Application of Pyrolysis, Gas Chromatography, and Mass Spectrometry for Identification of Alcohol and Carbonyl Isomers B. D. Boss‘ and R. N. Hazlett’ Chemistry Division, Code 6 180, Naval Research Laboratory, Washington, D.C. 20375
Pyrolysis-Gas Chromatography-Mass Spectrometry is used to identify isomeric alcohols and ketones. All of the straight chain 8- and 12-carbon alcohol isomers, two 12-carbon ketones and lauraldehyde have been pyrolyzed in a gold reaction tube at 600 ‘C to yield distinctive pyrograms, useful for qualitative analysis. The position of the functional group in these isomers exerts a strong effect upon the product patterns. Major products of alcohol pyrolysis are a-olefins, simple dehydration olefins, aldehydes, and methyl ketones. Significant products from ketone pyrolysis are a-olefins, vinyl ketones, and unsaturated ketones with the double bond not conjugated with the carbonyl group.
Pyrolysis research can be applied in several different areas. One can determine the products from vapor-phase pyrolysis of specific molecules in order to test current pyrolysis mechanisms ( 1 , 2) or apply pyrolysis techniques to the identification of unknown molecules by examination of the pattern (“fingerprint”) produced (3-7). To facilitate comparisons between laboratories, pyrolysis should be performed under standard conditions such as gold-tube controlled thermolytic dissociation ( 4 , 8) as used here. The fingerprint has been based on gas-liquid chromatography (GLC) (9, 1 0 ) or on mass spectrometry (MS) (11, 22). Simmonds and co-workers coupled these two instruments in developing an analysis of pyrolysis products (13, 1 4 ) . Dhont applied Py-GLC to some simple molecules (9) and Meuzelaar and coworkers have reported pyrograms for complicated biological materials and bacteria (10-12) using Py-GLC and Py-MS. Knowledge of pyrolysis reactions is vital to syntheses in the petrochemical industries and this same knowledge can be used to predict pyrolysis products for use in qualitative analysis. A t one extreme, a collection of pyrograms of reference compounds can be made under standard conditions, and then unknown molecules are identified by comparison with this data file. A t the other extreme, an unknown pyrogram could be interpreted by inspection of the pyrogram alone. As in mass spectrometry, a compromise between these two approaches is suggested in this paper for pyrolysis work. Previous research (8) has shown that partial identification of a molecule can be made a priori by study of the “small-molecule’’ pyrogram: CH4, CO, COS, C2H4, C2Hc;, “:I, HzS, H20, and C3H6. Information about the functional groups present or absent in the unknown molecule is contained in the concentration ratios of these molecules. This paper develops the concept of “large-molecule” pyrograms (Le., C4Hs and larger) to extend knowledge of pyrolysis mechanisms and to determine the potential for use of such “large-molecule” pyrograms for qualitative analysis. The technique is specifically applied to alcohol and ketone isomers formed during n-paraffin oxidation (15, 16).
’ Present address, Federal Energy Administration, Washington,
D.C. 20461.
Gas-liquid chromatography and mass spectrometry were combined to ensure positive identification of the pyrolysis products.
EXPERIMENTAL Apparatus. T h e apparatus is diagrammed in Figure 1. Selected samples are introduced into the pyrolyzer after initial separation in a gas chromatograph (Beckman Instruments Co., Inc., Fullerton, Calif., Model GC-4). T h e pyrolyzer is a 36-in. long section of lih-in. o.d. gold tubing with a volume of about 4 ml, manufactured by Chemical Data Systems (Oxford, Pa.). T h e flow rate in the pyrolyzer, near ambient pressure a t 30 ml min-’, provides a reaction time of 8 sec, a period too short to allow significant further pyrolysis of the initial stable pyrolysis products. T h e gold chamber used here exhibits reproducible, b u t not necessarily noncatalytic, pyrolysis ( I 7 ) . Pyrolysis products are separated by a temperature-programmed gas chromatograph (also manufactured by Chemical Data Systems) labeled GC-2 in Figure 1. This chromatograph uses a 10-ft, ‘/s-in. o.d. column of 10% SE-30 silicone rubber on 80-100 mesh Chromasorb-W-AW (Johns-Manville Co., New York, N.Y.). Pyrolysis products are concentrated by removing most of the He, utilizing a silicone-rubber membrane molecular separator a t 210 “ C (Model CG-5 from Cangal, Inc.. Lafayette, Calif.). Mass spectra are determined in a Finnigan Corp. (Sunnyvale, Calif.) quadrupole mass spectrometer model 3000-1 with a standard 1.0-mA filament current and 70-e\’ source energy. Complete spectra are recorded every 6 sec on magnetic tape by a Systems Industries (Sunnyvale, Calif.) d a t a acquisition system. Identification of products is aided by searching t h e spectra file stored on magnetic tape at the National Institutes of Health (Bethesda, Md.) as accessed through a teletype-telephone linkage with a PDP-12 computer (Digital Equipment Corp., Maynard, Mas Reagents. T h e dodecanol and dodecanone isomers used in this study were obtained from Chemical Samples Co. (Columbus, Ohio) as 99+% pure. T h e octanol isomers were obtained from the Aldrich Chemical Co. (Milwaukee, Wis.) as 99% pure. Lauraldehyde was J. T . Baker (Phillipsburg, N.J.) practical grade. Using gas-liquid chromatography (GLC) it was demonstrated t h a t the isomers contained no impurities which might interfere with t h e pyrolysis experiments. Procedure. Preliminary experiments tested t h e apparatus a n d techniques to determine the optimum conditions for pyrolysis. First. in order t o ensure the reliability of our data, a “large-molecule” pyrogram was obtained for n-dodecane a t 600 O C . These data, shown in t h e first part of Figure 2, are consistent with results in the literature ( 4 ) .Next, “large-molecule” pyrograms of n-dodecane were obtained from 5 i 5 to 675 “ C to establish an optimum pyrolysis temperature which was found to be 600 “C. This temperature affords significant amounts of primary products bu; only small amounts of secondary products. “Large-molecule” pyrograms were obtained at this temperature for each of the straight chain 8- and 12-carbon alcohol isomers (Figures 3 and 4), in addition to pyrograms for two dodecanone isomers and lauraldehyde (Figure 2 ) . Identification of the major products was made largely hy obtaining quadrupole mass spectra of each pyrolysis product and comparing these with spectra on file a t t h e National Institutes of Health. Confirmation of the identification was made by GC retention-time matching.
RESULTS A N D DISCUSSION The preliminary research indicated the optimum conditions for “large-molecule” pyrograms. Analysis of pyrograms obtained under these conditions is used in the dis-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
417
r
I-OCTANOL
3-OCTANOL
0 CONDUCTIVITY
LL HELIUM
-1
Fo 0
PLOTTER TIME SHARING COMPUTER TERMINbL
I-
1-34 5 6 7
r
2-OCTANOL
Figure 1. Flow chart of the pyrolysis-gas chromatography-mass spectrometry apparatus explained in text
5 0
i/i , il
CARBON
NUMBER
Figure 3. Pyrograms of the octanol pyrolysis products formed under conditions discussed in Figure 2
0
2
_I
0
1N EC ,-i ;4D :-1, ;6 7 8 9101112
ENONA; : ; :1-45 ;,
LAURALDEHYDE
0
"E
6 7 83 94 IO 11 12
5-DODECANONE
$ 1
CL W 1-45 6
7 8 9 1011 12
1-45 6 7 8 9 10 I1 12
CARBON NUMBER
Figure 2. Pyrograms of the products formed at 6OO0C f r o m n-dodecane and dodecanone pyrolysis for 8 sec. in a gold chamber as discussed in the text. The thin lines represent (i-olefins with carbon chain length indicated on the abscissa, while the other products are identified as follows: (1) 8-nonenal. (2) vinyl ethyl ketone, (3) l-octene-6-one, (4) l-nonene-7-one, (5) l-decene-8one, (6) vinyl butyl ketone, (7) vinyl heptyl ketone. Hydrocarbons with carbon numbers 1-4 were eluted as a single peak
cussion below to expand the current mechanism of pyrolysis reactions, and conclusions are drawn about the potential for analysis of a pyrogram. Large-Molecule Pyrograms. Based on the n-dodecane pyrograms, a 600 "C pyrolysis temperature was chosen because above 615 "C, n-dodecane primary pyrolysis products decrease in concentration relative to the secondary products which appear in trace amounts a t 600 "C. Pyrolysis Products. The major products of carbonyl and alcohol pyrolysis, as identified by GLC-mass spectrometry, are presented in Figures 2 through 4. Pyrolysis of (2-12 ketones produces ru-olefin and unsaturated ketones. As seen in Figure 2, the largest ru-olefin that forms corresponds to the largest alkyl group attached to the functional group carbon. The KIH search routine was used to identify most of the products formed but the mass spectra of the unsaturated ketones (except ethyl vinyl ketone) formed from ketone pyrolysis have not been previously reported. T h e identification was aided. however, by the knowledge that n-olefins are the product of paraffin pyrolysis. Thus vinyl ketones were expected from ketone pyrolysis. U'hat should the mass spectrum of a vinyl ketone look like? McLafferty rearrangement (18j of a vinyl ketone in a mass spectrometer ionization source should yield the ion
+0-H
II
.CH --C--CH=CH418
of ni e = 70
The thin lines represent (?-olefins as well as isomeric C g olefins. The repeated carbon number 8 is used to separate the products and show increasing retention time. Other products are identified as follows: (1) acetone, ( 2 ) propanal, (3) 2-butanone, (4) hexanal, ( 5 ) 2-heptanone, (6) butanai, (7) pentanal, (8) 2-pentanone, (9) 2-hexanone. Hydrocarbons with carbon numbers of 1-3 were eluted as a single peak
The vinyl group has an m/e of 27 and the vinyl group combined with a carbonyl group has an m/e of 55. These three m / e values are indeed the largest peaks for ethyl vinyl ketone as recorded by the NIH files. Several unidentified GLC peaks from dodecanone pyrolysis exhibited these three m/e values in their mass spectra. Examples are peaks 6 and 7 in the 5-dodecanone pattern (Figure 2). m/c
mle
--
70
27
Peak 6 41 13
29 39 57 100% 41°C 2 0 % 1 9 % 13% 1 2 % 9°C -=II C ~a
Peak 7 -7 0 2 7 4 1 4 3 2 9 4 2 83 7 1 100% 87% 164 1SR 1 2 % 10% 1 0 % 9 % 8% oo
These mass spectra, combined with GC retention data, indicate Peak 6 is vinyl butyl ketone and Peak 7 is vinyl heptyl ketone, both logical products from ,5-dodecanone rupture. Additional unidentified peaks had prominent nile values 2 units greater than the vinyl ketones, 57-72-29. Thus, Peak j from 3-dodecanone pyrolysis had the following mass spectra: m/e
n'i
100%
43 375
41 27%
55 24%
72 22$
29
167
56% 97
Peak 5 can therefore he identified as 1-decene-%one. an unsaturated ketone in which the double bond is not conjugated with the carbonyl group. All peaks for the ketone pyrolyses which are not (\-olefins exhibited the prominent m/e combinations 55-70-27 or 57-72-29 and thus can be identified as unsaturated ketones. Alcohol pyrolysis (Figures 3 and 1) yields the expected ci-olefins plus all the possible positional and cis-trans isomers from the simple dehydration of each specific alcohol. Thus. 3-dodecanol dehydrates by removal of the elements of water both from the 2-3 carbons and the 3-4 carbons yielding four olefins: cis- and trans- 2-dodecene plus (,isand trans- 3-dodecene. T w o unexpected major alcohol pyrolysis products were identified: aldehydes and methyl ke-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1 9 7 6
tones. T h e origin of these products is discussed in the theory section. Identification by Pyrolysis. The isomers in Figures 2 thru 4 are very clearly differentiated by their pyrolysis patterns, and, as with mass spectrometry, the isomers could be identified by comparison with a data file of pyrograms obtained under standard conditions. Moreover, the differences among the isomer pyrolysis patterns stand out more distinctly than most mass spectra fragmentation patterns. T h e biggest difficulty of this approach could be standardizing the GLC conditions, a problem t h a t has counterparts in high resolution mass spectrometry. The pyrograms of Figures 2 through 4 can be examined for many clues useful for a priori interpretation as follows. The largest olefin molecule t h a t appears as a pyrolysis product identifies the position of the functional group. For example, the pyrolysis of 3-dodecanone yields no olefin larger than 1-nonene by scission as follows:
Next, one observes that the alcohols are differentiated from the carbonyl compounds by the relative amounts of the two largest olefins formed by scission. For ketones and aldehydes, the second largest predominates presumably because of the resonance stabilization of the remaining free radical when the C-C bond is broken as follows for 3-dodecanone:
0
I!
CHC-CH.
+
1 t
C,H-
-
C.H COC , H I
hl y+
iic
C-H COCH--CH
1-
(11
C H CO--CH=CH It is conceivable that a ketene could form if the initial free radical splits a t the C-C bond between the carbonyl carbon and the methylene group on the opposite side as follows:
CH CH.
t
CO-t’H-C.H
cH
O=C=CH--C,H
No evidence for this was noted. Alcohol pyrolysis does not result in significant amounts of unsaturated alcohols nor are dienes detected. All the expected olefins are observed (Figures 3 and 4) as predicted for pyrolysis of the alkyl groups, ignoring the functional group. In addition, dehydration of the parent molecule results in a mixture of cis-trans isomers. The most intriguing products from alcohol pyrolysis are the very significant aldehydes and methyl ketones. The production of these products can be explained by the formation of a free radical a t the functional group, either on the oxygen or the carbon as illustrated for 6-dodecanol. Decomposition of one free radical affords the aldehyde by 8scission, whereas the methyl ketone arises from @-scission of the other radical followed by rearrangement. 0
CHCH
One also observes that the product peaks, other than olefins, which occur in both the alcohol and carbonyl compound pyrograms differentiate them from hydrocarbons. Additionally, other observations, such as peak area ratios, could be made which would help us identify the unknown pyrolyzed molecule. Pyrolysis Mechanism. In order to bring interpretation closer t o realization, the mechanism of hydrocarbon pyrolysis has been extended here to include the alcohol and ketone results. Current vapor phase theory ( I ) can be applied to n-dodecane as follows:
c H--CH-CH-
group which stabilizes formation of a free radical on the adjacent carbon as follows:
Ic-CH
11 -CH
=(
t OH
I
C H -CH-CH.-C
H
H
,C,H-
etc. Here, the initial formation of a free radical leads to C-C bond rupture to form an olefin and a primary alkyl radical, followed by isomerization of the alkyl radical to a secondary radical. Additional bond rupture by 8-scission and further isomerization steps occur until, finally, small alkanes will be formed by H atom abstraction when the free radical cannot readily isomerize. Here, this theory is extended logically to explain alcohol and ketone pyrolysis by deciding the influence the functional group has on which free radical will form initially, and then, which C-C bond will break. Ketone pyrolysis results in a-olefins from chain scission just as for dodecane above (Figure 2). Unsaturated (unconjugated) ketones are formed when the scission is remote from the functional group. However, vinyl ketones are important products because of the influence of the carbonyl
These reactions can account for the oxygenated products from all the dodecanol and octanol isomers. Suggested Application of Pyrolysis-GLC. A priori identification of molecules by pyrolysis-GLC (PGLC) is not possible a t this time. However, when PGLC is combined with other information, identification may be greatly facilitated in the following ways: I ) In many cases, pyrograms are more distinctive than mass spectra, especially for the very similar isomers studied here. 2) Mass spectra of pyrolysis products may be more readily interpreted than a mass spectrum of the parent molecule. 3) Appropriate reaction gas chromatography techniques (e.g., derivatization) could be added to analyze the pyrolysis mixture with a greatly increased ability to identify the parent (19). 4) Even if a molecule cannot be identified, the combination of a “small-molecule” and “large-molecule” pyrogram may reveal the functional groups present. ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
419
+
4-DODECANOL
LITERATURE CITED
?
x
mol
B. M. Fabuss, J. 0 . Smith, and C.N. Satterfield, Adv. Pet. Chem. Refin., 9, 156-195 (1964). D. Kunzru. Y . T. Shah, and E. 8.Stuart, ind. Eng. Chem., Process Des. Dev., 11, 605 (1972). D. L. Fanter, J. Q . Walker, and C. J. Wolf, Anal. Chem., 40, 2188 (1968). E. J. Levy and D. G. Paul, J. Gas Chromatogr., 5, 136 (1967). D. L. Fanter, R. L. Levy, and C. J. Wolf, Anal. Chem.,'44, 43 (1972). H . Groenendyk, E. J. levy, and S. F. Sarner, J. Chromatogr. Sci., 8, 599 (1970). C. Merritt, Jr., and D. H. Robertson, Anal. Chem., 44, 60 (1972). C. Merritt, Jr., and C. DePietro, Anal. Chem., 44, 57 (1972). J. H. Dhont, Analyst(London), 89, 71 (1964). H. L. C. Meuzelaar and R. A . in't Veld, J. Chromatogr. Scb, I O , 213 11972) -, H, R. Schuiten, H. D. Beckey, A . J. H. Boerboom, and H. L. C. Meuzelaar. Anal. Chem.. 45. 2358 11973). H. L. C. MeuzelaarandP. G. Kistemaker, Anal. Chem.. 45, 587 (1973). P. G.Simmonds. G. P. Shulmann, and C. H. Stembridge, J. Chromatogr. Sci., 7, 36 (1969). P. G. Simmonds. Appl. Microbiol., 20, 567 (1970). B. D. Boss and R. N. Hazlett, Can. J. Chem., 47, 4175 (1969). B. D. Boss and R. N. Hazlett. lnd. Eng. Chem., Prod. Res. Dev. 14, 135 (1975). J. E. Taylor, D. M. Kulich, D. A. Hutching, and K . J. Frech, Am. Chem. Soc., Div. Pet. Chem. Prepr , 17 (2). 847 (1972). F. W. McLafferty, "Interpretation of Mass Spectra", 2nd ed., W. A. Benjamin, Inc.. Reading, Mass., 1973, p 58. J. Guillot, H. Bottazzi, A. Guyot, and Y. Trambouze. J. Gas Chromatogr., 6 , 605 (1968).
1-45 6 7 8 9 IO I1 I2 12 12 12
5-DODECANOL; "7
6
3
s
:IE
6-OODECANOL
2 m
3
0 m
1-45 6 7 6 9 10 11 12 I2 12 12
CARBON
NUMBER
Figure 4. Pyrograms of the dodecanoi pyrolysis products under conditions discussed in Figure 2 The thin lines represent a-olefins as well as isomeric C I 2 olefins. The repeated carbon number 12 is used to separate the products and show increasing retention time. Other products are identified as follows: (1) acetone, (2) hendecanai, (3) propanal 2-butanone, (4) decanal, (5) 2-hendecanone, (6) butanal. (7) 2-pentanone, (8) nonanal, (9) 2-decanone, (10) pentanal. (111 2hexanone, (12) octanal, (13) 2-nonanone, (14) hexanal, (15) 2-heptanone, (16) heptanal. (17) 2-octanone
+
ACKNOWLEDGMENT The authors thank F. U'.Williams for considerable cooperation in obtaining the mass spectra.
RECEIVEDfor review April 23, 1975. Accepted October 22, 1975. This work was supported by the Naval Air Systems Command, Washington, D.C. 20360, and was presented t o the Petroleum Division, 167th National Meeting, American Chemical Society, Los Angeles, Calif., March 31-April 5, 1974.
Determination of Chlorophenoxy Herbicides in Air by Gas Chromatography/Mass Spectrometry: Selective Ion Monitoring S. 0. Farwell,* F. W. Bowes, and D. F. Adams Air Pollution Research, Washington State University, Pullman, Wash. 99 763
The electron-impact mass spectra for the chlorophenoxy family of herbicides were obtained and subsequently examined for their characteristic fragment Ions. These mass ions were used to establish selective ion monitoring techniques for the specific analysis of airborne 2,4-D compounds. A separate diffusion pump for the ion source of the quadrupole mass spectrometer allowed a direct Interface between the gas chromatograph and the mass spectrometer; thus, the total gas chromatographic effluent flows into the ion source. This total-effluent GC/MS system is extremely advantageous for maximum sensitivity and reproducibility in environmental analysis. For instance, if the base peak in the mass spectra of the chlorinated herbicides is monitored continuously, then the detection limit is in the 1-0.1 pg range. In practice, routine air samples were analyzed for 2,4-D butyl ester down to the 1-pg level. Data are also presented for a comparison study between GC/ECD-3H, GC/ ECD-63Ni, and GC/MS-SID quantitative results. 420
Both the agricultural application of 2,4-D herbicides and the concomitant atmospheric contamination from their use have recently increased in frequency and intensity over large areas of the United States and Canada ( I , 2). The most apparent environmental hazard in the use of the 2,4-D herbicides is due to the airborne movement of the chemical to nontarget vegetation during or after application. This family of herbicides are potent broad-leaved weed killers, but are unfortunately also extremely toxic t o grapes, cotton, tomatoes, and other susceptible crops (3-5). Therefore, severe injury to crops outside the treated areas can occur from the spray drift and/or the movement of the herbicide vapors. Because of these facts, it has become increasingly important to analyze qualitatively and quantitatively for trace concentrations of 2,4-D compounds in the air. The most popular technique for 2,4-D analysis is a gas chromatograph with an electron-capture detector (GC/
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
