Laser-Induced Degradation of Hydrocarbon Compounds Analyzed Using Gas-Liq uid Chromatography William Tonn Ristau and Nicholas E. Vanderborgh Department of Chemistry, University of New Mexico, Albuquerque, N . M . 87106
A high-power ruby laser operated in the normal pulse mode was used to degrade various solid hydrocarbons; the products were separated and detected with an online gas chromatograph. Fragmentation patterns were found to depend upon sample pretreatment. Low molecular weight products including methane, ethane, ethylene, acetylene, butadiyne, etc., come from the plasma produced in the plume while hi her molecular weight fragments robably result rom blow-off of the hot solid. The h i h e r molecular weight fragments can be explained in terms of simple bond cleavage. No correlation was found between these fragments and those resulting from mass spectral analysis. Characteristic, reproducible pyrograms were found for the compounds studied.
P
INTEREST IN PYROLYSIS gas chromatography as a means of analyzing nonvolatile materials is increasing. New methods for degrading samples include the use of pulsed laser radiation. At the present time there is a limited amount of information available concerning the type of mechanism, the types of compounds given off in the degradation process and the possible analytical applications of this technique. In order to provide more information on the application of laser-induced degradation for the analytical analysis of nonvolatile compounds, a study of the laser operating parameters and fragmentation patterns for various solid hydrocarbons was made. Thermally-induced degradation has been shown by Rice to proceed predominately by a free radical mechanism ( I ) . Early pyrolysis methods often required a long period of time in a reaction chamber which allowed secondary free radical reactions to take place before the products were analyzed. Recently attention has been directed at new techniques for degrading the sample more rapidly. The latest techniques include the Curie point apparatus and laser-induced degradation (2-4). These new techniques offer the advantage of a very short temperature rise time and in the case of the Curie point apparatus a reproducible temperature control (5). Thermal degradation is often characterized by the ultimate temperature achieved during the pyrolysis. Mild pyrolysis is assigned the temperature range of 300-500 “C. Normal pyrolysis is in the range of 500-800 “C while vigorous pyrolysis is above 800 “C. Vigorous pyrolysis causes the sample to be broken down into low molecular weight gases such as hydrogen, methane, ethane, ethylene, and acetylene which lowers the amount of information available from a pyrogram. (1) F. 0. Rice, J. Greenberg, C. E. Waters, and R. E. Vollrath, J . Amer. Chem. SOC.,56, 1760 (1934). (2) Wand Simon and H. Giacobbo, Chem.-Ing-Tech., 37, 709 (1965). (3) B. T. Guran, R. J. O’Brien, and D. H. Anderson, ANAL. CHEM., 42, 115 (1970). (4) 0. F. Folmer and L. V. Azarraga, “Advances in Chroma-
tography-1969,”
A. Zlatkis, Ed., Preston Technical Abstracts
Co., Evanston, Ill., 1969. ( 5 ) L. S. Ettre and A. Zlatkis, “The Practice of Gas Chromatography,” Interscience, New York, N. Y.: 1967. 702
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If the sample to be analyzed can be heated and cooled very rapidly, the opportunity for secondary reaction is minimized. The resulting pyrogram should be much simpler, thereby aiding in the use of the pyrogram for identification or structure determination. Normal pulsed ruby lasers have pulse widths of the order of several hundred microseconds (6). If operated in a Q-switched mode, the pulse width is reduced to nanoseconds. The interaction of laser radiation with matter is quite complex, and many unexpected reactions can occur (7). The Technology Application Center, University of New Mexico, Albuquerque, N. M., maintains a current survey of the literature on the interaction of laser radiation with matter, and the number of abstracts already exceeds several hundred. Various studies have been conducted to determine the temperature generated by the interaction of a laser beam with matter; a wide range of estimates is available. Figure 1 is a representation of a plume which develops when a pulsed laser beam interacts with a solid surface. Vaporized material is ejected from the surface in a direction back toward the source of the beam, and a crater is formed on the surface. Photographic studies on metal surfaces indicate that there is a short time period between the incidence of the beam of the surface and the development of the plume. In the case of Q-switched lasers, the pulse is ending before the plume develops (8, 9). Table I surveys measured temperatures reported for laser interaction with matter. Measurements conducted on the plume yield temperatures which are extremely high. Measurements confined to the surface of metals have indicated a temperature range of 1200 to 8000 OK. Even the lowest temperatures would correspond to the upper range of a vigorous pyrolysis. The earliest study of the products obtained from laserinduced degradation was done by Wiley (IO). Only the volatile degradation products up to C4 were included in the study. The system was not directly on line with the chromatograph but samples were taken from a borosilicate glass reaction capillary and analyzed. The major products consisted of methane, ethylene, acetylene, and butadiyne. Later, systems were developed in which the reaction vessel was on stream with the chromatograph (3, 4, 26). Folmer has shown that the pyrograms obtained from a laser-induced degradation are much simpler than the corresponding ones obtained from a filament or tube furnace pyrolysis (4). Guran has shown that the products obtained are a function of the focus of the laser beam on the sample (3). This has been attributed to the much higher thermal flux obtained with a focussed beam. No systematic study of laser-induced degradation of organic compounds has appeared, A series of aliphatic and (6) B. A. Lengyel, “Introduction to Laser Physics,” John Wiley and Sons, Inc., New York, N. Y.(1966). (7) J. D. Macomber, IEEE J Quantum Electron., QE-4, 1 (1968). (8) J. F. Ready, Appl. Phys. Lett., 3, 11 (1963). (9) T. J. Harris, IBM J . , Oct. 342 (1963). (10) R. H. Wiley and P. Veeraga, J. Phys. Chem., 72, 2417 (1968).
Table I. Estimates of Surface and Plasma Temperatures Achieved by Using Pulsed Laser Radiation Method Investigator References Material Temperature Spectroscopic Richards and Walsh (14 Q-switched; Ta 8500 iz 1500 OK Q-switched; Hz 104-105 OK Spectroscopic Litvak and Edwards (12) Graphite 2700 OK Spectroscopic Kushida (13) Verber and Adelman (14) Thoriated W 2000 OK Electron emission Narnba and Kim Electron emission (15) W, Mo, Ti, Ni 2700-4000 "K 1700-2180 OK Thermionic emission Verber and Adelman (16) Ta (17,ia) Q-switched; Au, Ta 1270-2710 OK Ion emission Knecht (19) Metals 7-8000 OK Ion emission Honig Plasma velocity Ehler (20) Q-switched; W 30 eV Q-switched 8 X lo4 "K Gas dynamics Afanasyev and Krokhin (21) Ambartsumyan and Basov (22) Q-switched; Li 9 x 104 OK Shock wave velocity Graphite 0.5-10 eV Optical thickness David and Weichel (23) Sun and Hicks (24) DrTz 200 eV Neutron Levine and Ready (25) Q-switched; alkali 100-200 eV Neutron emission metals
1
aromatic, solid hydrocarbons was analyzed to determine the practical application of laser degradation as an analytical tool. Fragmentation patterns and the effects of laser input energy as well as sample preparation were examined. Laser fragmentation patterns were also compared with those obtained from mass spectral degradation techniques.
LASER I N P U T
EXPERIMENTAL
A Hughes production microwelder, Model HM-20, with its associated optical system for focussing the beam, was used to degrade the samples. The Model HM-20 is a pulsed ruby laser with a rod 3/s-inch X 31/4-inchesand has a listed maximum output of 16 joules per pulse. The output of the laser is adjustable over a large range either by adjusting the voltage to which the capacitor banks are charged before firing the xenon flashlamp or adjusting the number of capacitors that are charged. The laser is operated in a normal pulse mode with a maximum pulse width of 2 milliseconds at the maximum rated output. Because of the configuration of the laser head, it was not possible to easily modify the system for Q-switching. The output of the laser was checked with a TRG Thermopile Model 100. The maximum output in a single pulse was found to be 8 joules while the minimum output was 0.8 joule. The energy was measured at the exit of the laser (11) F. A. Richards and D. Walsh, Brit. J . Appl. Phys., 2, 663 (1969). (12) M. M. Litvak and D. E. Edwards, IEEEJ. Quantum Electron. QE-2,486 (1966). (13) T. Kushida, Jap. J . Appl. Phys., 4, 73 (1965). (14) C. M. Verber and A. H. Adelman, Appl. Phys. Lett., 2, 220 (1963). (15) S . Namba and P. H. Kim, Jup. J . Appl. Phys., 4, 153 (1965). (16) C . M. Verber and A. H. Adelman, J . Appl. Phys., 36, 1522 ( 1964). (17) W. L. Knecht, Proc. ZEEE, 54, 692 (1966). (18) W. L. Knecht, Appl. Phys. Letters, 8 (IO), 254 (1966). (19) R. E. Honig, Appl. Phys. Lert., 3, 8 (1963). (20) A. W. Ehler, J . Appl. Phys., 37,4962 (1966). (21) Y . V. Afanasyev, 0. H. Krokhin, and G. N. Sklizkov, ZEEE, J . Quunt. Electron., QE-2 (9), 483 (1966). (22) R. V. Ambartsumyan, N. G. Basov, V. A. Roiko, V. S . Zuev, 0. N. Krokhin, P. G. Kryukov, Y . V. Senat-Skii, and Y. Y . Stoilov, Sor. Phys. JETP, 21, 1061 (1965). (23) C . D. David and H. Weichel, J . Afipl. Phys., 40, 3764 (1969). (24) K . H. Sun, I. M. Hicks, L. M. Epstein, and E. W. Sucov, J . Appl. Phys. 38, 3402 (1967). (25) L. P. Levine. J. F. Ready, and G. Bernal, E, IEEE, J . Quant. Electron., QE-4, 18 (1968). (26) W . T . Ristau and N. E. Vanderborgh, ANAL.CHEM., 42, 1848 ( 1970).
, I
LASER P L U M €
/
A 1-1-100
'
___SOLID
I
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RADICALS, I O N S
/ 2 0 0 0 -
800O0K
W-
LURIACI
Figure 1. Schematic representation of degradation temperatures optical system. This energy is then transmitted into the sample compartment. The repeatability and long-term stability of the laser was found to be acceptable, with the variation in a series of 10 shots at the same power setting yielding output energies differing by less than * l % and a long-term stability of better than +Z%. It was felt that the reproducibility of the output was sufficient and there was no need to monitor the output of the laser for each analysis. The pyrolysis chamber has been described previously (26). A Perkin-Elmer Model 800 gas chromatograph was used to separate the degradation products. The products were detected with a standard flame ionization detector. Various columns were used with the chromatograph for different molecular weight ranges. For the low molecular weight gases, a 3-meter, l/s-inch 0.d. stainless steel column packed with 80-100 mesh activated alumina was used. For higher molecular weight fractions either a 6-meter, '/s-inch 0.d. stainless steel column packed with 80-100 mesh Chromosorb P coated with 10% by weight carbowax 20 M or a 3-meter, '/s-inch 0.d. stainless steel column packed with 80-100 mesh Chromosorb P coated with 10% Apiezon N was used. Flow rates were adjusted to 20 ml of carrier gas per minute. Both helium and nitrogen were used as the carrier gas. Mass spectral analysis was done with a Bell and Howell Model 21-491 double focussing mass spectrometer. Samples were inserted in the heated solid sample probe and the spectra were obtained using a 70 eV ion source voltage. Materials. With the exception of the polystyrene and paraffin which were commercial grade, all other hydrocarbons were Eastman White Label, Two different types of carbon filler were used, Darco (R) G-60 Activated Carbon from ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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Table 11. Repetitive Runs of pTerpheny1 Degraded with a 4-Joule Laser Pulse Initial pulse5 Acetylene, ethylene, ethane, methane Butadiyne Benzene Styrene 1A
2A 3A 4A 5A
71.36 70.16 70.54 69.21 72.26
14.82 13.92 14.72 16.66 15.17
70.70 rt 1.60
15.05 rt 1.00
Methylstyrene
10.97 11.82 11.18 9.10 9.92
2.05 3.80 2.12 3.18 1.35
0.76 0.25 1.86 1.82 1.26
10.60 rt 1.08
2.50 =k 0.95
1.19 3Z 0.69
Second pulseb 1B 2B 3B 4B
65.19 69.04 62.32 65.74
14.73 16.09 15.20 17.90
15.79 13.02 17.30 10.72
2.06 1.91 2.55 3.89
2.20 1.91 2.60 1.73
65.57 3Z 2.75
15.98 Z!Z 1.39
14.20 =k 2.92
2.60 Z!Z 0.90
2.11 Z!Z 0.37
Data obtained with new sample. b Data obtained by pyrolysis of sample by a second pulse, Le., 1B shows results from second pulse (aimed in same location) of sample 1A. Data shows percentage of each peak along with relative standard deviation, 0
2b
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i
i,
e
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1
4
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6
10
Figure 2. Comparison of degradation pattern of p-terphenyl with different additives Chromatogram run isothermal at 150°C on the carbowax 20 M column
Za. No additives 2b. 5 Ultrapure graphite added 2c. 5 Carbon Black added
Fisher Scientific and spectra quality graphite supplied by Spex Industries. Procedure. The technique of adding graphite or carbon to the sample to increase the absorption of the laser radiation was avoided wherever possible. It was necessary to incorporate additives only in docosane and 1-docosene. Intimate mixing was obtained with a mortar and pestle. Five 704
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
PeakIdentification--2a 1. Methane, ethane, ethylene, and acetylene 2. Butadiyne 3. Benzene 4. Toluene 5. Xylene or ethyl benzene 6. Methyl styrene per cent by weight of the additive was used. For samples with low melting points, it was necessary to cool the inlet tube with dry ice to prevent the sample from melting. The flash lamp pumping the laser rod was fired at 3600 V with full capacitive load giving a measured 4-Joule pulse at the focal point of the optical system. Data analyses were done graphically. The percentages
Table III. Low Molecular Weight Fraction Analysis of Various Hydrocarbons Methane Ethane ethylene Propane propene 4.64 f 1.80 2.47 f 0.83 0.96 f 0.51 0.96 f 0.51 4.16 f 1.80 52.14 f 2.55 1.95 f 0.53 4.48 f 1.82 60.48 =t 2.78 2.56 f 0.52 12.78 f 2.05 8.43 f 1.83 1.31 f 0.51 1.31 f 0.52 17.16 f 1.56 39.05 f 2.01 8.13 f 1.03 57.93 f 2.53 2.20 f 0.53 5.68 f 1.54 9.00 f 1.52 1.58 f 0.81 1.19 f 0.83 0.78 f 0.51 0.78 f 0.51 1.39 f 0.52 1.39 f 0.51 0.17 f 0.10 0.21 f 0.10
+
+
Acetylene Biphenyl 92.87 f 2.58 98.07 f 1.06 Biphenyl Docosenea 33.29 f 1.51 1-Docosane" 22.40 f 1.81 Durene 78.77 f 2.53 Naphthalene 97.36 f 1.04 Paraffin 43.17 f 3.06 Polyethylene 31.72 f 2.03 Polystyrene 85.30 f 3.04 Pyrene 97.22 f 1.51 o-Ter phenyl 98.42 f 1.05 m-Terphenyl 97.21 f 1.08 p-Terphenyl 99.61 f 1.24 a Graphite was added. Values give per cent distribution of components in the low molecular weight peak along with relative standard deviation. Table IV. Molecular Weight Fractions Analyzed on Carbowax 20 M Column for Various Hydrocarbons Acetylene, ethylene, ethane, Methylmethane Propiynea Butadiyne Benzene Toluene Xylene Styrene styrene Anthracene 5.7 f 1.01 0.3 f 0.11 91.6 f 2.55 2.3 f 0.51 Bibenzyl 1 9 . 6 f 1.04 5.3 f 0 . 8 4 8 . 9 f 1.03 66.0 f 2.06 T Biphenyl 83.9 f 2.53 T 8 . 2 f 1.06 7 . 3 f 0.96 0.4 f 0.10 Chrysene 7.2 f 1.03 0 . 4 =t 0.10 90.2 f 2.54 2.1 f 0.53 Dureneb 62.0 f 2.01 T 25.0f 1.58 11.0f 1.08 0 . 8 f 0 . 2 0 0.2f0.13 Naphthalene 82.7 zk 2.58 10.3 f 1.56 2.7 f 0.51 3.5 f 1.06 Phenanthrene 91.6 f 2.59 2.5 f 0.61 5.8 f 1.01 T @-Phenyl naphthalene 84.9 f 2.03 1 . 6 f 0.30 5.8 f 1.04 7.7 f 1.45 53.8 f 2.03 T 6 . 3 f 1 . 1 3 7.0f1.31 1.2f0.41 0.4f0.11 23.1f1.01 2 . 1 f 0 . 5 1 Polystyrenec 14.9 f 1.54 6.4 f 1.26 1 . 0 f 0.30 75.4 f 2.04 2.3 f 0.54 Pyrene Quarterphenyl 81.5 f 2.06 1.9 f 0.31 0 . 9 It 0.22 5.3 f 0.66 10.1 f 1.89 71.2 f 2.05 T trans-S tilbene 11.7 f 1.84 10.4 f 1.88 0 . 2 f 0.10 3.4 f 0.86 2.9 f 0.60 o-Terphenyl 15.4 f 1.98 5.2 f 0.71 0.4 f 0.11 74.2 f 2.04 T]4. 7d 83.2 f 2.03 T rn-Terphenyl 12.0 f 1.53 4.7 f 1.03 p-Terphenyl 70.7 f 1.58 T 2.50 f 1.31 1.2 i 0.82 15.0 f 1.61 10.6 f 1.22 0.9 f 0.21 a Tentatively assigned as propiyne. Also traces of trimethyl benzene. c Entry under xylene column is for ethylbenzene. Additional unassigned peak between propyne and butadiyne. Values show percentage of each component along with relative standard deviation; each data point shown represents a minimum of three determinations.
indicated are for the peak area uncorrected for detector response. Peak identification was done by using retention times on isothermal columns. Percentages reported are tabulated from a minimum of three runs. RESULTS
Weight loss studies of the pyrolysis tube showed typical weight losses of 4-5 mg for each shot. No difficulty was experienced due to low absorption of the laser radiation by white crystalline aromatic compounds. A carbon residue was left in the tube after each shot. Table I1 shows results for repetitive samples of p-terphenyl, No carbon additives were used. Generally only a small portion of the entire sample is degraded by the laser beam, and the tube is blackened with carbon deposits. If the sample is then reirradiated close to the original degradation, the products are of the same composition but the percentage distribution differs. The largest effect is a decrease in methane and a corresponding increase in benzene. Figure 2 illustrates the significant differences obtained in the pyrolysis patterns. Figure 2a shows pyrolysis with no additions. Figure 26 shows results with high purity graphite, while Figure 2c shows the pyrolysis pattern resulting from the addition of carbon. Variations of the program as a function
of sample pretreatment are not unique to laser-degraded systems. However, the extent of the variation, in this case, is large and indicates that careful control of the type of additive is necessary. Additional experiments are now in progress on this problem. Figure 3 is a pyrogram of bibenzyl showing the separation of low molecular weight fractions on an alumina column. Table 111 shows the results of the analysis of the low molecular weight fraction from several hydrocarbons. The percentages given in Table I11 are normalized for components eluted on this column. These gases form the first peak of a pyrogram using the carbowax 20 M column. Table IV shows the analysis of the higher molecular weight fractions for various hydrocarbons. The first peak consists of methane, ethane, ethylene, and acetylene. The second major peak is assigned to butadiyne while other peaks can be assigned to benzene, XI, toluene, xylene, styrene, and methyl styrene. The peak immediately following benzene, X I , is unassigned but probably is a CSor C 6 olefin. Figure 4 is the pyrogram of polystyrene again obtained with a carbowax 20 M column. The column was operated isothermally at 150 O C and the flow was adjusted to 25 ml/ min of He. Products from the laser degradation of aromatic hydroANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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carbons were also separated using the Apiezon N column. The column was temperature programmed from 100 to 350 "C at 2O"Jmin and yielded information on the higher molecular weight fractions. Figure 5 shows the type of separation achieved on the Apiezon N column. Fused aromatic ring systems yield, in addition to the previously mentioned products, naphthalene, methyl, and dimethyl substituted naphthalene. Mass spectral analyses were made on the same series of the hydrocarbons. The spectra, when normalized to 100 for the major peak, showed no significant peaks below a molecular weight of 30. DISCUSSION
These experiments clearly show the marked differences obtained when different graphitic additives were pyrolyzed along with a hydrocarbon sample. The reproducibility found between identical runs shows a standard deviation from the mean of approximately ~k2.0%. It is not known presently if this limit results from nonreproducibility in the degradation process, from errors introduced in the data analysis, or nonreproducibility in the sample. We suspect that part of the reproducibility limit is caused by our graphical data analysis and are currently exploring electronic integration methods to reduce this error. Our studies were done using a highly focussed laser beam and resulting high thermal flux. Guran (3) has shown that larger molecular weight fragments can be obtained by defocussing the laser beam. Thus, we would expect a different pattern distribution if the beam were less highly focussed; however, in this case sample homogeneity requirements would be less stringent. This aspect of the reproducibility problem is also under investigation. It must be noted here, however, that these studies do uniquely characterize the samples and indicate that this degradation method shows promise for routine characterization. Our results for the low molecular weight fractions are generally in agreement with those obtained by Wiley (IO). Pure aromatic compounds yield predominantly acetylene with only a small amount of the other low molecular weight fragments, methane, ethane, or ethylene. Mixed aromatic/ aliphatic compounds give a larger percentage of methane and ethylene but acetylene is still the predominant product. 706
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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8
IO
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Figure 3. Pyrogram of bibenzyl 1. Methane 2. Ethane and ethylene 3. Acetylene
4
Figure 4. Pyrogram of polystyrene 1. Methane, ethane, ethylene, and acetylene Propiyne 3. Butadiyne 4. Benzene 5. C5 or Ce olefin 6. Toluene 7. Ethyl benzene 8. Styrene 9. Methyl styrene 2.
Degradation of aliphatic compounds gives predominantly (in the lower molecular weight region) ethane and methane. These types of differences are also detected for the higher molecular weight fragments. Using the temperature programmed Apiezon N column, we found that fused ring aromatic systems yield degradation products (in the higher molecular weight region) of naphthalene and mono- and dimethyl-substituted naphthalene. Compounds with the biphenyl type of linkage gave a significant amount of methyl substituted styrene. The 0, m, and p-isomers of terphenyl yield significantly different fragmentation patterns. The reported product distributions (of the higher molecular weight fragments) all can be explained by simple bond cleavage. No aromatic compounds were found in the degradation distribution of aliphatic compounds. This indicates that aromatic compounds are not formed from recombination reactions after the plasma is quenched. With the assumed plume temperatures, one concludes that higher molecular weight fragments cannot exist within the plume environment. Such a conclusion is based on thermal equilibrium arguments. The degradation process is accompanied by a severe shock wave; measurements in liquids have shown that this pressure wave is an excess of several hundred atmospheres (27). Thus, the overall process may not be an equilibrium one. The larger molecular weight fragments may result from degradation processes occurring at the cooler, peripheral region near the pyrolysis crater; alternately, they may be fragments which are blown off by the thermal shock wave. At present, we have no method to distinguish between these two mechanisms. No significant changes in the product distributions were found when the carrier gas was changed from nitrogen to helium. This agrees with the conclusions of Wiley (IO), and indicates that there is little interaction between the (27) C. E. Bell and J. A. Landt, Appl. Phys. Lett., 10 (2), 46-8 (1967).
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IL
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Figure 5. Separation of degradation products from phenanthrene on the Apiezon N Column. Temperature programmed from 100 O to 350 OC a t 20 O/min 1. 2. 3. 4. 5. 6.
Methane, ethane, ethylene, acetylene, and butadiyne Benzene Naphthalene Methyl naphthalene Dimethyl naphthalene Phenanthrene
pyrolysis atmosphere (here, the carrier gas) and the high energy plume. Studies on carbon and hydrogen plasmas generated in a high intensity arc in the 4000-6000 "C temperature range have been made (28). Quenching of a plasma in this temperature range, which is believed to contain Cl, Cz, Cs, H, Hz, C2H2, C2H, C3H, C4, CH, CH2, CH3, and C ~ H Zyields , acetylene as a principal product. Hydrogen and butadiyne are also produced in smaller amounts. This is similar to the products obtained from a laser-generated plasma. Mass spectral data for the hydrocarbon were analyzed to determine if there was any similarity in the fragmentation pattern for the two degradation techniques. Fragments below molecular weight 30 are low in concentration indicating small amounts of CH4+ or CH3+ and CzH2+or C2H+ are produced. This contrasts markedly with the laser degradation results. Even the higher molecular weight mass spectral fragments do not correspond to those obtained from the laser studies, and one must conclude that these two techniques degrade in quite dissimilar mechanistic ways.
In conclusion, we note that laser-induced degradation does give simpler fragmentation patterns than those obtained with older thermal degradation techniques. The products seem to result from two different processes. Low molecular weight fragments probably result from recombination of simple atomic fragments while higher molecular weight fragments result from fragmentation of the parent species. Sample treatment prior to degradation plays an important part in the composition of the degradation fragments. The area in which the energy is deposited is small, making homogeneity in the sample quite important. The ratio of the lower molecule weight products appears to be a function of the carbon and hydrogen content of the original sample. There is no recognized correlation between electron impact fragmentation (mass spectral data) and laserinduced degradation. The simplified pyrograms obtained from laser degradation seem to yield unique pyrograms and indicate that this method should be useful in identifying a variety of materials.
(28) R. F. Baddour and J. L. Blanchet, Ind. Eng. Chem., Prod. Res. Develop., 3(3).
RECEIVED for review November 30,1970.
Accepted February
9, 1971.
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