Fragmentation Gas Chromatography J. C. STERNBERG, 1. H. KRULL, and G. D. FRIEDEL eeckman Instruments, Inc., Fullerton, Calif.
b
Qualitative characterization of individual component peaks separated by gas chromatography has been based upon retention data, or upon the use of such nonchromatographic identification techniques as infrared, nuclear magnetic resonance, or mass spectrometry. This work demonstrates a new and entirely chromatographic technique, employing an electrical discharge at atmospheric pressure within a GC system for qualitative identification based on the principle of characterization of structures by their breakdown fragments, as in mass spectrometry. The fragmentation patterns obtained with this technique were highly indicative of the structure of the investigated molecules. Fragmentation patterns obtained using different carrier gas mixtures as the discharge atmosphere produced patterns different from those obtained with a pure helium atmosphere. For molecules with certain functional groups, a comparison of the patterns obtained with pure helium and with a hydrogenhelium gas mixture was particularly informative.
F
many years the fragmentation of substances has been examined as a possible qualitative analytical tool; these techniques have reached their highest development in mass spectrometry. Since the advent of gas chromatography, investigators have searched for a qualitative tool compatible with the chromatographic system, and it has become apparent that pyrolysis is an effective basis of an analytical system which can be employed in this connection. I n mass spectrometry, fragmentation is produced a t low pressures by bombardment with electrons of controlled energy. Some investigators have attempted to expand the use of fragmentation to atmospheric or higher pressures and to obtain the patterns or “spectra” of molecules from the stable products found, rather than from the charged particle measurement needed in inass spectrometry. There are essentially five methods of fragmentation-photolysis, radiolysis, sonic degradation, thermal cracking (pyrolysis), and the electrical discharge methods. The first three tJechniques are rather specialized and not widely used as general methods. Investigators OR
El
SOLMOID
RECORDER
ELECTROMETER
~ O U T P. U T I H F I I ..
blRH
H F
”... -
I -
I I - L ~ I . ~ I 1 9
-AIR
H2 FLAME I OUTPUTAIR
-ELECTROMETER
D-
CONTROL
I
VARIAC
-HE
RECORDER
ELECTROMETER
TRANSFORMER INJECTION PORT
THERMOTRAC
OUTPUT ELECTROMETER
(H.F.I11
COLUMN EFFLUENT
Ha + SAMPLE
M THERMOTRAC
Figure 1.
Fragmentation chromatograph
have used various means for providing the energy in thermal pyrolysis, such as the coating of a hot wire with a liquid or solid, or the feeding of a sample into a heated cylindrical tube. Investigations have been carried out using samples from each of the three states of matter, although the means or methods employed by various experimenters have been different. The results have been interestmg and in some ways similar. Chromatography has been used fairly extensively to detect the fragments formed during pyrolysis, particularly of nonvolatile samples. The separating power of gas chromatography serves well to analyze the mixture of substances formed during pyrolysis of a given substance, so that the combination of the two systems affords a highly potent tool for qualitative identification. Keulemans et al. (5) were among the first to incorporate the two systems into a single unit by having individual sample components flow through a heated tube into a chromatographic system. The present investigation used the electrical discharge in a similar manner; however, in this study the discharge, unlike Keulemans’ pyrolyzer, was placed within the chromatographic system, so that purification or isolation of the sample species of interest and identification of its breakdown fragments both could be accomplished within a single system. EXPERIMENTAL
Apparatus. The fragmentation chromatograph (see Figure 1) employs the Beckman D Analyzer [equipped with the standard thermal
conductivity cell (TC), two hydrogen flames (HF) with their electrometers, two Reckman Thermotracs, three 1-mv. potentiometric recorders, a high-voltage transformer capable of furnishing 15,000 volts and 30 ma., and a 10-watt Variac]. The oven compartment of the D Analyzer serves as a focal point of the system housing the three detectors (TC, H F KO. I, and H F KO. 11), the discharge chamber, and most of the columns with their associated valving. I t is connected to the first Thermotrac, where the sample is injected and vaporized, the second Thermotrac, where the temperature programming of the alumina column occurs, and two electrometers. The Variac and high-voltage transformer provide the power for the fragmentation discharge. The electrical outputs of both hydrogen flames are fed to the electrometer,, where they are amplified and transferred to two of the recorders for readout. The thermal conductivity cell output is displayed on the third recorder. h device now available on the GC-4 as the fragmentation inlet and its associated equipment can conveniently be used in the fragmentation chromatography of solids as well as gases and liquids. One of the column configurations employed (see Figure 2) made it possible to provide a fairly low pressure a t the discharge while still determining the hydrocarbon fragmentation products. The vapor sample flowed through the silicone column C-0 (located in Thermotrac I) to the thermal conductivity cell. In the sampling system, the thermal conductivity cell, TC, provided data on the purity of the sample and the timing needed to pick off a sample peak from column C-0. This made it possible t o separate the sample vapor from air before it reached VOL 38, NO. 12, NOVEMBER 1966
1639
t,he discharge. After the compound was fragnient'ed in t'he discharge, the products were distributed between C-1 and C-2 (the Apiezon and alumina columns) leading to t'he flame detectors HF-2 and HF-1, respect'ively. A second standard Beckman Thermotrac provided a means of programming the temperature of t'he alumina column. The alumina column was programmed at a rate of 47" C. per minute for the first 2 minutes, 9' C. per minute for the next 3 minutes and 1.3' C. per minute thereafter, until all components up to and including C6 hydrocarbons were observed on the recorder. DETECTIONASD READOUT. Three detectors were employed in the fragmentation system. Two of these detectors were standard Beckman laboratory hydrogen flame units with their associated electrometers. One hydrogen flame detector was always connect.ed to the alumina column, while the other provided t'he detection of the effluent, of the Apiezon column. The third detector was the t'hermal conductivity cell, provided with and located in the D Analyzer. The TC cell was connected to a 1-mv. Brown recorder, and the two HF outputs were fed to 1-mv. Brist,ol recorders. Discharge. ELECTRICAL CosFIGURATION. Primary voltage to the power transformer used for the discharge was controlled by a Variac. Discharge current was limited by the use of a 1-megohm power resistor installed in the high-Yoltage lead between the power transformer and the discharge; discharge voltage was monitored with a voltmeter. The two means of current me ployed are shown in first employed a true r.m.s. milliammeter in t'he ground leg of the t'ransformer, while the other used a voltmeter across a 35-kilohni resistor in series with the discharge. CONSTRUCTION. Four main considerations governed the design of t'he discharge device (see Figure 4): the need for minimization of dead volume to prevent excessive spreading of the sample slug; t,he requirement that' the highvoltage electrode and connections be adequately isolated from ground to withstand the increase in breakdown
R- I
-3
n
He SUPPLY
I
1
c- I
H 2 +OTHER ADDED GASES
He MAKE UP FLOW
Figure 2.
Figure 3.
1640
n
'14" l/g0
l/m"
silicone (Thermotroc I) Apiezon alumina (Therrnotrac II)
voltage that occurs when the sample enters the gap; the desirability of making the reaction chamber (region between electrodes) disposable t'o eliminate deposit,s which build up during use and which may interfere with the fragmentation process; and, finally, the desirability of providing the ability to see within the discharge so that the reaction could be visually monitored. The silica tube B which served this last purpose was mounted with insulation washers of Teflon, C, bet'ween the stainless steel rear end plat'e, D, and a brass cup, E , which fit into t'he insulator of Teflon, F . The latter joined the front end plate, G , of stainless steel. Platinum wires, H, served as electrodes; one was silver-soldered to the brass cup and one wound helically and held by compression in the channel of the rear end plate. The rear end plate was tapped to receive the t'hree mounting bolts, I , with which the insert' is assembled. Current mas supplied from the secondary of the power transformer via a bolt', J , which contacted the brass cup or t,he platinum mire soldered bo the brass cup. Standard '/*-inch Swagelok fittings were welded to each end plate, permit'ting easy installation. Two types of discharge t'ube were employed in this study-an open tube and a constricted t'ube. A fused quartz t,ube, having the same 2-mm. diameter throughout, contained the platinum electrodes and served as the open form of the reaction chamber. The constrickd tube shown in Figure 4 as B' had the same outside shape as that of the open tube and started as a tube of the identical inside diameter. The wall t,hickness \vas slowly increased so that t,he inside diameter was diminished to approximately 0.75 mm.
This decrease in diameter provided a region of high linear gas velocity, and permitted the discharge to fill the cross section of the tube. The electrodes in the constricted tube were placed as closely as possible to the decreased diameter region without contacting the tube. DISCUSSION OF RESULTS
With the open tube it was originally hoped that the amount of secondary breakdown could be limited by the use of electrodes oriented to provide a discharge perpendicular to, rather than along, the tube axis, thus limiting the residence time of the sample within the gap itself. However, the amount of actual sample contacting the discharge was only 10 to 20Qj, of the entire sample injected. Since roughly 10% breakdown was obtained by exposing merely 10% of the compound to the discharges, almost 100% breakdown of the portion of the sample in contact with the discharge gas was occurring. It therefore seemed desirable to change the electrode configuration to provide more efficient use of the discharge. Use of the constricted tube (see previous section) permitted the exposure of virtually the total sample to the discharge. The increase in breakdown, holding all parameters constant with the exception of the discharge tube configurations described above, is indicated below for 2,3-dimethylbutane. With the open tube, 7% fragmentation of 2,3-dimethylbutane was observed. With the constricted tube possessing an inside diameter of 0.030 inch (constricted portion)
P ""'$0
D
Electrical configuration (ax.) ANALYTICAL CHEMISTRY
Column configuration
Column 0, 8' X Column 1 , 8' X Column 2, 8' X
R-l
115 L V
AIR
C- 2
Figure 4.
Fragmentation discharge
5470 breakdown was noted at the same current. The diameter of the con-
stricted portion of the discharge tube is very critical, with a decrease in the diameter leading to an increase in fragmentation. The breakdown patterns obtained with the constricted tube and the open tube are essentially similar. Less current is required in the constricted tube to obtain the same per cent breakdown. When the same current density is used with the different tube configurations, both the patterns obtained and per cent breakdown are similar, and fragmentation patterns can then be compared on an equivalent basis. In practice, little seems to be gained by employing a constricted tube and diminishing the current as opposed to using the open tube with the electrode gap oriented perpendicular to the tube axis. Effect of Operating Current. The effect of operating current on the per cent breakdown has been investigated s i t h a constricted discharge tube and utilizing 2,3-dimethylbutane as the sample. Figure 5 displays the increase in the extent of fragmentation observed with increasing operating current, with all other parameters held constant. A similar curve shape was obtained when the open tube was eniployed. Figure 6, A and B , shows the changes in i,he numbers of moles of the individual components formed per unit amount of sample a t various current levels. The fragmentation patterns observed with rising current and increasing breakdown show several characteristic changes. The continuous rise in the amount of CH4 and C2H2 formed indicates that these are products of secondary breakdown, which also increases with increased current. The increase of secondary breakdown with increased current is also indicated by the decline of the larger CS fragments. Many of the components show a constant number of moles formed per unit amount of sample over a wide range, indicating that the operating current
100
90
80
70 e./
B R E A 6o
K D
0
w
N
50
40
30
2c
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0
I
I
10
6
30
20 C U R R E N T , MA
Figure 5. Per cent breakdown vs. current
within this range need not be held to a precise setting (see Table I). The ratio of CzH4 C2H6,designated as Cz, to C3H6 CaHs, designated as C3, is a main factor in tlhe characterization of isomeric 6: ( compounds; the C2/C3 ratio continuously rises with increasing operating current, showing a tendency for Cz rather than C3 fragments to be formed from secondary breakdown. Effect of Carrier Gas Composition. The effect of gas composition was
+
studied by employing several different gas mixtures for the discharge. The breakdown voltage of a gas discharge depends on the pressure of the gas in the discharge, the length of the gap between the electrodes, and the gas composition. The characteristic current and voltage wave shapes of an a.c. discharge are shown in Figure 7. The voltage curve shows the initial rise of the voltage to the breakdown potential characteristic of the gas
+
Table I. Effect of Current on Fragmentation Pattern of 2,J-Dimethylbutane
120 1.4 0.18 0.11 0.82 0.084 0.15 220 0.15 0,097 320 1.4 400 2.7 0.61 0.31 0.73 500 3.0 0.35 0.82 600 3.9 0.45 0.72 4.1 0.43 700 0.82 4.6 0.50 800 0.86 4.2 0.50 900 0.96 0.62 1000 5.9 0.83 9.1 0.73 1400 Constricted discharge tube.
0.38 0.25 0.19 0.54 0.70 0.73 0.60 0.74 0.69 0.71 0.53
0.54 0.33 0.33 1.1 1.1
1.2
1.1
1.2
1.1 1.1
0.93
0.39 0.29 0.28 1.1
1.2 1.6 1.5 1.7 1.8 2 . Q n
+0
1644
ANALYTICAL CHEMISTRY
I
cluded straight-chain (n-hexane and n-octane), branched-chain (2,2- and 2,3-dimethylbutane), and alicyclic (cyclohexane) hydrocarbons. I n the data obtained with helium carrier and presented in Table IV the most significant differences appear to arise in the relative amounts of CZ (C2H4 C2H6,as before) and C3 (C3Hs C3Hs) fragments formed; as might be expected, the ratio of C2/C3gives an indication of chain branching. Thus the straightrchain compounds n-hexane and n-octane show very similar total Cz to C3 ratios of 1.5 and 1.7, respectively; the branched chain 2,3-dimethylbutane, which should cleave preferentially to give C3 fragments gives a ratio of 0.47 and the branched isomer 2,2-dimethylbutane, which should yield Cp preferentially to C3, gives a ratio of 2.8. The ratio thus indicates not only the presence but also the position of branching. The Cp and C3 ratios of olefin to saturated among the unbranched paraffins studied also indicate the carbon to hydrogen ratios in the parent compounds. When less hydrogen is present, higher olefin to saturate ratios are found. Thus the Cz and C3olefin-saturate ratios of cyclohexane are 5.0 and 2.5, respectively, whereas n-octane yielded ratios of 2.4 and 1.8 and n-hexane yielded ratios of 1.6and 1.5. Fragmentation of paraffin hydrocarbons with 10% hydrogen in helium carrier did not furnish fragmentation patterns more easily interpreted than those obtained in helium carrier alone. To some extent characteristic differences tended to be masked rather than amplified by the presence of hydrogen. One differing feature, however, is the CSproduction of 2,2- and 2,3-dimethylbutane with helium and helium plus hydrogen carriers. The C5 production from 2,3dimethylbutane remains essentially the same, 0.061 in helium and 0.066 in helium plus hydrogen carrier. However, the CS production obtained from the 2,2-dimethylbutane fragmentation patterns are extremely different, 0.016 with helium and only 0.0025 in helium plus hydrogen. Thus the Csformation in 2,3-dimethylbutane appears to be a primary step, occurring upon cleavage of any of the four equivalent methyl groups, while the C5 production from 2,2-dimethylbutane must result primarily from a recombination step, which is quenched by the presence of excess hydrogen. An investigation was undertaken to determine which C4 product was predominant in the patterns of these two molecules. The evidence found indicates that 2,2-dimethylbutane produces primarily normal C4 products, while isoC4 products predominate in the breakdown of 2,3-dimethylbutane. Results obtained by Maillard, Deluzarche, and Gruseck-Lutz (3) on the electrical discharge breakdown of the liquid 2,2- and
+
+
2,3-dimethylbutanes, in a long-contact time batch system very different from lhat used here, showed characteristics of the breakdown pattern similar to those obtained in the present vapor phase fragmentation chromatographic study. Aromatics. Two aromatic compounds, chlorobenzene and benzene, were run, and they showed a breakdown level which is distinctly characteristic. Because of the benzene ring stability, the exit of breakdown is appreciably lower for these compounds (see Table IV) than for all other compounds run under the same discharge conditions. Chlorobenzene, as would be expected, produces a large quantity of benzene upon fragmentation. Aromatics are also distinguished by the high proportion of unsaturates in their fragmentation patterns. Thus Cz olefin-to-saturate ratios found for chlorobenzene and benzene are 30.4 and 13.4, respectively, as compared to a ratio of 5.0 for cyclohexane. This characteristic of a high proportion of unsaturated products is partially destroyed with helium carrier containing 10% hydrogen which tends to produce saturation of the fragments. Functional Group Effects. The effects of oxygenated functional groups on fragmentation patterns were studied by comparing dimethyl ether, sec-butyl alcohol and 2-butanone. These compounds show differences in C3 production, falling in the order sec-butyl alcohol (0.127), 2-butanone (0.073), diethyl ether (0.050). This order can be explained by considering the possible modes of C3 formation for the three compounds. The recombination of Cz and C1 radicals to form a C3 product is more probable in sec-butyl alcohol, because of its greater methane production and the relatively favorable cleavage of the C-0 bond, leaving a radical which can disproportionate to a C3 product. The greater C-0 bond energy in 2-butanone and its lower yield of methane during fragmentation makes the corresponding process improbable for 2-butanone. The low C3 production from diethyl ether results from the essential impossibility of heteroatom extraction in this molecule. The C4 production is higher in the fragmentation pattern of diethyl ether (0.41) than for sec-butyl alcohol (0.11) or 2-butanone (0.064). Sec-butyl alcohol can form a C4 product either by hydroxyl group extraction or by dehydration, as well as by recombination. 2-Butanone can form C4 fragments by recombination and by heteroatom extraction; however, this extraction is more difficult than in secbutyl alcohol because of the greater carbon-oxygen bond energy in a carbonyl than in a carbinol group. The only possible formation of C4 from diethyl ether fragmentation is from recombination. The higher C4 production
from diethyl ether is understandable upon examination of its Cz production with helium plus hydrogen carrier (see Table V), which is seen to be appreciably greater (0.41) than that given by secbutyl alcohol (0.22) or Zbutanone (0.24). The C4production with heliumhydrogen carrier is higher for sec-butyl alcohol (0.074) than for 2 butanone (0.012), with both being appreciably lower than found with pure helium carrier; diethyl ether produces no detectable amount of Ca with heliumhydrogen carrier. It can thus be concluded that the Czrecombination, which contributes a large portion of the Cc yield in pure helium carrier, is virtually quenched by reaction of the Cz radicals with hydrogen when helium-hydrogen carrier is used. In an additional study of compounds containing various functional groups, five related compounds were compared with 2-methylbutane. The per cent breakdown increased slightly with the presence of olefinic groups in the molecule, while the presence of halide (chlorine) and hydroxyl groups diminished the per cent breakdown observed (see Table VI). A most important characteristic of the fragmentation of hydrocarbons containing halide and hydroxyl groups is that of the production of a large quantity of the parent chain or carbon skeleton; thus, the last peak to elute prior to the parent molecule itself, and containing an equal or greater quantity of material than previous peaks, can almost certainly be assumed to be the parent chain of the molecule. Fragmentation in helium carrier containing 10% hydrogen provides very useful information on the presence of functional groups in the sample molecules. Thus, olefins are found to be saturated to a significant degree to their parent saturated hydrocarbon chain (see Table VII), and other functional groups such as chloride or hydroxyl also tend to be extracted to leave the parent hydrocarbon. In addition, the fragments formed in the principal cleavages of the molecules tend to become “frozen” in structure by simply adding hydrogen to become stable molecules. In these cases a close parallel is seen between fragmentation in the presence of hydrogen and the “carbon-skeleton” hydrogenation method of Beroza and coworkers ( 2 ) . The two olefinic molecules studied, 2-methyl-2-butene and 2-methyl-1-butene, showed fairly significant amounts (28.0 and 7.9%, respectively) of their direct hydrogenation product, 2-methylbutane. In addition, the large distinction in c2/c3 ratio found between the two samples in straight helium carrier became essentially nullified in the helium-hydrogen carrier system (see Tables VI and VII). These observaVOL 38, NO. 12, NOVEMBER 1966
1645
8 d
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c c 3 0
a 3 d C
0 L a
A
3 .-E"
Ea c
1646
0
ANALYTICAL CHEMISTRY
tions suggest that in the presence of hydrogen the primary attack on the molecule is by a hydrogen atom, which forms identical radicals from the two structures if attack occurs a t the more favorable end of the double bond, away from the tertiary carbon atom in the sample molecule; when this occurs, both structures give the same distribution of products, including identical C2/C3 ratios. The less probable initial attack by the hydrogen atom a t the tertiary carbon atom leads to a more reactive radical with a higher probability of attaching further hydrogen to form the parent 2-methylbutane structure. The initial attack a t the trrtiary carbon is less favorable in 2-methyl-l-butene, where an unstable primary radical would form, than in 2-methyl-2-butene, where a more stable secondary radical would form; this accounts for the greater saturation of,the latter olefin (see Table VII). The increase in the breakdown of 2chloro-2-methylbutane was particularly great with helium and 10% hydrogen carrier. The fact that chlorine located in the tertiary position has a lorn bond energy ( 7 ) may explain this greater ease of fragmentation. This is confirmed in Tablr VI1 by the percentages of the functional groups evtracted in the fragmentation of the two chlorinated molecules. The larger per cent extraction in the halide molecule> relative to the alcohols is probably due largely to greater strength of the C-0 bond relative to the C--Cl bond. METHODS OF DETERMINING MOLECULAR STRUCTURE
Investigators in pyrolysiq and other fragmentation studies have used various means of identifying a substance by the fragmentation pattern it yields. However, as in mass spectrometry, the principal method of identification is the comparison of its pattern with patterns obtained from various known compounds. I t is desirable to be able to perform this comparison with the smallest possible number of previously obtained patterns. Comparison of Results to a Plot of Moles of Product Formed/Total Parent Moles Fragmented us. Peak Concentration of Sample at Discharge. One practicable method of comparison of fragmentation patterns is through the comparison of logarithms of the mole ratios of individual products to the total parent moles fragmrnted. The method would be ideal if the log plot of this ratio us. peak concentration yielded a set of parallel curves (preferably with slopes of zero). However, the curves are similar, particularly within the center of the concentration range, but not parallel. Therefore, the fragmentation pattern of a molecule is not completely independent of sample size, and a single set of numbers (break-
down product/total parent moles fragmented) may be insufficient t o provide a comparison. If, in these cases, the set of calibration curves as a function of sample peak concentration is available, a characterization can still be made. The comparison of data on an unknown sample with known references would proceed as follows. The peak areas of the breakdown products for the unknown sample are obtained and laid off on a logarithmic scale. It is unnecessary to divide by the total breakdown, because this represents only a vertical displacement of the entire logarithmic pattern. This scale is then compared with the calibration curves by matching the ordinate for a particular fragment,and sliding the point along the appropriate curve until a simultaneous match of all the fragment points to their corresponding curves is obtained. If the peak concentration is known, however, the comparison need be made only a t this concentration. It appear9 then that a general method for characterization has been found. It is probable that it can be based on threeor four-point (sample concentration) calibration data, and the graphical matching procedure should be quite rapid; if desired, automation could even be employed. Comparison of Fragmentation Patterns in Helium and Helium-Hydrogen. Much information is required to produce a clear characterization of the molecular structure. The use of Comparison identification techniques in characterizing structure in the fragmentation analytical method makes possible a partial identification, even without a complete understanding of a large number of the variables involved. Work in the fragmentation field is progressing, and it is conceivable that the future may hold the key for a more complete understanding of fragmentation. In the meantime, it is very helpful to augment the basic method by the use of wpplemental techniques which give further structural information. In particular, it has been found here that the use of hydrogen-helium mixtures as the discharge carrier leads to changes in patterns (see Tables IV and V) which can produce conclusive evidence for the identification of molecules containing functional groups. The comparison of results obtained with and without addition of hydrogen to the carrier gas is also helpful, although much less definitive, in the characterization of hydrocarbon compounds. Various Other Comparisons. The importance of the Cz/C3 ratio, demonstrated previously in relation t o the isomeric C6 compounds, can be utilized in other compound comparisons as well. The olefin-to-saturate ratios and the per cent breakdown also provide information helpful for
a comparison. These three items may not produce a complete match by themselves but may provide the extra evidence needed in labeling an unknown. Determination of Structure from Fragmentation Patterns and Prediction of Patterns from Structure. A complete prediction of fragmentation patterns from molecular structure would require the thorough understanding of the fragmentation process. Since the reactions in the discharge or pyrolysis fragmentation of molecules are obviously not thoroughly understood, it would seem to be impossible to predict the fragmentation patterns. However, certain portions of the fragmentation patterns appear to be reasonably predictable from a combination of theory and experience. Portions of the patterns obtained from 2,2-dimethylbutane and 2,3-dimethylbutane are completely compatible with present theory-e.g., the greater C3 than Cz production from the 2,a-isomer (see Table IV) and the greater Cz than Cs production from 2,2-dimethylbutane. The breaking of the tertiary bond, the weakest structural link ( 7 ) in the 2,2isomer, leads to this greater C? yield. In 2,3-dimethylbutaneJ the breaking of the sterically weaker C3H7-C3H7 secondary bond produces the relatively high C3 content. The extremely high C2/Ca ratio observed in the fragmentation pattern of diethyl ether (see Table IV) was predicted prior to the experimentation, since the C2HsO-C2Hs bond has the greatest tendency t o rupture, forming Cs fragments directly, while all C3 fragments must arise from recombinations. From the position of the oxygen on 2-butanone1 a high C2/C3 ratio was expected and was obtained. From the comparative energies of the carbonoxygen bonds in sec-butyl alcohol and in 2-butanoneJ higher heteroatom extractions would be expected in the alcohol than in the ketone. This was indicated by the relatively lower C4 production (see Tables IV and V) obtained in the fragmentation pattern of 2-butanone. The olefin-to-saturate ratios in cyclohexane were greater than found for any other aliphatic compound employed (see Table IV). The hydrogen deficiency of cyclic compounds compared with openchain molecules accounts for this greater olefin fragment population. Of greater importance to the analytical field, however, is the determination of structure from fragmentation patterns obtained from unknown molecular species. The arguments presented in determining patterns from structure apply conversely to the interpretation of fragmentation patterns. Wherever a comparatively weak bond occurred, the fragments produced indicated the relsr tive position of the bond within the molecular structure. VOL 38, NO. 12, NOVEMBER 1966
1647
The fragmentation pattern of an aliphatic hydrocarbon provides less of a lever for interpretation. However, chain branching can be recognized in the fragmentation pattern, as it was in the lower C2/C8ratio in the fragmentation pattern of 2,3-dimethylbutane. It should also be possible to distinguish between terminal branching and branching along the chain. CONCLUSION
The fragmentation techniques can be usefully employed a t several different levels of sophistication of instrumentation and of interpretative effort. , Thus, in its simplest form, fragmentation may be used to furnish a fingerprint-like pattern of a compound for identification by comparison with patterns obtained for known compounds; this is essentially the mode of operation in solids pyrolysis, and may be employed with
liquid or gaseous samples as well. In a more refined form, a more thorough study and more complete column system may be employed to furnish a pattern of identified peaks, from which the structure of the molecule may be inferred by mechanistic considerations and correlations with patterns obtained for related compounds. Finally, the enterprising physico-organic chemist may wish to employ the technique to attempt to obtain basic information on relative bond strengths and mechanistic influences. The value of the fragmentation system will certainly appreciate as more information is obtained on the mechanism of the fragmentation processes and as a greater number of compounds have been investigated. Similarly to mass spectrometry, the value of the atmospheric pressure discharge fragmentation method will increase with the growth of libraries of breakdown patterns.
LITERATURE CITED
(1) Benson, S. W., “The Foundations of Chemical Kinetics,” pp. 662-3, McGraw-Hill, New York, 1960. (2) Beroza, M., Sarmiento, R., ANAL. CHEM.37, 1040 (1965). (3) Gruseck-Lutz, H., Deluzarche, A., Maillard, A., Bull. Sac. Chim. France 1963, pp. 1616-18. (4) Herzberg, G., “Molecular Spectra and
Moleylar Structure-Diatomic Molecules, 2nd ed., pp. 532-53, Van Nostrand, New York, 1951. (5) Keulemans, A. I. M., Perry, S. G., in “Gas Chromatography 1962,” M. van Swaay, ed., pp. 356-67, Butterworths, Washington, 1962. (6) Moore, C., National Bureau of Standards Circular No. 467, p. 5, June 15, 1949. (7) M,orrison, T. T., Boyd, R. N., “Organic Chemistry,” p. 39, Allyn and Bacon, Boston, 1959.
RECEIVEDfor review May 24, 1966. Accepted August 22, 1966.
Separation and Distribution of Normal Paraffins from Petroleum Heavy Distillates by MolecuIar Sieve Adsorption and Gas Chromatography J. V. BRUNNOCK The British Petroleum Co ., Ltd., BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex, England
A method has been developed for studying the normal paraffins distributed in the heavy distillates of a crude oil. The method consists of dewaxing the distillate fractions and selectively adsorbing the normal paraffins present in the waxes by refluxing their benzene solutions with Type 5 A pelleted molecular sieve. The sieve crystal structure is then destroyed by the action of hydrofluoric acid, and quantitative recoveries of the liberated normal paraffins are achieved. The carbon number distributions of the separated normal paraffins are then established and these results, when combined with the easily obtainable corresponding data from light distillates, give an almost complete profile of the carbon number distribution of the normal paraffins in a crude oil. Examples of the application of the method to two contrasting waxy crudes from Libya and Nigeria are given. The distribution data show a relative abundance of odd- over even-numbered normal paraffins in the C2&0 range and of even- over odd-numbered normal paraffins above GO. 1648
a
ANALYTICAL CHEMISTRY
T
of high molecular weight normal paraffins on the flow properties of crude oils, and their effect on the qualities of a lubricating oil, have made the estimation of these paraffins of importance to the petroleum industry. Furthermore, since the normal paraffin contents of those petroleum distillates which boil below 371” C. can already be determined directly by gas chromatography using a subtractive technique ( 1 , 5 ) , the successful estimation of the higher boiling normal paraffins will complete the normal paraffin profile of a crude oil, and thus enable a better assessment of its potential to be made. The first requirement is a preliminary concentration of the normal paraffins from the complex hydrocarbons and asphaltenes present in the material boiling above 371” C. This is achieved by preparing a series of vacuum distillates and crystallizing the normal paraffins from them under conditions of cooling and solvent/sample ratio, which ensure that their segregation is complete. Direct mass spectrometric and chromatographic analyses of the resulting waxes are possible, but the accuracy of HE IKFLUENCE
the determinations deteriorates with a fall in normal paraffin concentration, so that a further separation based upon the selective retention of normal paraffins by a Type 5A molecular sieve is required. The recovery of the adsorbed normal paraffins by desorption requires a long time, and good recoveries are hard to achieve. To overcome this difficulty a recovery technique other than desorption ha5 to be employed. Since molecular sieve adsorption is 3, physical process in which the noniial paraffins are trapped within the cavities of the sieve, destruction of the sieve lattice will result in their liberation and greatly simplify their subsequent recovery. The high silica content of the sieve, together with the known action of hydrofluoric acid on this oxide, was found sufficient to destroy the sieve lattice (3). The successful quantitative recovery of an adsorbed synthetic blend of normal paraffins then led to the adoption of a procedure which, combined with temperatures programmed chromatography, enabled the complete normal paraffin carbon number distributions of the waxes and hence of most of the crude to be evaluated.
