Programmed Temperature Gas Chromatography for Identification of Ester Plasticizers Anoop Krishen The Goodyear Tire & Rubber Co., Research Division, Akron, Ohio 44316
with a Moseley-Hewlett-Packard 7127A 1-mV recorder. Two stainless steel columns each 6 f t X '/s in. 0.d. packed with 10% UCW-98 on 60-80 mesh Diatoport S were employed in the dual operation mode. The initial column oven temperature was 100 "C and after 4 minutes of isothermal operation, the temperature was programmed at a rate of 8 "C per minute to a maximum of 330 "C. The final temperature was held constant for 8 minutes. The injection block and the detector were maintained a t 270 "C. The helium, hydrogen, and air pressures were 60 (Flowrator 0.8), 14,and 30 psi, respectively. Procedure. Samples of the plasticizers were dissolved in tetrahydrofuran and then injected into the gas chromatograph. When the plasticizers were present in PVC, the polymer sample was dissolved in tetrahydrofuran; insoluble components were allowed to settle out; and a sample of the clear solution was injected into the gas chromatograph. A 1 % solution of the polymer was suitable for this purpose when the plasticizer content of the polymer was between 10 and 40$. In order to obtain the alcohols and the methyl esters of the acids from the plasticizers, the plasticizers were first separated from the polymeric materials by one of the following techniques. If the polymer was soluble in tetrahydrofuran, 10 ml of a 1 solution of the polymer in tetrahydrofuran were added to 40 ml of methanol. The precipitated polymers were removed by filtration and the filtrate containing the plasticizers was evaporated to 25 ml.
PLASTICIZERS REPRESENT a n important class of materials which are essential ingredients in a variety of applications of poly(vinylch1oride) (PVC) and other polymers. The esters derived from alcohols and dibasic acids constitute the largest single category of plasticizers. These materials are also used in coatings, propellants, and films. Gas chromatographic techniques have been used for the analysis of these plasticizers (1-4). Isothermal column conditions have commonly been employed since relative retention data have been available for these conditions. Only limited use of temperature programming has been made for a few of the plasticizers (2). One of the major drawbacks of isothermal column operation has been the necessity for using either two different columns or two different temperatures for the identification of both the original plasticizer and the products obtained from it by hydrolysis and esterification. EXPERIMENTAL Apparatus. , The gas chromatographic unit was a HewlettPackard 5750B dual flame ionization chromatograph equipped (1) F. I. H. Tunstall, ANAL.CHEM., 42, 542 (1970). (2) J. M. Trowel1 and M. C. Philpot, ibid., 41, 166 (1969). (3) W. Fischer and G . Leukroth, Plustoerurbeiter, 20, 107 (1969). (4) H. Haase, Kuur. Gummi Kunstst., 20, 501 (1967).
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Column TemperatureOC 196 228 260
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Figure 1. Gas chromatography of ester plasticizers 9. Butylbenzyl phthalate 10. Trioctyl phosphate 11. Di(2-ethylhexi1)adipate 12. Di(2-ethylhexy1)phthalate 13. Di(2-ethylhexy1)azelate 14. Di(2-ethy1hexyl)sebacate 15. Di-n-decyl phthalate
1. Tetrahydrofuran 2. Triethyl citrate 3. ~lethylphthalylethylglycolate 4. Ethylphthalylethyl glycolate 5. Dibutyl phthalate 6. Dibutyl sebacate 7. .4cetyltributyI citrate 8. Butylphthalylbutyl glycolate 1130
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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Table I. Relative Retention Data for Plasticizers Retention relative to DOP (100.00) Plasticizer Phthalates 76.9 Dibutyl 92.8 Butyl benzyl Di-(2-ethylhexyl) 100.0 Di-n-decyl 120.0 Adipates Di-( 2-ethylhexyl) 95.0 Azelates Di-(Zethylhexyl) 105.4 Citrates 62.5 Triethyl 89.1 Acetyltribut yl Sebacates 85.9 Dibutyl 108.7 Di-(2-ethylhexyl) Glycolates 73.6 Methylphthalylethyl Ethylphthalylethyl 76.4 91.3 Butylphthalylbut yl Phosphates 92.8 Trioctyl
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Carbon Number d the Alcoholic Constituents
Figure 2. Carbon number-relative retention relationship for phthalate esters Upper line. Phthalates of n-alcohols Lower line. Phthalates of 2-ethyl alkyl alcohols 124
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Table 11. Relative Retention Data for Hydrolysis Products Retention relative Component to DOP (100.00) Methanol 0.034 Butanol 1.82 Pentanol 3.98 8.04 Hexano1 Heptanol 15.5 21 .o 2-Ethylhexanol Octanol 24.5 Dimethyl adipate 37.0 Decanol 40.0 Dimethyl-o-phthalate 51.1 Dodecanol 53.0 Tetradecanol 64.3 Methyl palmitate 76.1 Methyl stearate 84.8
If the polymer was insoluble in tetrahydrofuran, a gram sample of the polymer was extracted with 75 ml of methanol in a Soxhlet extractor for 8 hours. A suitable aliquot of the extract was used for hydrolysis. For hydrolysis, a freshly cut 0.1-gram piece of lithium metal was added t o the methanol solution of the plasticizers; and the mixture was refluxed for 2 hours. At the end of this time, the solution was allowed t o cool, and then it was carefully acidified by dropwise addition of concentrated sulfuric acid. When the solution became acidic as noted by p H indicating paper, it was refluxed again for 1 hour in order to convert the acids to their methyl esters. After cooling, the solution was neutralized by addition of dry sodium carbonate, and the p H was checked by p H indicating paper. A suitable sample of this solution was injected into the gas chromatograph for detection of the alcohols and methyl esters of the acids. The gas chromatographic conditions used were identical t o those used for chromatographing the original plasticizers. RESULTS AND DISCUSSION The gas chromatogram obtained with a mixture of ester plasticizers is shown in Figure 1. Most of the commonly used materials are well separated and can be identified by their retention times. The retentions calculated relative to di(2-ethylhexy1)phthalate (DOP), are shown in Table I. For these data, the column dead space was measured by
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Carbon Number of Acid
Figure 3. Carbon number-relative retention relationship fo di-(2-ethy1hexyl)esters of acids 1. Adipic 2. Azelaic 3. Sebacic
injecting methane. D O P was chosen as the reference material as this is one of the most generally encountered plasticizers. Under programmed temperature conditions, relative retentions plotted against the carbon number produce a straight line. These relationships which are helpful for the identification of unknown components, are shown in Figure 2 for a series of phthalate esters. The relative retentions for the phthalate esters of straight chain alcohols from butanol to decanol follow the straight line relationship. The relative retentions of the phthalate esters of branched chain alcohols homologous to 2-ethylhexanol are expected to fall o n the lower line. Phthalate esters of other series of alcohols with different branching will similarly form a family of parallel straight lines. These plots are extremely helpful in identifying components of complex mixtures of phthalate esters. Similar relationships for the relative retentions of di-(2-ethylhexyl) esters of various acids are shown in Figure 3. Since 2-ethylhexanol is extensively used in ester plasticizers, the relative retention of a n unknown plasticizer may be utilized to make a tentative identification of the dibasic acid by using the plot in Figure 3. Although the relative retentions of the plasticizers are helpful for the identification, the complexity of mixtures, normally encountered, necessitates hydrolysis and esterification to obtain information about the components of plasticizers. A definite identification of the original plasticizer can be obtained only after the identification of the constituent alcohols and acids has been made. The simple hydrolysis and esterification scheme used in this study, followed by gas chromatography is helpful in identifying these products ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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Figure 4. Gas chromatography of alcohols and methyl esters of acids 1. Methanol 2. Butanol 3. Pentanol 4. Hexanol 5. Heptanol 6. 2-Ethylhexanol 7. Octanol
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Figure 5. Carbon number-relative retention relationship for n-alcohols as shown in Figure 4. Since the gas chromatographic conditions used for the alcohols, and the methyl esters of the acids are identical to those used for the original plasticizers, it is very easy to establish whether the plasticizers have been completely reacted. Presence of nonhydrolyzable components in a mixture can also be detected by examining the gas chromatograms of plasticizers before and after hydrolysis as the original gas chromatographic peak will still be observed. The identification of unknown alcohols and methyl esters of acids is facilitated by their relative retentions given in
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8. Dimethyl adipate
9. Decanol
10. Dimethyl-o-phthalate 11. Dodecanol 12. Tetradecanol 13. Methyl palmitate 14. Methyl stearate
Table 11. The relative retention and carbon number relationship for straight chain primary alcohols shown in Figure 5 can be used for identification of members of this homologous series of alcohols. Slight deviations from linearity a t the lower end of the plot may be caused by polarity of the alcohols and their lower boiling points. The acid component of the plasticizers, derived from inorganic acids like phosphoric acid, cannot be identified directly by this technique but their presence may be inferred from the relative retention of the original plasticizer and that of the alcohol obtained by hydrolysis. Similarly, although the epoxidized vegetable oil plasticizers cannot be chromatographed directly under the gas chromatographic conditions used, identification of a group of methyl esters derived from these plasticizers on hydrolysis and esterification, can be used to establish their presence. The programmed temperature gas chromatographic conditions used to chromatograph the plasticizers and their alcoholic and acidic components along with the relative retention-carbon number relationships established for various compounds represent a scheme of significant help in identification of ester plasticizers. RECEIVED for review February 19, 1971. Accepted April 20, 1971. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 2, 1971. Authorization by The Goodyear Tire and Rubber Company to publish is gratefully acknowledged.
