite ways. Since C-C bonds are weaker than C-H bonds, the split occurs at a C-C bond and not at a C-H, C=C, or CEC bond. The small free radical thus formed continues the chain; only the chain propagating steps need be considered in order to determine the product distribution. Three additional assumptions are made: the rate of decomposition of the large alkyl radical is faster than a bimolecular reaction with another radical, the exact nature of the radical initially formed by the abstraction reaction depends on the relative ease with which hydrogen can be removed from different sites in the parent molecule, and the free radicals formed by these processes undergo C-C bond rupture at the bond @ to the carbon atom from which the hydrogen is missing. Further, they assume that the preexponential factor is the same for all hydrogen abstraction reactions and that the removal of a secondary hydrogen requires 2 Kcal less activation energy than a primary hydrogen while abstraction of a tertiary hydrogen requires 4 Kcal less energy. Thus, at 600 “C, the relative rates of hydrogen abstraction from a primary, secondary, and tertiary position are in the ratio of 1:3.2 :10.1, respectively. Therefore, the initial rate of formation of each type of radical is taken to be proportional to this ratio times the number of C-H bonds of that type present in the parent molecule. The agreement between the three laboratories and with the Kossiakoff-Rice theory for n-hexane decomposition is good. The interlaboratory reproducibility of the thermal fragmentation patterns and the agreement with the theoretical predictions of the Kossiakoff-Rice theory indicate that the vapor phase pyrolysis gas chromatography of hydrocarbons can be used for qualitative identification. The technique is of particular value when it is necessary to identify extremely small amounts (of the order of g) of compounds which are difficult to identify by other sensitive methods, such as mass spectroscopy. ACKNQWLEDGMENT
We greatly acknowledge the technical assistance of Thomas C. Harby, Jr. We are also indebted to Michael A. Grayson for his help with the mass spectrometer. RECEIVEDfor review June 17, 1968. Accepted September 19, 1968. VOL. 40, NO. 14, DECEMBER 1968
rn
2175
