Generation of Retention Index Standards by Pyrolysis of Hydrocarbons Kent J. Voorhees, Fred D. Hileman, and lrvlng N. Einhorn Flammability Research Center and Departments of Materials Science and Engineeringand Chemistry, University of Utah, Salt Lake City, Utah 84110
A method for generating a complete series of reference compounds in situ for gas chromatographic retention indices is described. I-Alkenes from ethylene to any desired molecular weight are produced by pyrolyzing the approprlate n-alkenes or polyethylene at 1000 O C In an inert atmosphere. The alkenes are related to the corresponding alkane by a 61 correction factor for a particular phase. A solid state copyroiysis technique is described along with a computer program for retention index searching of a master 11brary.
The technique of GC/MS has become one of the most popular tools for identifying components of complex mixtures (1 ). Traditionally, gas chromatography has been used as the separation tool with mass spectrometry used for identification of eluting peaks. Until recently (2, 3 ) , the idea of using retention indices from the gas chromatography as an ancillary qualitative tool has been overlooked. This paper describes how the combination of mass spectral analysis and retention indices can be used in a precise and convenient procedure for identifying the products from a solid-state pyrolysis. Several problems have been encountered in the general usage of retention indices in connection with solid-state pyrolysis of polymers in our laboratory. These problems are: 1) the difficulty in coinjecting hydrocarbons during an injection-port solid-state pyrolysis; 2) the necessity of having a wide variety of normal hydrocarbons as standards; 3) the cost of these hydrocarbons; and 4) the inconvenience, due to differences in volatility, of injecting an entire spectrum of hydrocarbons. Ideally, one would like to find an inexpensive precursor which could be added to the material under investigation such that during pyrolysis a complete series of reference standards would be produced simultaneously with the regular pyrogram. We have worked out a general procedure to fulfill these requirements using as a precursor an appropriate normal hydrocarbon or a low-density, highly-branched polyethylene. EXPERIMENTAL Apparatus. All samples were pyrolyzed, using a Hewlett-Packard Model 80 or a Chemical Data System Pyroprobe Model 120 pyrolyzer, directly into the injection port of a Hewlett-Packard Model 7620A gas chromatograph equipped for subambient operation. All eluted peaks were split 1O:l between a thermal conductivity and flame ionization detector, and integrated from the TC detector using a Hewlett-Packard Model 3370B integrator. A Hewlett-Packard Model 5930A mass spectrometer was connected in series following the thermal conductivity detector and was used for all qualitative identification. In order to facilitate data acquisition and refinement, the mass spectrometer was coupled to a HewlettPackard 5933A computer system equipped with the appropriate data-handling software. Reagents. Compounds used in this study have been purchased from the following companies: Chromosorb 101 and Dexsil 300GC, Supelco, Inc.; UC-98, Hewlett-Packard Company; methane-butane, Matheson Company; pentane-octadecane and tetracosane,
Aldrich Chemical Company; and tetratetracontane, Chemical Samples Company. Polyethylene was obtained from the National Bureau of Standards (Standard Reference Material 1476). The urethane foam was formulated from 75 wt % of propoxylated trimethylolpropane (mol wt = 312), 106.7 wt % polymethylenepolyphenyl isocyanate, 0.9 wt % of silicone surfactant, 21 wt % trichlorofluoromethane, and 2.7 wt % of triethylene amine. Gas Chromatography. The most satisfactory separation conditions for compounds with less than C12 reference hydrocarbon was achieved using a l/s-in. X 8-ft stainless-steel Chromosorb 101 column programmed between 0 and 220 "C a t 10°/min. For mixtures containing compounds with hydrocarbon references above C12, either a ?$-in. X 8-ft stainless-steel, 10% Dexsil on Chromosorb W or a lk-in. X 8-ft stainless-steel, 1Wo UC-98 on Chromosorb W column was used. In all cases, the flow rate was set a t 40-60 ml/min measured a t 25 "C. The retention indices were calculated using procedures outlined for programmed temperature gas chromatography by ASTM (4), with n-alkanes as reference compounds. Pyrolysis Procedure. Typically, a 2-mg sample of pure hydrocarbon or polyethylene was pyrolyzed directly into the injection port of the gas chromatograph. For solid-state copyrolysis, 10-30 wt % of the solid hydrocarbon was sandwiched in the quartz pyrolysis tube with the polymer sample. Liquid hydrocarbons (10-30%) were injected with a microsyringe directly into a 2-mg sample of the polymer under investigation. Copyrolysis of the polyethylene and polymeric foam was done by placing approximately 30% polyethylene (by weight) directly into the pyrolysis probe with the polymer sample.
RESULTS AND DISCUSSION In selecting materials to be used as reference standards, it was considered necessary to have all compounds present, in reasonable concentrations and approximately equal amounts. This would allow for precise values to be obtained with interpolation between nearest neighbor standards, especially with the use of either temperature programming techniques or the use of capillary columns (5). Second, a precursor could not generate products with excessively long retention times or products that would not be eluted and thus contaminate and change the column characteristics. A suitable choice for a precursor was found to be normal hydrocarbons of Cl2 and greater (6). With the described pyrolysis equipment, it was very difficult to obtain good pyrolysis of normal hydrocarbons below '212. They would tend to vaporize before pyrolysis and thus would give very low concentrations of pyrolysis products. Figure 1 shows the 1000 OC pyrogram from n-dodecane. The products from this pyrolysis listed in Table I were identified using a coupled mass spectrometer. The major products were 1alkenes with traces of low molecular weight alkynes. The pyrogram of the higher boiling pyrolysis products of tetracosane (C24) and tetratetracontane (C44) are shown in Figure 2. 1-Alkenes were again produced from these pyrolyses as identified by GCMS. Clearly, through the appropriate selection of molecular weight of the hydrocarbon, any range of 1-alkene standards can be obtained. Table I1 is a summary of the observed retention indices for the alkenes and corresponding alkane. It is evident for purposes of comparison on this particular column that the
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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Flgure 1. Thermal decomposition of Modecane at 1000 O C in helium separated on Chromosorb-101
retention indices are essentially equal for the alkene vs. its corresponding alkane. For exacting work or for columns with a large 6 1 for unsaturates, the 1-alkenes could be used as secondary standards with correction factors used to correct all indices to the normal hydrocarbon values (7). The copyrolysis of a polymer and hydrocarbon is shown in Figure 3, in which a sample of rigid urethane foam was pyrolyzed with and without the added normal C24 precursor. The peaks attributed to the standards are labeled and can be identified using GCIMS techniques or by backcounting from the larger C24 peak. Since the column was temperature programmed, the standard compounds assume a linear relationship, thus aiding in their identification. Table I11 summarizes the retention indices for peaks 1-6 in Figure 3. Ratios of hydrocarbon to polymer were varied from 20-50 wt % in these four pyrolyses. Comparing these results to pyrolysis with subsequent injection of standards, the error is reduced drastically. With the injection
TIME (minutes)
J
C26
c24
w
I
I
Figure 3. Comparison of alkene spiked and unspiked urethane PYrograms (Top) Urethane foam. (Bottom) Urethane foam copyrolyzed with n-tetracosane
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procedure, an error of f 5 index units is common, probably resulting from slight variations in the temperature programming and flow control. With the copyrolysis procedure, the error is reduced to f 2 index units, as shown in Table 111. The problems previously encountered with evaporation of the low molecular weight hydrocarbons (
