Note 6: Direct Thermal Analysis of Plastic Food Wraps Using the Short Path Thermal Desorption System

by John J. Manura
2000

Introduction

There is presently a great deal of concern for the potential health risks due to the consumption of food products contaminated with packaging material residues. When using these packaging materials for food preparation at high temperatures (the boiling point of water) such as those encountered in microwave ovens, the problem may become exacerbated by the thermal leaching of the packaging material residues from the packaging film and eventual diffusion into the food products. This concern for the contamination of food products by leaching of residual components from the packaging materials will continue to be of major concern for both the scientific community and the food industry especially with the increased use of recycled papers and plastics. A study was undertaken to detect and identify the low level volatile and semi-volatiles present in commercially available Microwave safe plastic food wraps, and to determine the viability of this technique for the analysis of other packaging material analysis. The new Short Path Thermal Desorption system from Scientific Instrument Services was utilized to permit the direct volatilization of samples into the GC injection port without any prior solvent extraction or without the use of solvent or vapor traps. This technique permits the maximum sensitivity of analysis due to this direct injection technique and permits the capillary Gas Chromatograph/Mass Spectrometer (GC/MS) analysis of these samples. The aim of this study was to demonstrate the versatility of the technique to determine the volatile and semi-volatile residues from the plastic food wraps when they are subjected to conditions such as those encountered in microwave ovens. This work closely correlates with the study done at Rutgers Food Science Department by Thomas Hartman (1), in which Dr. Hartman studied the applicability of this technique to identify the off-odors in packaging films.

Instrumentation

The Scientific Instrument Services Short Path Thermal Desorption System, Model TD-1, as described previously (2 & 3), was attached to the injection port of a Hewlett Packard 5971 GC/MSD. The equipment utilized is identical with the exception that the plastic samples to be analyzed were placed directly inside the desorption tube. The Short Path Thermal Desorption System is placed directly over the top of the GC injection port to permit direct injection in the GC injection port. The GC is equipped with cryo cooling capabilities using liquid CO2 to permit cooling of the GC oven down to -40 degrees C. Data was acquired utilizing the Hewlett Packard ChemStation DOS Series Software.

A J&W DB-5, .25 mm x 25 meters, 0.25u film thickness capillary GC column were utilized for this study. The GC was used in the splitless mode of operation for the maximum sensitivity of analysis. The column terminated in the mass spectrometer source without any splitting. The column flow was adjusted to 0.5 ml/min.

The Hewlett Packard 5971 GC/MSD was operated under standard E.I. conditions. The mass was scanned from 40 to 350 daltons, at a rate of 3 scans per second. All mass spectra were identified through a search of the Wiley NBS library of Spectra.

Figure 1 - Desorption Tube for Direct Thermal Analysis

Experimental

Three inch square samples of four commercially available plastic food wraps (Glad Cling Wrap, Handi-Wrap, Reynolds Plastic Wrap, and Saran Wrap) were placed inside of the 4 inch long by 4 mm I.D. Glass Lined Thermal Desorption Tubes (Figure # 1). These tubes are then attached to the Short Path Thermal Desorption System and a syringe needle is attached to the other end of the GLT Desorption Tube. The desorption tube with sample and needle was flushed with He carrier gas for 2 minutes at a rate of 3 ml/min at room temperature to remove all traces of oxygen from the sample. The GLT Desorption Tube with needle attached (Figure # 2) was injected into the GC injection port. The Short Path Thermal Desorption heater blocks (which were preset to 100 degrees C), were closed around the GLT Desorption Tube which permits the rapid ballistic heating of the sample therein to the preset temperature. This combination of heat and the continuous flow of He carrier gas through the sample, sparge any volatiles and semi-volatiles thermally eluted from the sample directly into the GC injection port. A temperature of 100 degrees C was selected, since this is the boiling point of water and is the temperature that the plastic wraps would be exposed to under microwave oven conditions. The GC injection port was set to 250 degrees C. The GC column was cryo-cooled to -40 degrees C, which permits the cold trapping of the sample in a narrow band at the front of the GC column. The sample was collected and trapped for 10 minutes total time, after which the desorption tube assembly was withdrawn from GC injection port and GC column was temperature programmed from -40 degrees C to 280 degrees C at 10 degrees per minute, to elute the sample components from the GC column. The sample was subsequently analyzed via the mass spectrometer.

Figure 2 - Short Path Thermal Analysis-Theory of Operation

Results and Discussion

Each of the four plastic wraps produced its own distinctive pattern of peaks as observed in the comparison of the total ion chromatograms (Figure # 3, # 4, # 5, and # 6). A negative control sample was also run and found to contain only two minor peaks at 21.8 and 25.2 minutes which were identified as organosilicones originating from column and septa bleed. These background peaks produced no interference with the sample peaks. Each plastic food wrap sample exhibits in excess of 100 peaks which vary in intensity from strong to very weak. The majority of the compounds present were identified as both branched and linear chain hydrocarbons and alcohols. In addition a few cyclic chains and aromatics were also identified as well as a few phthalates. The hydrocarbon chain lengths varied from C6 to C19. Due to the relatively low temperatures used for sample collection (100 degrees C), it was not expected that compounds from the base polymer plastic films would be observable, as was the case. The plastic films analyzed were polyethylene and polystyrene polymers which are not pyrolyzed at these temperatures. All the peaks observed are due to additional compounds added to the plastic polymer including aliphatic and olefinic hydrocarbons, plasticizers, antioxidants, mold release agents, and other contaminants during the manufacturing and packaging processes. Differentiation of these plastic food wraps would have been otherwise impossible by other techniques such as infrared analysis and GC pyrolysis which analyze the basic polymer, whereas this thermal analysis technique identifies the trace surface components. This analysis of the trace surface residues makes it possible to distinguish between these samples.

Figure 3 - Gladwrap Plastic Food Wrap Total Ion Chromatogram.

Figure 4 - Handiwrap Plastic Food Wrap Total Ion Chromatogram.

Figure 5 - Reynolds Plastic Wrap Total Ion Chromatogram.

Figure 6 - Saran Wrap Plastic Food Wrap Total Ion Chromatogram.

Figure #7 A through D, examines in more detail the nature of the residues heat extracted from the Glad Wrap Plastic Food Wrap sample. Figure #7-A is the total ion chromatogram (masses 40 through 350) of the plastic food wrap sample.

Figure 7 - Gladwrap Plastic Wrap

7A - Total Ion Chromatogram.

Figure #7-B is the mass chromatogram of the 43, 57, and 73 ions which are indicative of the linear and branched paraffinic hydrocarbons which are commonly used as mold release agents during the manufacture of these films. These hydrocarbons and many of their corresponding alcohols comprise the bulk of the residues detected in all the plastic samples analyzed. Linear and branched chain hydrocarbons from C6 through C19 were identified. In addition alcohols from nonanol through decanol were identified as were several aromatics including phenol.

7B - Mass Chromatogram for masses 43, 57, and 71 Characteristics of hydrocarbons.

Figure #7-C is the mass chromatogram of the 149 ion characteristic of the phthalate ester plasticizers. The two major phthalates at 34.4 minutes and 38.6 minutes were identified as diethyl phthalate and dibutyl phthalate respectively.

7C - Mass Chromatogram for mass 149 Characteristics of phthalates.

The final figure of this series, Figure #7-D, is the mass spectrum of the dibutyl phthalate peak observed at 38.6 minutes.

7D - Mass Spectrum of peak at 38.6 minutes identified as dibutylphthalate.

Figure #8 is a similar series of chromatograms of the Handi-Wrap plastic food wrap. Mass Chromatograms of the same classes of compounds as above were studied to pattern the hydrocarbons and phthalates. The mass spectrum at the bottom identifies the peak at 31.7 minutes as tetradecane one of the many long chain hydrocarbons identified. In the Handi-Wrap plastic food wrap samples analyzed branched and unbranched hydrocarbons from C6 through C19 were identified, as were many alcohols, and trace amounts of some aromatics including phenol. Noteworthy is the large group of peaks between 25 and 27 minutes which were not observed in the Glad Wrap sample. Most of these peaks were identified as branched chain hydrocarbons. The major peaks at 30.2 minutes, 31.7 minutes, 33.1 minutes, and 34.3 minutes were identified as Tridecane, Tetradecane, Pentadecane, and Hexadecane respectively. The minor peak just after Pentadecane at 34.4 minutes was identified as diethyl phthalate.

Figure 8 - Handiwrap Plastic Wrap

8A - Total Ion Chromatogram.

8B - Mass Chromatogram for masses 43, 57, and 71 Characteristics of hydrocarbons.

8C - Mass Chromatogram for mass 149 Characteristics of phthalates.

8D - Mass Spectrum of peak at 38.6 minutes identified as dibutylphthalate.

Figure #9 and #10 are the same series of total ion chromatograms and mass chromatograms on the Reynolds Wrap and Saran Wrap samples consecutively. The same mass chromatograms for the hydrocarbons and phthalates were plotted out to compare these two samples. Both of these plastic food wraps exhibited a major peak at 33.5 minutes which was identified as BHT (Butylated Hydroxytoluene). BHT is commonly used as an antioxidant in the food industry to protect the foods as well as the film itself. The hydrocarbon patterns for both plastic food wraps are quite unique. BHT was the major component in both samples, while the bulk of the remaining peaks were identified as linear and branched chain hydrocarbons between C6 and C19. Alcohols including among others hexanol, octanol, and nonanol were also identified as were a few aromatics.

Figure 9 - Handiwrap Plastic Wrap

9A - Total Ion Chromatogram.

9B - Mass Chromatogram for masses 43, 57, and 71 Characteristics of hydrocarbons.

9C - Mass Chromatogram for mass 149 Characteristics of phthalates.

9D - Mass Spectrum of peak at 38.6 minutes identified as dibutylphthalate.

Figure 10 - Handiwrap Plastic Wrap

10A - Total Ion Chromatogram.

10B - Mass Chromatogram for masses 43, 57, and 71 Characteristics of hydrocarbons.

10C - Mass Chromatogram for mass 149 Characteristics of phthalates.

10D - Mass Spectrum of peak at 38.6 minutes identified as dibutylphthalate.

Conclusion

This work complements a study which was previously done by T. Hartman (1) who used the same technique to identify off-odors in plastic food wraps using this technique. Utilizing the Short Path Thermal Desorption System it is possible to analyze plastic film samples, as well as other packaging materials, by directly sparging the volatiles eluted from the surfaces of these materials into the GC injection port. The technique is unique in that it can be utilized to analyze the trace surface components and impurities present on polymers which are not pyrolyzed or thermally effected at the relatively low temperatures used in this sampling technique. By analyzing the samples directly via the shortest transfer line possible and without the use of chemical extraction or other sample preparation much sample setup time is saved and the maximum sensitivity of sampling is obtainable. This technique can easily be applied to other applications such as analyzing recycled papers and plastics. From the above study it is quite evident that this technique can be used to:

1. Identify volatiles and semi-volatiles present on the plastic film surfaces to determine their safe use.

2. Identify manufacturing contaminants

3. Identify off-odors due to manufacturing processes or recycling

4. Characterize plastic films for forensic or other identification purposes.

The use of the Short Path Thermal Desorption System sample introduction system, used in conjunction with the GC/MS is a powerful tool for the analysis of trace levels of volatiles and semi-volatiles via the direct thermal analysis for plastic food wraps as well as other plastic materials and other solid type samples. The amount of sample required for the analysis is very small, and due to the direct injection and volatilization into the GC injection port, maximum sensitivity is attained via this technique. In addition the labor time for sample prep or the use of solvents for extraction is eliminated with this technique. This same technique has also been utilized for the analysis of spices, peanuts, residual solvents in pharmaceuticals, and synthetic fibers (3).

References:

(1) Determination of Off-Odors and Other Volatile Organics in Food Packaging Films by Direct Thermal Analysis-GC-MS, Thomas Hartman, The Mass Spec Source, Vol. XIII, No. 4, pg. 30 (December 1990).

(2) Short Path Thermal Desorption - Design and Theory, John Manura, The Mass Spec Source, Vol. XIII, No. 4, pg. 22 (December 1990)

(3) Direct Thermal Analysis, John Manura, The Mass Spec Source, Vol. XIV, No. 1, pg. 22 (March 1991).