Determination of PAHs in the Tires by GC/MS, 08-R8402
Inclusive Dates: 07/01/13 – Current
Background — In 2007, the European Union issued 1906/2007/EC (REACH), a regulation requiring, as of January 1, 2010, all process oils isolated from the tire treads contain less than 1.0ppm benzo(a)pyrene, BaP, and that the sum of eight specific PAHs be less than 10ppm. ISO 21461, the EU's official method, uses nuclear magnetic resonance analysis to determine whether a tire meets these new standards. The EU regulation states that if the Bay-H NMR signal of the process oil isolated from the tire treads is < 0.35 percent of the total H NMR signal, the tire is regarded as having met the EU's PAH standards for the tires.
Approach — We set out to develop a GC/MS method that would accurately and precisely determine each individual PAH concentration in the process oils isolated from tires. The approaches included:
- using the GC/MS with isotope dilution technique
- studying the effects of the sizes of the rubber bits on PAH extraction efficiency
- comparing various solvents for their efficiencies of extracting the PAHs from the rubber bits
- fine tuning the sample extract cleanup procedures with liquid-liquid partition and silica gel liquid column chromatography
- studying the accuracy and precision of the developed GC/MS method.
Accomplishments — Rubber bits of five sizes were tested: 2.8 to 2.36mm, 2.36 to 1.70mm, 1.70 to 1.00mm, 1.00 to 0.425mm, and 0.425 to 0.125mm. No significant differences in PAH extraction efficiency were observed. The solvent extraction efficiencies of the five-ring PAHs from the rubber bits were: toluene > carbon disulfide > acetone = 1,4-dioxane > propionic acid > carbon tetrachloride > cyclohexane. The DMF (N,N-dimethylformamide) or NMP (N-methyl-2-pyrrolidone) each with 10 to 15 percent water extract PAHs well from hexane, separating PAHs from the non-PAH hydrocarbons. The liquid chromatography column packed with activated silica gel also separates target PAHs well from the non-PAH hydrocarbons. The method average accuracy for the determination of 18 PAHs and the five-ring PAHs in the tire oils were 52.1 percent and 48.5 percent, respectively. The method average %RSD for the determination of 18 PAHs and the five-ring PAHs in the tire oils were 15.3 percent and 18.8 percent, respectively.
Forty-three passenger car tires made in 15 countries were purchased locally. These tires were cut up and their treads extracted for PAH analysis. The tire extract oils were analyzed using the developed GC/MS method for quantifying 18 PAHs using 16 deuterated PAHs as internal standards. The split extract oil of each tire was sent to a commercial laboratory for NMR analysis by ISO 21461. A pass/fail decision was made for each tire based on BaP, sum of the eight PAHs, and NMR data.
Results of Commercial Tires Tested for PAHs
- 81 percent (35 tires/43 tires) passed based on NMR data.
- 23 percent (10/43) passed based on the level of Total EU 8 PAHs by GC/MS.
- 7 percent (3/43) passed based on the level of BaP by GC/MS.
- 100 percent (3/3) passed based on the BaP level also passed the Total 8 PAHs level.
- 70 percent (7/10) passed based on the Total 8 PAHs level failed the BaP level.
- 25.6 percent (11/43) agreed between the NMR and GC/MS data on pass/fail decisions, of which 27.3 percent (3/11) agreed on passes (i.e. pass/pass) and 72.7 percent (8/11) agreed on "fails" (i.e. fail/fail).
- 74.4 percent (32/43) disagreed on pass/fail decisions between the GC/MS and the NMR methods, of which 100 percent passed by the NMR data and 100 percent failed by the GC/MS data.
The data for individually tested tires are given in Table 1.
In conclusion, the NMR method tended to pass tires easily (35/43 or 81.4 percent), many of which (32/35 or 91.4 percent) would be viewed as "false negative" based on the GC/MS (BaP) data.