Polycyclic aromatic hydrocarbons (PAHs) are a class of naturally occurring chemicals comprised of two or more benzene rings fused together that have varying degrees of negative impacts on human health. PAHs are also a component in crude oils and find their way into hydrocarbon process/extender oils used in the manufacturing of tires. The European Union (EU) issued a directive limiting the PAHs present in the process/extender oil used to manufacture tires. This directive became effective on January 1, 2010. In recent years, due to the advancement in PAH carcinogenicity studies, a list of 30 PAHs has been identified that most likely will be in the EU’s next PAH regulation for process oils and require their detection limits (DL) to be in the range of 0.1 ppb to 40 ppb. Thus, there is a need to develop a PAH method that delivers DLs of 0.1 ppb to 5 ppb in process oils.
Our technical approach was four-fold: 1) using a sample preparation process comprised of liquid-liquid partition (LLP) coupled with deactivated silica gel column chromatography (DSGC) (100-200 mesh, 60Å), 2) increasing the sample size from 25 mg to 250 mg and at the same time decreasing the sample final extract volume from 1,000 uL to 250 uL, 3) using both High Resolution Gas Chromatography/Low Resolution Mass Spectroscopy (HRGC/LRMS) and High Resolution Gas Chromatography/High Resolution Mass Spectroscopy (HRGC/HRMS) to achieve the DLs of 0.1-5.0 ppb, and (4) using a GC/MS with isotope dilution technique in which a PAH is quantified against its deuterated counterpart, such as quantifying benzo(a)pyrene against benzo(a)pyrene-d12.
On average, the LLP process removed 97.2% of the oil in just two rounds of LLP operations. Out of the four non-polar solvents tested in the LLP process, pentane gave the highest PAH recoveries, followed by isooctane, hexane, and lastly heptane. The average efficiency of the DSGC with 7% deactivation on removing the oil matrix was 98.5% using either the 10 mm or 14 mm i.d. glass columns. Four DSGCs were tested with 7%, 8%, 9%, and 10% deactivations. Interesting PAH elution patterns were documented. These PAH elution patterns, especially those of the 9% and 10% deactivations, could be incorporated into future lab practices to produce cleaner sample extracts and thereby help the lab meet demanding DL requirements such as those set for the dibenzopyrenes. With the tandem LLP and DSGC sample prep, a 1 ppb DL was achieved for all 12 PAHs, and the 0.1 ppb DL goal was achieved on two oils doped with dibenzopyrenes. The data collected in this project strongly suggest that any lab’s ability to quantify PAHs accurately and precisely in process oils at very low concentrations such as 0.1-1.0 ppb largely hinges on, in addition to the high sensitivities of its GC/MSs, its ability to remove the maximum amount of oil matrix in the samples while preserving the maximum amounts of PAHs.