Qualitative Analysis of Gas Chromatographic Eluates by Means of Vapor Phase Pyrolysis 1. Small Molecule Pyrograms Charles Merritt, Jr., and Carmine DiPietro Pioneering Research Laboratory, U.S. Army Natick Laboratories, Natick, Mass. 01760 The use of small molecules (CHI, CzH4,COz, Con, H20, etc.) coupled with retention volume/carbon number data for homologous series of functional group type compounds can be employed to deduce the identity of gas chromatographic eluates. Typical pyrolysis atterns are found which serve to determine the unctionality, and individual components may be identified by interpolation on retention volume carbon number graphs. The small molecule pyrograms of gas chromatographic eluates may be obtained from replicate chromatograms or by means of interrupted elution.
P
THECONCEPT OF DEGRADING gas chromatographic eluates into smaller molecules by thermolytic dissociation was first described nearly a decade ago by Dhont (1,2). It was expected initially that unique gas chromatographic patterns of the pyrolysis products simulating the aspects of a mass spectrum might be obtained that would allow each compound to be characterized. In fact, the early approach was sometimes called the “poor man’s mass spectrometer.” The products of thermolytic decomposition, however, were not themselves easily identified by gas chromatographic retention data, and patterns were represented by assigning Kovats index values (3) to the pyrolysate components. Pyrolysis techniques were not well developed, patterns were difficult to discern, and because of its complexity, the method did not find much application. Subsequently, several workers (4-6), improving both the methods of pyrolysis and developing data that revealed the mechanisms of decomposition, showed that thermolytic dissociation of molecules under appropriately controlled conditions would yield reproducible patterns. Moreover, the decomposition products are systematically related to the parent compound. These facts led both Levy and Paul (5) and Fanter, Walker, and Wolf (6) to reassert the utility of vapor phase pyrolysis as a means of identifying gas chromatographic eluates. Although the chromatograms of the pyrolysis products, called pyrolysis patterns or pyrograms, are related uniquely to the parent compound, the patterns are complex. This feature of vapor phase pyrolysis behavior has tended to discourage use of the technique for qualitative analysis. The use of small molecule pyrograms under appropriate circumstances (7,8) may also be used to achieve qualitative analysis. (1) J. H. Dhont, Nature, 192, 747 (1962). (2) Ibid.,200,882 (1963). (3) E. sz. Kovats, “Advances in Chromatography,” Vol. I, J. C. Giddings and R. Keller, Ed., Marcel Dekker, New York, N.Y. 1965, p 229. (4) C. A. M. G. Cramers and A. I. M. Keulemans, J . Gas Chromatogr., 5 , 58 (1967). (5) E. J. Levy and D. G. Paul, ibid.,5, 136 (1967). (6) D. L. Fanter, J. Q. Walker, and C. J. Wo1f;ANAL. CHEM., 40, 2168 (1968). (7) J. Franc and J. Blaha, J. Chromatogr., 6 , 396 (1961). (8) H. Groenendyk, E. J. Levy, and S. F. Sarner, J . Chromatogr. Sci., 8, 599 (1970).
This paper and its companion (Part 11) describe two approaches which have been utilized to develop simpler, yet more systematic methods for identifying gas chromatographic eluates from vapor phase pyrolysis patterns. EXPERIMENTAL
The vapor phase pyrolysis system used in this study was a Model 1100 Pyrochrom Analyzer (Chemical Data Systems, Inc., Oxford, Pa. 19363). This device is a commercial version of the controlled thermolytic dissociation apparatus described by Levy and Paul (5). The gas chromatograph employed for the separation of mixtures prior to vapor phase pyrolysis (GC-1) was a Barber-Colman Series 5000 modular system comprising a column chamber equipped with a subambient temperature programming control chassis and with appropriate heating controls for the inlet and detector cells. Dual thermal conductivity and flame ionization detectors were available for the separation column (GC-1) but the F I D was used for most of the experiments. When purification of reagents (especially for removal of water) was required, the thermal conductivity detector was used. The gas chromatographic columns employed were appropriate to the types of mixtures taken for separation. These were, in in. 0.d. columns packed with 3 % general, either 6 ft x SE-30, or OV-17, on 80-100 mesh Chromosorb or 50 ft X 0.02 in. SCOT columns coated with SE-30 or TRIS. The eluates were transferred to the pyrolysis unit through a l/win. 0.d. stainless steel transfer tube heated to 250 “C. The separated components of the test mixtures employed were individually transferred to the pyrolyzer by means of the interrupted elution (stopped flow) technique described by Walker and Wolf (9) in their GC-pyrolysis-GC analysis system. The decomposition products from the pyrolysis unit were collected in a delay coil and transferred to a gas chromatograph column (12 ft x in. 0.d. packed with mixed porous polymer beads) designed primarily for the separation of small molecules such as CO, C o n , HzO, CHI, etc. This column is an integral part of the Pyrochrom Analyzer. When desired, the entire pyrolysate could be transferred to a second gas chromatograph (GC-2) for complete separation, or the eluate from either the small molecule column or GC-2 could be transferred to a mass spectrometer if confirmation of component identification was required. The complete analysis system has recently been described (10). The data for establishment of typical pyrolysis patterns for various functional group types were obtained by injecting pure, known compounds into the analysis system and obtaining the corresponding small molecule pyrogram. The purity of all chemicals used was verified by gas chromatography, and compounds were likewise purified by gas chromatography when necessary. Representative analysis (9) J. Q. Walker and C. J Wolf, ANAL,CHEM., 40, 711 (1968). (10) C. Merritt, Jr., C. DiPietro, and M. L. Bazinet, Proceedings . of 19th Annual Conference on Mass Spectrometry and Allied Topics, American Society for Mass Spectrometry, Atlanta, Ga., May 1971, p 224.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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acetates
aldehydes
CO
CH4 CO,
.
GH,
HO ,
CO
I
‘ 3 .
I
I
I
‘ 4 .
I
I
.
I
6‘
I
I
c
I
I
I
cfJ
I
I
I
‘6.
I
I
I
CH, C02 $H4
HO ,
CO
Figure 1. Graph showing adjusted retention volume us. carbon number relationship for a set of homologous series of compounds of various functional group types. R = normal alkyl group
CH4 CO,
c 3 1
1
‘ 4 1
I
6
CARBON NUMBER
‘ 5 1 ‘ 7 1
I I
5
D
I
RESULTS AND DISCUSSION
The inability t o accomplish identification of gas chromatographic eluates from retention data alone is due to the fact that members of different homologous series may have identical retention characteristics, thus preventing optimum use of the gas chromatograph for the analysis of heterologous mixtures. If the functionality of a n eluting component can be established, its identity may be established from the relationship between retention volume and carbon number. The situation is depicted in Figure 1. The linear plots of log retention volume (isothermal elution) cs. carbon number are shown for several typical homologous series of compounds. Two compounds having the same retention time are represented by the dashed line. If the functionality may be determined, the identity of the component may be ascertained by interpolation on the corresponding retention volume carbon number plot. The basis of the method is essentially the same as that employed by Walsh and Merritt (22,ZZ) using qualitative reagents t o determine the functionality of the compound. In this study, functionality may be deduced from the small molecule pattern obtained by vapor phase pyrolysis of the GC eluate. A graphic representation of the small molecule pyrograms obtained from several members of various functional group classes is shown in Figure 2. The pyrograms of each class have common attributes and, in general, the composition of the pyrolysate is sufficiently different so that each compound type may be delineated. The distinguishing features are summarized in Table I. In most cases, the determination of functional group can be made simply on the basis of the presence or absence of certain compounds. In some cases as with compounds of a similar nature such as aldehydes and ketones or mercaptans and sulfides, a consideration of the relative amount of certain components may be necessary to complete the diagnosis. (11) J. T. Walsh and C. Merritt, Jr., ANAL.CHEM., 32, 1378 (1960). (12) C. Merritt, Jr., in “Ancillary Techniques of Gas Chromatography,’’ L. S. Ettre and W. H. McFadden, Ed., John Wiley and Sons, Inc., New York, N.Y., 1969, p 325. 58
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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alcohols
C2H4 H20
CO ‘2
I
I
.
I
I
‘3
I
I
I
I
I
4‘
I
I
I
1
6‘
I
I
I
.
ethers
CO CH, c4-‘6
of unknown mixtures were accomplished by injecting the sample into GC-1, and employing interrupted elution as each component was transferred to the pyrolyzer in order to obtain its small molecule pattern.
H20
I
ketones
4
CO, CH ,4
I I
c 3 1 4‘
2
CH,
I
I
C q CH ,4
.
I
H20 I
Figure 2, Small molecule pyrograms for selected representative members (designated by carbon number) of some typical functional group compound types: Aldehydes; propanal, butanal, hexanal, octanal. Ketones; 2-propanone, 2-butanone, 2-pentanone, 2-heptanone. Acetates; methyl acetate, ethyl acetate, propyl acetate, butyl acetate. Alcohols; ethanol, n-propanol, n-butanol, n-hexanol. Ethers; diethyl ether, ethylpropyl ether, dipropyl ether
Table I. Functional Group Characterization from Small Molecule Patterns. Hydrocarbons No CO, C02,H20,etc. CO is present, ratio CO/CH, > 1 Aldehydes CO is present, ratio CO/CH4< 1 Ketones CO and H 2 0are present Alcohols CO, C02 and H 2 0are present in small amounts, Ethers C2H4 is predominant component, C2H6 is present CO, C02,and H 2 0are present, C& is absent Esters (and acids) Mercaptans H2Sis present, ratio HzS/CZH, 1 Sulfides H2Sis present, ratio HzS/CZHG >> 1 a CHa, C2H4, and C2H6 present in all cases except where noted.
An example of the characterization of the components of a four component mixture is seen in Figure 3. In this situation, each component of the mixture was trapped successively from each of four replicate separations of the mixture as indicated by the arrows. The pyrograms for each are then compared with the pyrogram patterns corresponding to the various functional groups. Following determination of the functionality, each component is subsequently identified by interpolation on a n appropriate graph of retention volume cs. carbon number. If sufficient sample is available for repeating the separation enough times in order to collect the components individually for vapor pyrolysis, the procedure is the simplest to use since the retention data are obtained a t the same time. In fact, the replication of the chromatograms can provide a statistical measure of the precision of the retention data if desired. An alternative approach as described above is t o use interrupted elution. In this case, retention data must be acquired independently of the stopped flow chromatogram. Here the retention times of the separated components can be acquired from a chromatogram obtained separately with a second in-
IFigure 3. Pyrograms (lower trace) obtained by trapping GC peaks from successive chromatograms (upper trace) a. First peak, butyl acetate; b. second peak, octanol; c. third peak, dihexyl ether; d. fourth
peak, octanol
jection using the desired conditions of temperature and carrier gas flow rate, or using a sample splitter with a dual column arrangement where one column is operated with a continuous flow and the other, connected to the pyrolyzer, in the stopped flow mode. It is obvious that both columns need not have identical separation characteristics except for the order of component elution.
the use of functional group reagents (11,12), but fails to provide definitive information for unresolved or partially separated components. The pyrogram can suffice, however, for the identification of chemically inert classes such as hydrocarbons and ethers, and in many cases may be more sensitive than the use of qualitative reagents. ACKNOWLEDGMENT
SUMMARY
The use of small molecule patterns coupled with retention volume-carbon number data for homologous series of functional group type compounds can be employed to deduce the identity of GC eluates. The method follows the rationale of
The authors are indebted to their colleague, Dr. J. T. Walsh, for advice and many helpful suggestions.
RECEIVED for review July 16, 1971. Accepted September 3, 1971.
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