Pyrolysis Gas Chromatography of Phenyl Polymers and Phenyl Ether M. T. Jackson, Jr. Central Research Department, Monsanto Company, St. Louis, Mo. 63166
J. Q. Walker McDonneN Douglas Research Laboratories, St. Louis, Mo. 63166
Pyrolysis in tandem with capillary column gas chromatography-mass spectrometry is a unique and definitive analytical technique for the characterization of various hydrocarbon polymers. We studied the effect of varying parameters affecting the nature of products formed and relative product distribution in routine pyrolysis. These parameters include the effects of pyrolysis temperature rise times, pyrolysis temperatures up to 985 OC, and pyrolysis duration. Temperature rise time (0.1 to 1.5 sec) is not a critical factor in the Curie point pyrolysis of a styrene-isoprene copolymer, either with regard to the products formed or the relative distributions. Additionally, the variation of pyrolysis duration or hold time (2.0 to 12.5 sec) at a fixed Curie temperature reflected no change in the nature of components formed; however, changes in product distributions were observed. Variations in Curie temperature at a fixed pyrolysis duration produced drastic changes in product distributions such as a three-fold change in isoprene dimer formation; however, temperature variance did not change the nature of the products formed. A “polymer-like” compound, bis[m-(m-phenoxy phenoxy)phenyl] ether, produced two primary pyrolysis products, diphenylether and dibenzofuran. PHENYL POLYMERS of commercial importance are not readily amenable to chemical degradation. However, they can be readily pyrolyzed to yield the original monomers and/or other volatile fragments. Vinyl or phenyl polymer pyrolysis has been studied by a number of workers (1-3). The major difficulty in pyrolysis studies is obtaining sufficient reproducibility to permit meaningful structure determinations. Good reproducibility requires that all pyrolysis conditions be accurately controlled ( 4 ) . Simon et af. ( 5 ) described that the following pyrolysis conditions must be met. The fragmentation must be conducted a t a high dilution sample size in carrier gas and the products quickly transferred to the column to prevent recombination of the products; the sample must be rapidly heated to a constant reproducible temperature; and the pyrolysis temperature must not be so high as to cause catastrophic fragmentation or produce very low molecular weight secondary products. Rapidly achieved and reproducible pyrolysis temperature can be obtained by the high frequency inductive heating of a wire of ferromagnetic material. With sufficient power input the wire rapidly reaches its Curie point and thereafter absorbs
(1) J. C. F. Williams, J. Chem. Sac., (London), 15, Pt. 10 (1862). (2) G. M. Brauer, J. Polym. Sci., 8, 3 (1965). (3) M. P. Stevens, “Characterization and Analysis of Polymers of Gas Chromatography,” Marcel Dekker, New York, N. Y., 1969, p 64. (4) S. G. Perry, Aduan. Chromatogr., 5 , 221 (1968). (5) W. Simon, P. Kriemler, J. A. Vollmin, and H. Steiner, J. Gas Chromatogr., 5 , 5 3 (1967). 74
negligible energy (6). Such a n apparatus is now commonly called a Curie Point pyrolyzer. Because degradation mechanisms are highly temperature dependent, substances which give similar thermal degradation patterns at one temperature will often yield different patterns when the temperature is changed. Hence, a second pyrolysis at a different temperature is often necessary to distinguish clearly between two different materials. Quantitative analysis of pyrolysis products requires much more stringent control than qualitative identification techniques (5). Deviations in handling procedure, such as variation of total sample size and film thickness, can often significantly affect the decomposition pattern (7). The metal surface of the wire could have a catalytic effect on the product distribution. However, this has been discounted in at least one instance where the surface area was increased by addition of finely ground metal to the polymer sample and found to have no apparent effect (8). This paper describes Curie point pyrolysis experiments applied to a complex copolymer, polystyrene-polyisoprene, and a polymer-like compound, bis[m-(m-phenoxy phenoxy)phenyllether, molecular weight 538.6. These experiments included a detailed calibration of Curie point wire composition and temperature, a comparison of three rf power supplies with rise times of 0.1 to 1.5 sec, a comparison of pyrolysis time durations of 0.5 to 12.5 sec, and the studies of the effect of temperature on the pyrolysis of copolymer and the phenyl ether, mol wt 538.6. The combination of a Curie point pyrolyzer and opentubular capillary column yields adequate separation of most of the olefinic, aromatic, and/or oxygenated products. The flame ionization detector and digital integrator provide quantitative analyses. A medium-resolution mass spectrometer permits positive identification of trace concentrations of pyrolysis product. EXPERIMENTAL
Apparatus. Three pyrolyzers were used for this work. The three systems showed that Curie point thermal fragmentation patterns were repeatable, at least in the two laboratories. The three units consisted of the following: a pyrolyzer (Philips Electronics) equipped with a 0.05 to 10.0-sec duration timer and a 30-W power supply; a pre-column pyrolyzer (PyeUnicam) equipped with a 0.2- to 15.0-sec duration timer and a 30-W power supply; and a lab-built pyrolysis chamber equipped with a manual timer and a 2.5-kW rf generator (Lepel High Frequency Laboratories, Model T-2.5-1 -MC-
( 6 ) W. Simon and H. Giacobbo, Angew. Chem., Int. Ed. Eng., 4,
938 (1965). (7) R. S. Lehrle and J. C. Robb, J. Gas Chromafogr.,5,89 (1967). ( 8 ) C. E. R. Jones and G. E. J. Reynolds, ibid., p 25.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
AP-B). Curie point wires, obtained from Philips Electronics and Varian Associates, were used for all experiments. The data from the quantitative experiments were obtained using a dual flame detector gas chromatograph (HewlettPackard Model 5750) and two digital integrators (Infotronics Models CRS-100 and CRS-108). The qualitative data were obtained with a dual flame detector gas chromatograph (Varian Aerograph Model 1700) and a zero dead-volume column effluent splitter was connected in tandem for gas chromatography/mass spectrometry (GC/MS) operation. A Biemann-Watson type molecular separator provided the interface between a mass spectrometer (Atlas CH-7) and the gas chromatograph. A block diagram of the pyrolysis GC/MS system is shown in Figure 1. Samples for pyrolysis were obtained from several different sources: bis[m-(m-phenoxy phenoxy)phenyl]ether (MW 538.6) was obtained from Varian-Aerograph. The polymer reagents can be obtained from Waters Associates, Inc., (Polystyrene) and Shell Oil Company, (Kraton 1107, styrene-32 %, isoprene-68 %). The separation column(s) used for the polymer products were 50 ft X 0.02 in. (i.d.) support-coated open tubular (SCOT) column (Perkin-Elmer Corp.) containing 5-ring polyphenyl ether liquid phase with Apiezon L Igepal C O 880 added; and 200 ft X 0.02 in. (i.d.) diisodecylphthalate capillary column. The column used for the separation of the pyrolysis products from the bis[m-(m-phenoxyphenoxy)phenyl]ether pyrolysis was a 100 ft X 0.02 in. (i.d.) SCOT column (Perkin-Elmer Corp.) containing free fatty acid phase (FFAP) liquid phase. In the preliminary experiments we used packed separation in. with 5 methylvinyl silicone columns such as a 10 f t x liquid phase (UCW-98) on 80-100 mesh chromosorb W-AW. However, this column did not separate the xylene or dimer (C10H16) isomers (9). Both the diisodecylphthalate and the 5-ring polyphenyl ether-coated open tubular column gave adequate separation of most of the polymer pyrolysis products (IO). The copolymer pyrolyzate contained 13 compounds having boiling points in a 14 "C range. Thus, detailed analysis of pyrolyzates was necessary to differentiate between very similar structures. Procedure. The polymer sample is dissolved in a volatile solvent (benzene) to form a 10 weight per cent solution. The chosen pyrolysis Curie point temperature wire is dipped 0.25 in. into the polymer solution. Several samples may be pre-
+
(9) I. N. Nazarov, A. I. Kuznetsova, and N. V. Kuznetosov, Z h Obshch. Khim., 25, 307 (1955). (10) W. 0. McReynolds, "Gas Chromatographic Retention Data," Preston Technical Abstracts, Evanston, Ill., 1966, pp 71 and 137.
CURIE PT.
-
I
I ____
I
INTEGRATOR
I
A
HEATED
Figure 1. Pyrolysis GC-MS system Note that the dashed line indicates the heated carrier gas transfer lines and the gas chromatograph heated areas pared simultaneously. The polymer-coated wires are then placed in a vacuum oven at 75 to 80 "C for 30 min to remove the solvent. Multiple coatings of polymer on the wire were necessary for tandem GC/MS identification of trace products; however, multiple coatings are not recommended for quantitative data. After the sample is positioned in the pyrolyzer, a period of 5 min is allowed for flow rate equilibration. An auxiliary heating device (heat gun or heating tape) is used to pre-heat the carrier gas line before entry into the pyrolyzer. The pyrolysis chamber is maintained at 174 "C. Asbestos tape and a heating tape were used to eliminate cold spots between the pyrolysis chamber and the G C injection port so that sample and products would not condense prior to entering the column. RESULTS AND DISCUSSION
A prime advantage of the Curie point method (5,11) is the well defined pyrolysis temperature one can obtain with iron or a nickel alloy wire in a rf field. Our analyses included a determination of the wire alloy composition by X-ray fluorescence and the determination of Curie temperature by thermal gravimetric analysis. The manufacturer's specifications of eight different wire compositions were similar to our X-ray fluorescence composition data with the exception of the 985 "C wire (see Table I). The Curie temperatures obtained were in (11) R. L. Levy, Chromatogr. Rev., 1966,49.
Table I. Comparison of Curie Point Wires Manufacturer's specification, OC 358~ 480~
TGA,a
Curie point range,
Manufacturer's composition Ni 100 48
X-Ray fluorescence compositionb Fe Ni 0 100 53.5 46.5 50.6 49.4 42 42 29.2 70.8 33 33 100 0
"C "C Fe co 352 1-2 0 0 474 1-2 52 0 510~ 482 1-2 49 51 0 600d 597 f12 42 42 16 610~ 601 1-4 30 70 0 700d 707 1- 12 33 33 33 770" 782 =t4 100 0 0 980~ 985 f 5 0 60 40 39 1 a Thermal Gravimetric Analyses, (Magnetic transition point, corrected) Perkin-Elmer TGS-1. Av of three X-Ray Fluorescence analyses from Siemens Crystalloflex IV X-Ray Generator with Vacuum Tunnel Spectrometer. Philips Electronic Instruments, Mt. Vernon, N. Y . Fischer Labs (Varian-Aerograph, Walnut Creek, Calif.).
co 0 0
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
0
16 0
33 0
60
75
-
I
2 17
Time
- Minutes
Figure 2. Pyrogram of isoprene-styrene copolymer at 601 O C ; 10.0-sec duration Note that the numbers correspond to the components listed in Table 11. The relative areas are also given at 601 "C in Table I1
Table 11. Repeatability of Thermal Fragmentation Patterns from 601 OC Pyrolysis of Isoprene-Styrene, Copolymer Run No. Component 1. C2-G 2. Isoprene 3. Cyclohexadiene benzene 4. Methylcyclohexadiene 5. Toluene 6. Cyclooctadiene 7. Isopropylcyclohexene 8. Ethyl benzene 9. p-Xylene 10. m-Xylene 11. o-Xylene 12. Isopropylbenzene 13. Styrene 14. 1,4-Dimethyl-l-vinylcyclohexene 15. CI Alkyl benzene 1,3dimethyl-l-vinylcyclohexene 16. Diprene and Csbenzene 17. Dipentene 18. Dimethylcyclooctadienes + isopropenyl benzene Total area,
+
+
(Relative Per Cent Areas) 3 4 5 6
7
8
9
10
AV
1.5 31.7
1.6 32.0
1.4
1.4
1.4 31.8 1.4
1.6 32.1 1.5
1.5 31.6 1.3
1.56 31.83 1.35
0.4 0.7 0.4 0.6 0.3 0.2 0.3 0.3 0.2 16.0 7.3
0.4 0.6 0.5 0.6 0.3 0.3 0.2 0.3 0.2 15.8 7.1
0.4 0.7 0.5 0.7 0.3 0.3 0.2 0.2 0.2 16.3 7.4
0.4 0.7 0.6 0.7 0.3 0.2 0.3 0.4 0.2 16.0 7.2
0.3 0.7 0.5 0.6 0.3 0.2 0.3 0.3 0.2 15.9 7.2
0.7 0.5 0.5 0.6 0.3 0.2 0.3 0.2 16.0 7.3
0.4
0.39 0.68 0.49 0.61 0.32 0.24 0.25 0.30 0.20 16.06 7.25
1.6
1.7
1.7
1.6
1.7
1.6
1.8
1.66
3.1 32.9 0.6
2.7 33.7 0.5
2.8 33.4 0.6
3.1 33.7 0.6
3.0 32.6 0.6
2.9 33.2 0.6
2.6 33.5 0.6
2.8 33.4 0.6
2.88 33.34 0.58
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
1
2
1.6 31.8 1.3
1.5 31.6 1.2
1.7 32.0 1.4
1.7 31.8 1.3
1.6 31.9 1.3
0.4 0.7 0.5 0.6 0.3 0.2 0.3 0.4 0.2 16.1 7.3
0.4 0.7 0.5 0.5 0.2 0.3 0.2 0.3 0.2 16.4 7.1
0.4 0.6 0.5 0.7 0.3 0.2 0.3 0.2 0.2 15.9 7.4
0.4 0.7 0.4 0.6 0.3 0.2 0.2 0.3 0.2 16.2 7.2
1.6
1.7
1.6
3.0 33.2 0.5
2.8 33.8 0.6
100.0
100.0
good agreement (within 8 to 12") with the manufacturer's specifications for the wires except for a negative 28" error in the 510 "C wires. Figure 2 shows a pyrogram of the copolymer (isoprenestyrene) resulting from a 10-sec pyrolysis at 601 "C. One might expect that such a copolymer could yield a product distribution similar to the sum of the two constituent product distributions. F o r example, when the polymer polyisoprene is pyrolyzed, Cp,Ca,C4,isoprene, and CloHle dimers are produced (9). When polystyrene is pyrolyzed, styrene and aromatic hydrocarbons are the products (12). Figure 2 and Table I1 show that the copolymer product distribution and relative area basis resemble the two individual polymer product distributions. (12) G. F. L. Ehlers, K. R. Frisch, and W. R. Powell, J. Polym. Sci., 7,2931 (1969). 76
u
0.09
0.17 0.11
0.37
The pyrolysis patterns observed from the pyrolysis of the copolymer, isoprene styrene, a t 601 "C from 10 separate determinations is shown in Table 11. These data were obtained over a period of 10 days. The repeatability of the normalized per cent areas is excellent. The average value of the 10 determinations and the standard deviation (u) are shown at the right. The per cent deviation from the average value for the most abundant products, peak numbers 2,13,14, and 17, is 0.28,1.02,1.52, and 1.12 %,respectively. The next series of experiments investigated the effect of Curie point rise time o n the pyrolysis of the copolymer, isoprene-styrene. Curie point pyrolysis rise time is related to the wattage of the rf'power supply (13). F o r example, a pure iron wire requires 1.5 sec to reach the final temperature (782 "C) with a 30-W power supply. However, the same iron (13) R. L. Levy and D. L. Fanter, ANAL.CHEM.,41,1465 (1965).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
Table 111. Effect of Pyrolyzer Power on Products (Copolymer Sample). (Relative per cent area) Unit Philies Pve-Unicam LeDel 30 W 30 W 2 . 5-kW Component 1.7 1.5 1.4 C2&4 Isoprene 32.1 31.5 30.2 Cyclohexadiene benzene 1.7 1.3 1.3 Meth ylcyclohexadiene 0.4 0.4 0.4 Toluene 0.7 0.7 0.9 Cyclooctadiene 0.2 0.5 0.5 Isopropylcyclohexene 0.3 0.7 0.6 Ethyl benzene 0.3 0.3 0.3 p-Xylene 0.2 0.3 0.2 m-Xylene 0.4 0.2 0.3 o-Xylene 0.2 0.4 0.2 Trace Isopropylbenzene 0.2 0.2 18.4 15.7 17.6 Styrene 1,4-Dimethyl-17.5 vinyl-cyclohexene 7.3 7.2 Ca Alkyl benzene 1,3-dimethyl- 11.7 1.7 2.0 vinyl cyclohexene 2.8 Diprene and C3 benzene 3.2 2.5 32.0 33.0 32.4 Dipentene Dimethylcyclooctadienes 1.1 isopropenyl benzene 0.6 0.6 100.0 100.0 100.0 Total area, Styrene-isoprene at 601 "C, 10 sec, 200 ft X 0.02 in. (i.d.) diisodecylphthalate (DIDP) capillary, 85 "C, isothermal, flow = 12 ml/min (he!ium).
+
+
+
wire requires only 0.12 sec t o reach 782 "C with a 2500-W power supply. O u r results shown in Table 111 indicate that there is little difference in product distribution normalized on a per cent area basis with this copolymer using the 30-W (rise time of 1.5 sec) and the 2500-W power supply (rise time 0.12 SeC).
Table IV. Effect of Pyrolysis Durationsa (Relative per cent area) Time, sec Component
5.0
7.5
10.0
12.5
1.2 1.4 2.3 3.8 3.4 Isoprene 31.6 32.0 27.0 24.1 21.9 Cyclohexadiene + benzene 1.9 1.4 1.8 2.3 4.0 Methylc yclohexadiene 0.4 0.3 0.4 1.0 1.2 Toluene 0.7 0.2 0.3 1.4 1.6 Cyclooctadiene 0.4 0.8 1.1 1.0 1.3 Isopropylcyclohexene 0.3 0.5 0.7 0 . 1 Trace Ethyl benzene 0.2 0.2 0.4 0.2 0.3 p-Xylene 0.3 0.1 0.3 0.2 0.3 m-Xy 1ene 0.2 0.2 0.3 0.2 0.3 a-Xylene 0.3 0.2 0.3 0.1 0.1 Isopropylbenzene 0.2 0.4 1.1 2.3 2.7 Styrene 16.4 16.0 16.0 18.1 20.0 1&Dimethyl-1-vinylcyclohexene 9.7 9.3 9.2 9.2 9.5 Cs Alkylbenzene 1,3-Dimethyl-l-vinylcyclohexene 2.9 2.6 3.3 2.0 . . . 2.0 2.1 2.4 Diprene and C3 benzene 1.8 2.4 Dipentene 31.3 32.6 33.4 31.1 31.0 Dimethylcyclooctadienes isopropenylbenzene Trace Trace Trace 0 . 5 Trace Total area, 100.0 100.0 100.0 100.0 100.0 Copolymer (styrene, 32Z-isoprene 68 %); 782 "C pyrolysis temperature 20 ft X 0.02 in. (i.d.). Diisodecylphthalate (DIDP) capillary, 85 "C isothermal; flow = 12 ml/min (helium). c2-c4
+
+
However, it has been suggested (14) that faster rise times, on the order of 0.01 t o 0.02 sec will yield different polymer product distributions. The effect of pyrolysis duration has not been reported in the (14) F. FarrC-Rius and G. Guiochon, ANALCHEM., 40,998 (1968).
Table V. Effect of Pyrolysis Temperatures" (Relative per cent areas) 474 482 601
Temperature "C Component
2.0
782
985
1. crc4 0.2 0.4 1.5 3.8 7.6 2. Isoprene 38.0 39.2 31.5 24.1 33.3 3. Cyclohexadiene 0.3 0.4 1.3 2.3 2.6 benzene 4. Methylcyclohexadiene 0.7 0.7 0.4 1.0 0.8 5. Toluene 0.2 0.2 0.7 1.4 0.9 6. Cyclooctadiene 0.3 0.2 0.5 1 .o 1.2 7. Isopropylcyclohexene 0.2 0.4 0.6 0.1 0.5 8. Ethyl Benzene 0.1 0.3 0.3 0.2 0.4 9. p-Xylene 0.2 0.2 0.3 0.2 0.8 10. m-Xylene 0.2 0.2 0.3 0.2 0.3 11. o-Xylene 0.4 0.1 0.4 1.1 Trace 12. Isopropylbenzene 0.2 Trace 0.2 2.3 1.4 13. Styrene 2.0 2.2 17.6 18.1 22.9 14. 1,4-Dimethyl-l-vinyl6.1 6.8 7.2 9.2 6.5 cyclohexene 15. C3 Alkyl benzene 1.9 1.4 1.7 2.0 2.2 1,3-dimethyI-l-vinylcyclohexene 16. Diprene and CI benzene 2.1 1.9 2.5 2.4 1.4 17. Dipentene 45.9 44.8 32.4 31.1 16.6 18. Dimethylcyclooctadienes 0.4 0.6 0.6 0.5 0.6 isopropenyl benzene Total area, 100.0 100.0 100.0 100.0 100.0 Copolymer (styrene, 32Z-isoprene 6873; 10 sec, pyrolysis duration 200 ft X 0.02 in. (id.). Diisodecylphthalate (DIDP) capillary, 85 "C isothermal; flow = 12 ml/min (helium).
+
+
+
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
77
Table VI. Effect of Pyrolysis Temperatures. (Relative per cent areas) Temp, "C Component
+ Benzene COZ
c2-c4
985
782
601
17.2 11.1 1.5
6.2 19.1 1.0
1.0 11.2
482
474
Trace Trace Toluene Trace Trace Indan Trace Indene Benzofuran 2.0 5.5 4.1 Me Indan 0.9 0.9 Me Indene Naphthalene 13.4 12.3 6.4 Diphenylether 42.3 41.0 52.5 61.6 65.4 Dibenzofuran 12.5 13.1 24.8 38.4 34.6 Total area, 100.0 100.0 100.0 100.0 100.0 Bis [m-(m-phenoxy phenoxy)phenyl]ether. Mol wt 538.60. 100 ft X 0.02 in. (Ld.) free fatty acid phase, (FFAP), SCOT program, 75 to 210 "C at 8 "C/min; flow rate = 20 ml/min (medium). Q
literature. We studied the effect of pyrolysis duration o n the product distribution of the copolymer, isoprene-styrene. Table IV shows the relative product distribution variation as the pyrolysis duration is changed from 2.0 to 12.5 sec a t a temperature of 782 "C. (Note the automatic timer intervals less than 2.0 sec (0.05, 0.1, 0.2, 0.5, and 1.0 sec) were not useful o n the Philips and Pye-Unicam pyrolyzers since the rise time to 782 " C was a t least 1.2 sec.) The results shown in Table IV show small increases in the relative amounts of polystyrene products (benzene, toluene, styrene, and isopropyl benzene) with increasing pyrolysis time. There are also small changes in the polyisoprene relative pyrolysis product distributions. The effect of pyrolysis temperature is the most important parameter in pyrolysis gas chromatography (14). Our next experiments compared the relative pyrolysis product distributions of the copolymer, isoprene-styrene, a t five different temperatures a t a constant pyrolysis duration of 10 sec (See Table V). These data show that as the pyrolysis temperature is increased from 474 to 985 "C, there is a three-fold decrease in one polyisoprene degradation product, dipentene, and a n increase in Cs to C4products. The polystyrene products also significantly increase with increases in temperature.
78
The effect of Curie temperature on pyrolysis product distribution was also studied for a phenyl "polymer-like" compound bis[m-(m-phenoxy phenoxy)phenyl]ether. Table VI shows the relative product distribution variation from 474 to 985 "C. This thermally stable substance yields primarily two high molecular weight compounds, diphenyl ether and dibenzofuran, a t 482 and 474 "C. However, at higher temperatures, lower molecular weight products are produced.
CONCLUSIONS Variation of pyrolysis duration or hold time (2.0 to 12.5 sec) a t a fixed Curie temperature reflected no change in the nature of components formed; however, changes in product distributions were observed. Additionally, temperature rise time (0.1 to 1.5 sec) is not a critical factor in Curie point pyrolysis in a styrene-isoprene copolymer, either with regard to the products formed or the relative distributions. Variations in Curie temperature (from 474 to 985 "C) a t a fixed pyrolysis duration produced drastic changes in the principal products from the Curie pyrolysis of the copolymer and phenyl ether compound. I n general, the reproducible pyrolysis product distributions of the copolymer resembled the combination of the distributions of the constituent polymers. If this phenomenon is observed in the pyrolysis of other copolymers, it represents a n added advantage in copolymer structure determination. If one uses the wire alloy that Simon originally defined, one obtains the Curie temperature he states with exception of the 510 " C wire.
ACKNOWLEDGMENT We thank A. H. Bell for technical assistance with the initial experiments. We wish to acknowledge the contributions of J. A. Schoeffel for the X-ray fluorescence analyses and C. E. Scott for the TGA. analyses. Also, we express thanks to W. E. Koerner, C. J. Wolf, J. D. Kelley, and D. R. Beasecker for helpful suggestions concerning the manuscript. RECEIVED for review July 17, 1970. Accepted October 12, 1970. This research was partially conducted under the McDonnell Douglas Independent Research and Development Program.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
