Note 37: Volatile Organic Emissions from Automobile Tires

Santford V. Overton & John J. Manura
1999

INTRODUCTION

Numerous reports have appeared describing the "sick building syndrome" which has been associated with the quality of indoor air in public buildings. Building related health problems may be due to contamination of indoor air by emissions of volatile organic compounds (VOC'S) from a variety of sources including construction materials, fabrics, furnishings, maintenance supplies, combustion by-products, adhesives, paints, caulks, paper and cleaning products. Recently new allegations have appeared suggesting increased danger to the population due to emissions from automobile tires. Because many of the volatile emissions and by-products from these products are toxic, additional knowledge of the levels of these organic chemicals is required in order to determine the human health impacts. This is particularly important in highly populated cities where these compounds can not escape the maze of high rise office buildings and gradually affect the quality of indoor air in public buildings. In the next few years, it is expected that the quality of indoor air will come under closer scrutiny by the public in both the domestic home and industrial workplace environments. New methods will be required to accurately determine the identity and to accurately quantify the levels of these volatile organics to help identify potential health risks. If manufacturing processes are contributing to poor air quality, then the manufacturing processes will need to be improved to limit the emission of VOC's. For this study, samples of automobile tires were analyzed by "Direct Thermal Extraction". This new technique using a thermal desorption apparatus attached to the injection port of a GC/MS system permits the direct thermal extraction of volatile and semi-volatile organics directly from small sample sizes (mg) without the need for solvent extraction or other sample preparation. The samples are ballistically heated and together with the carrier gas flow through the samples the volatiles are outgassed into the injection port and onto the front of the GC column for subsequent analysis via the GC and/or GC/MS.

Instrumentation

A new technique called "Direct Thermal Extraction" which utilizes a thermal desorption apparatus attached to the injection port of a GC/MS system permits the direct thermal extraction of volatile and semi-volatile organics directly from small sample sizes (mg) without the need for solvent extraction or other sample preparation. The sample is placed inside a preconditioned glass-lined stainless steel desorption tube between two glass wool plugs which simply hold the sample in place. The desorption tube containing the sample is attached to the Short Path Thermal Desorption System and a syringe needle attached. The desorption tube with sample is injected into the GC injection port, ballistically heated and together with the carrier gas flow through the sample the volatiles are outgassed into the injection port and onto the front of the GC column for subsequent analysis.

All experiments were conducted using a Scientific Instrument Services model TD-3 Short Path Thermal Desorption System accessory connected to the injection port of an HP 5890 Series II GC interfaced to an HP 5971 Mass Selective Detector. The mass spectrometer was operated in the electron impact mode (EI) at 70 eV and scanned from 35 to 550 daltons during the GC run for the total ion chromatogram.

A short 0.5 meter by 0.53 mm diameter fused silica precolumn was attached to the injection port end of a 60 meter x 0.25 mm i.d. DB-5MS capillary column containing a 0.25 um film thickness. The GC injection port was set to 260 degrees C and a direct splitless analysis was used. The head of the column was maintained at -70 degrees C using a GC Cryotrap model 951 (Scientific Instrument Services, Ringoes, NJ) during the desorption and extraction process and then ballistically heated to 200 degrees C after which the GC oven was temperature programmed from 35 degrees C (hold for 5 minutes) to 80 degrees at 10 degrees C/min, then to 200 degrees C at 4 degrees C/min and finally to 260 degrees C at a rate of 10 degrees /min.

Experimental

Several brands of automobile tires were analyzed by "Direct Thermal Extraction" to determine the most sensitive technique for possible development of a quality control method for the tire industry. Tire samples measuring approximately 30 mg were placed into an inert thermal desorption tube on top of a glass wool plug (Fig. 1).

The desorption tube with sample was then attached to the Short Path Thermal Desorption System and a syringe needle attached. The desorption tube was injected into the GC injection port at desorption block temperatures of 150 degrees C, 175 degrees C, 200 degrees C, 225 degrees C and 250 degrees C for 5 minutes each. The extracted organics are subsequently cryo-trapped at the front of the GC injection port using the GC Cryotrap at a temperature of -70 degrees C. After the 5 minute desorption period, the Cryotrap was heated to 200 degrees C to elute the volatiles and begin the GC analysis and identification via the mass spectrometer.

Results and Discussion

New tires from two different manufactuters as well as a used tire were analyzed by "Direct Thermal Extraction" to identify and compare the volatile organics present. A series of studies were conducted to determine the optimum conditions for desorbing the volatile organic compounds in one brand of tire. Samples of tires from Brand A were desorbed at temperatures of 150 degrees C, 175 degrees C, 200 degrees C, 225 degrees C and 250 degrees C, in order to determine the optimum temperature for the complete extraction of all the VOC's (Figs. 2-6). Temperatures lower than 225 degrees C were determined to be insufficient for the detection of all the VOC's (Figs. 2-4). However, the compounds 1,2-dihydro-2,2,4-trimethyl-quinoline and 2,4-dimethyl-quinoline were only detected at a desorption temperature of 150 degrees C (Fig. 2). Generally, desorption temperatures of 225 degrees C to 250 degrees C were determined to be the optimum temperatures for the thermal desorption of the VOC's (Figs. 5&6). At these higher temperatures, the tire samples from Brand A were found to contain numerous straight and branched chain hydrocarbons, aldehydes, alcohols, ketones, furans and benzene derivatives in addition to much higher concentrations of the compounds previously identified at the lower desorption temperatures. Compounds such as benzene, cyclohexene and 4-methyl-2-pentanone were detected at much higher concentrations at desorption temperatures 225 degrees C as compared to the lower temperatures (Figs. 5&6). Methanethiol, which occurs in coal tar and in petroleum distillates, and xylene were only found in tire samples desorbed at the higher temperatures.

Although they possessed many common compounds, significant differences existed in samples of Brand B desorbed at 250 degrees C as compared to Brand A (Fig. 7). There was significant variation in the quantities as well as the variety of volatile organics present in Brand B which may be attributed to the differences in the manufacturing processes. Brand B was found to contain high concentrations of the compounds sulfur dioxide (DOT), 2-methyl-1-propene, 2-propanone, 2-methyl-2-pentene, 2,4-pentanedione, acetic acid and 2,4-(1H, 3H) pyrimidinedione which were not detected in Brand A.

There was also variation in the quantities and variety of VOC's present in a used Brand A tire (Fig. 8). Significant increases were found in the aromatic compound toluene, the siloxanes octamethyl-cyclotetrasiloxane and decamethyl-cyclopentasiloxane as well as the compound benzothiazole which is used in organic synthesis and has an odor similar to that of quinoline. Decreases were detected in benzene, cyclohexene and 4-methyl-2-pentanone. Not only can differences in VOC's be attributed to the differences in the manufacturing processes from tire to tire by the same manufacturer, but road wear and build-up may also contribute to these changes.

Conclusion

"Direct Thermal Extraction" using the Short Path Thermal Desorption System attached to the injection port of a GC/MS system permits the direct thermal extraction of volatile and semi-volatile organics directly from small sample sizes without the need for solvent extraction or other sample preparation. This technique offers several unique advantages over other time consuming solvent extraction techniques routinely used in the laboratory. New methods which reduce or eliminate the solvents required for sample purification provides many advantages to the laboratory. The thermal extraction of volatiles and semi-volatiles using the "Direct Thermal Extraction" technique is thorough and complete with no trace of foreign contaminates from the extraction technique, solvents or other memory effects contributing to the chromatogram. This technique can be easily incorporated into a troubleshooting technique to detect problems in various commercial tires due to potential toxic emissions, to compare competing manufacturers' products, as well as implementation into a quality control program.