In the Laboratory
Use of a Dynamic Headspace GC–MS Method for the Study of Volatile Organic Compounds in Polyethylene Packaging
W
An Undergraduate Experiment in Polymer Analysis Steven C. Hodgson, R. John Casey, John D. Orbell, and Stephen W. Bigger* School of Life Sciences and Technology, Victoria University of Technology, Footscray Park Campus, P.O. Box 14428, Melbourne City MC, Melbourne 8001, Australia; *
[email protected] Rationale
Experimental Methods
The topic of odor and taste has often been considered in the undergraduate chemistry teaching literature (1–19). Perhaps the earliest reference to the topic in this literature is the paper by Saul (1) that appeared in 1946 and is concerned with making students aware of the association between certain classes of chemicals and their corresponding odors. Since this early study there have been a number of papers that describe the organoleptic properties of antiperspirants and deodorants (7), candy (8), catalysts (9), decaying fish (10), fir trees (10), fruits (10, 11), lilacs (10), perfumes (12–16 ), chemicals (17–19), and even the striped skunk (10). The organoleptic activity originating from these sources is usually assessed by sensory evaluation (1, 7, 9, 10, 12–15, 17–19). Only a few papers, such as the ones by Kjonas et al. (8), Ramussen (11), and Quigley (16 ), demonstrate to students the use of instrumental techniques to identify volatile organic compounds (VOCs) that contribute to an aroma originating from a given material or source. This paper provides a novel experiment that introduces students to an instrumental technique used to identify organoleptically active VOCs that are emitted at ambient temperatures from everyday materials such as polymers. In the experiment, food-grade low-density polyethylene (LDPE) resin, which is cheap, readily available, and familiar to most students, is used as a source of VOCs. The VOCs are investigated using a dynamic headspace technique in which the VOCs are (i) purged with nitrogen, (ii) trapped at ambient temperature on Tenax–GC (2,6-diphenyl-p-phenylene oxide polymer), and (iii) identified by dynamic headspace gas chromatography–mass spectrometry (GC–MS). Students investigate the relationship between the total area under the VOC chromatogram and the temperature at which the VOCs are desorbed from the Tenax–GC sorbent. The effect of the desorption temperature on the intensities of the chromatographic peaks due to the individual VOCs is also investigated and modeled using a modified form of the van’t Hoff equation. The recommended procedure requires the use of a headspace sampler connected to a GC–MS system and, for this reason, the experiment is, perhaps, most suitable for senior undergraduate students who have access to such equipment. As the experiment is concerned with investigating the adsorption/desorption of VOCs in LDPE, it will be of most interest to students who are studying physical chemistry or food science. The methodology is simple and the additional equipment required for the experiment (listed in the supplemental materialW) is readily available in most laboratories.
Chemicals The following chemicals and polymers are needed for the experiment: low-density polyethylene (LDPE) pellets, nitrogen, Tenax–GC (2,6-diphenyl-p-phenylene oxide polymer), sampling ampoules. Procedure A 50-g sample of pelletized LDPE is sealed in a glass jar with a screw-top lid that is fitted with inlet and outlet ports (see Fig. 1). Volatile compounds present in the headspace of the jar are concentrated onto a Tenax–GC trap (Air-Met Scientific Ltd., Australia) by passing 200 L of ultra-highpurity nitrogen through the jar at a constant flow rate of 220 mL min᎑1. After the concentration step, the charged sorbent is transferred to a 20-mL headspace vial and sealed with a silicone septum that is faced with Teflon. Another five Tenax– GC samples are prepared using this procedure. Each charged sorbent is isothermally desorbed for 150 s, a Varian-Genesis automated headspace sampler connected to a Varian Saturn-2 mass spectrometer being used for this step. Desorption temperatures ranging from 100 to 200 °C at 20-°C intervals are recommended. Details of the GC column and recommended temperature profile to be used are given elsewhere (20) and appear in the supplemental notes.W The area counts of the chromatographic peaks are obtained by integration at the 1% peak threshold limit. The tentative assignment of each chromatographic peak is made using a mass spectral library such as the NIST library. To identify VOCs associated with background, another Tenax–GC ampoule is purged with nitrogen using an empty glass jar and subsequently desorbed at 200 °C for analysis.
Figure 1. Schematic diagram of apparatus used for purging VOCs from LDPE pellets at ambient temperature.
JChemEd.chem.wisc.edu • Vol. 77 No. 12 December 2000 • Journal of Chemical Education
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In the Laboratory
Hazards To the best of our collective knowledge, all chemicals used in this experiment are inert. The VOCs collected from LDPE are present at trace levels that lie below the limits considered to be injurious to health. The reader is advised, however, to consult relevant safety information on these compounds if attempting to reproduce the experiments. The reader is also reminded of the routine safety precautions that need to be taken when handling hot glassware and cylinders of compressed gases. Results and Discussion The chromatogram of C6 to C14 VOCs in LDPE that were desorbed from Tenax–GC is shown in Figure 2. The tentative assignment of each major peak in the chromatographic window was accomplished by searching a NIST GC– MS mass spectral library. Of the 14 peaks that originated directly from LDPE (see Table 1), four are known to cause undesirable organoleptic properties in packaged foodstuffs (23). These are n-heptanal, n-octanal, n-nonanal, and n-dodecanal. The branched and unbranched alkanes and alkenes rarely impart such undesirable effects (24, 25). Other peaks appearing in the chromatogram are not related to the LDPE sample and are believed to originate from either the GC column (20, 26 ) or the Tenax–GC sorbent itself (20, 27, 28). The area under a given chromatographic peak is proportional to the partial pressure of the corresponding species in the headspace and the instrumental response factors. Since the VOCs in the series are of a similar chemical nature, the instrumental response factors would be expected to be effectively the same in each case. Thus, the effect of the desorption temperature on the intensity of each of the peaks in the chromatogram shown in Figure 2 can be modeled by a modified form of the van’t Hoff isochore (20): d ln[a i (T )]/d(1/T ) = ᎑∆Hi /R
(1)
where a i (T ) is the area under the peak due to component i at the thermodynamic temperature T, ∆Hi is the enthalpy of desorption of this component, and R is the ideal gas constant. Figure 3 shows a plot of ln[a i(T )] vs 1/T for two selected compounds whose peaks appear in the chromatogram shown in Figure 2. The temperature dependence of the peak area of the other VOCs in the chromatographic window (see Table 1) is also fitted remarkably well by the van’t Hoff isochore over the temperature range investigated (20). As the enthalpy of vaporization, ∆Hvap, of each VOC is similar to its ∆Hi , this suggests that the process of desorption involves the volatilization of a physically adsorbed multilayer (20). An interesting exercise for students is to investigate the applicability of the van’t Hoff equation to the total area under the VOC chromatogram, a(T ), in the C6 to C14 region. Figure 4 shows a plot of ln[a(T )] vs 1/T for VOCs originating from LDPE that were desorbed from the Tenax–GC sorbent. The linearity of this plot indicates that the behavior of this multi-component system may be treated, to a good approximation, as a single-component system that has an “average” desorption enthalpy of 45.5 kJ mol ᎑1. This is believed to result from the similarity in the physical adsorption parameters, which, in turn, is due to the similar chemical nature of the
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Figure 2. Chromatogram of C6 to C14 LDPE VOCs desorbed from Tenax–GC: 50 g of LDPE purged with 200 L of N2; desorption at 200 °C for 150 s.
Figure 3. Plots of ln[ai(T )] vs 1/ T for (䊊) n-octanal and (䊉) n-dodecanal, which are organoleptically active VOCs derived from LDPE. The solid lines are linear least-squares fits to the data in accordance with the van’t Hoff isochore (eq 1).
Figure 4. Plot of the natural logarithm of the total area under the chromatogram in the C6 to C14 region, ln[a(T )], vs the reciprocal of the desorption temperature, 1/ T, for LDPE VOCs desorbed from Tenax–GC: ( 䊊 ) experimental data; ( –––) linear least-squares fit to the data in accordance with the van’t Hoff isochore (eq 1).
Journal of Chemical Education • Vol. 77 No. 12 December 2000 • JChemEd.chem.wisc.edu
In the Laboratory Table 1. Identities and Some Thermodynamic Quantities for VOCs Isolated from LDPE at Room Temperature Peak Retention No. Time/min
Tentative Peak Assignment
∆Hi a/ kJ mol ᎑1
∆Hvapb/ kJ mol ᎑1
21.45
5,5-dimethylhex-1-ene
49.8
37.7 c
2
24.24
d
n-heptanal
—
—
3
24.38
n-heptane
41.1
36.6e
1
4
24.56
n-nonanold
5
27.26
n-decene
6
27.44
7
—
—
76.6
50.5 c
8-methyldec-1-ened
—
—
28.15
9-methyldec-1-ene
87.1
—
8
28.32
2,4-dimethyl-n-heptane
33.7
42.9 c
9
28.54
n-octanal
87.5
53.7 f
10
30.36
n-nonanal
52.9
58.5 f
11
30.50
n-undecane
69.5
56.3 e
12
31.46
n-dodecanal
54.2
72.7 f
13
34.02
n-dodecane
43.8
61.3 c
14
35.07
6-methyltridecane
42.7
—
a∆H
i is the enthalpy of desorption from Tanax–GC of the tentatively assigned compounds. b∆H vap values are those of the tentatively assigned compounds as discussed in text. c Values obtained from Wilhoit and Zwolinski (21). d Minor peak in chromatographic window. e Consistent value given in Wilhoit and Zwolinski (21 ) and Lide (22). f Values obtained from Steinmetz (Steinmetz, W. Thermodynamics Research Center, Department of Chemistry, University of Texas, College Station, TX, 1997, private communication) .
TAFE, Victoria University of Technology, for access to the GC– MS system. We are also grateful to the ARC Small Grants Scheme for providing funding to investigate the degradation of synthetic polymers. WSupplemental
Instructor notes, student materials, experimental procedures, and additional information needed to implement the experiment are available in this issue of JCE Online. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.
adsorbed species (20). At temperatures above ca. 175 °C the values of ln[a(T )] depart from the established trend and possible reasons for this have been discussed in the literature (20).
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Conclusions
21.
This experiment demonstrates to students a dynamic headspace sampling technique that is used to analyze odoriphores derived from LDPE at ambient conditions. In particular, the experiment illustrates that (i) the technique of GC–MS may be used as both a qualitative and a quantitative tool in polymer analysis, (ii) the intensity of the chromatographic signal during dynamic headspace sampling is enhanced by increasing the desorption temperature and a systematic approach to the adjustment of experimental variables will be needed in choosing optimum conditions for any subsequent quantitative analysis, and (iii) mathematical models can be used to obtain thermodynamic quantities from experimental data.
Material
22. 23.
24. 25.
Acknowledgments
26.
We would like to thank W. Stirk Kyle, Department of Biology and Food Sciences, Victoria University of Technology, and Peter Thomson, Head of Science and Food Technology,
27. 28.
Saul, E. L. J. Chem. Educ. 1946, 23, 296. Phillips, J. P. J. Chem. Educ. 1958, 35, 35. Ferguson, L. N.; Lawrence, A. R. J. Chem. Educ. 1958, 35, 436. Roderick, W. R. J. Chem. Educ. 1966, 43, 510. Guild, W. Jr. J. Chem. Educ. 1972, 49, 171. Tseng, K.-c.; He, H.-z. J. Chem. Educ. 1987, 64, 1003. Schamper, T. J. Chem. Educ. 1993, 70, 242. Kjonas, R. A.; Soller, J. L.; McCoy, L. A. J. Chem. Educ. 1997, 74, 1104. Richman, R. M. J. Chem. Educ. 1998, 75, 315. Graedel, T. E. J. Chem. Educ. 1984, 61, 681. Ramussen, P. W. J. Chem. Educ. 1984, 61, 62. Levey, M. J. Chem. Educ. 1954, 31, 373. Moore, D. R. J. Chem. Educ. 1960, 37, 434. Grant, N.; Noves, R. G. J. Chem. Educ. 1972, 49, 526. Sbrollini, M. C. J. Chem. Educ. 1987, 64, 799. Quigley, M. N. Educ. Chem. 1994, 19. Brower, K. R.; Schafer, R. J. Chem. Educ. 1975, 52, 538. Slabaugh, W. H. J. Chem. Educ. 1980, 57, 72. Risley, J. M. J. Chem. Educ. 1996, 73, 1181. Hodgson, S. C.; O’Connor, M. J.; Casey, R. J.; Bigger, S. W. J. Agric. Food Chem. 1998, 46, 1397. Wilhoit, R. C.; Zwolinski, B. J. Handbook of Vapour Pressures and Heats of Vaporization of Hydrocarbons and Related Compounds; Department of Chemistry, University of Texas: College Station, TX, 1971. CRC Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996. Bigger, S. W.; O’Connor, M. J.; Scheirs, J.; Janssens, J. L. G. M.; Linssen, J. P. H.; Legger-Huysman, A. In Polymer Durability; Clough, R. L.; Billingham, N. C.; Gillen, K. T., Eds.; Advances in Chemistry Series 249; American Chemical Society: Washington, DC, 1996; p 249. Tice, P. In Food Taints and Off-Flavours; Saxby, M. J., Ed.; Blackie Academic & Professional: London, 1996; pp 226–263. Yam, K. L.; Ho, Y. C.; Young S. S.; Zambetti, P. F. Polym. Plast. Technol. Eng. 1996, 35, 727. Hartman, T. G.; Lech, J.; Karmas, K.; Salinas, J.; Rosan, R. T.; Ho, C.-T. In Flavor Measurement; Dekker: New York, 1993; pp 37–60. Cao, X.-L.; Hewitt, C. N. J. Chromatogr. 1994, 688, 368. Macleod, G.; Ames, J. M. J. Chromatogr. 1986, 355, 393.
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