3. Separation of Linear Alkanes from the UCM in Lube Oil and PM

The Carbon Preference Index (CPI) is used in assessing the relative contribution from anthropogenic and biogenic sources in particulate matter that is found in ambient air. The CPI is determined by calculating the ratio of the sum of odd- to the sum of even-carbon alkanes found in the soluble organic fraction of particulate matter samples. However there are many cases where the CPI cannot be reliably calculated because GC/MS analysis of their linear alkanes is difficult. This arises from the similar nature of the non-polar compounds, i.e., linear and branched alkanes and cycloalkanes, found in these samples, which are difficult to resolve by gas chromatography. Typical chromatograms contain many overlapping peaks that produce a large hump commonly described as the unresolved complex mixture (UCM) interfering with the identification of alkanes. A project was initiated to therefore minimize or eliminate this UCM. The size and shape selective properties of molecular sieves were investigated to separate alkanes from the UCM. Result from tests using molecular sieves will be presented.

Several sieves were tested on pure alkanes and lube oils. Lube oils were used because it has been shown that they contribute to particulate matter in ambient air and contain many of the compounds that give rise to a large UCM. Data obtained with a 5Å pore size sieve showed the most promising results and it was selected for further testing. Diluted mixtures of pure alkanes were exposed to the 5Å molecular sieve and resulted in a 95% uptake. It was also shown that the alkanes could be extracted (ca. 67%) from the sieve using a mixture of acid and hexane. Further tests with lube oil samples were done using deuterated alkanes as recovery surrogates. However, data from the experiments indicated that the extraction of the alkanes from the lube oil samples is more difficult. Modifying the extraction procedure using a sonicator bath has yielded a method where alkanes can be obtained in higher yields from lube oil samples with a significant reduction of the UCM.