QUANTITATION OF PAHS IN USED ENGINE OIL USING GCXGC AND TIME OF FLIGHT MASS SPECTROMETRY


Peak tailing due to interactions with GC columns and MS sources and reproducibility of internal standard injections often make the quantitative analysis of polycyclic aromatic hydrocarbons (PAHs) particularly challenging. The addition of a complex matrix, often found in petroleum products that have thousands of distinct components, only makes experiments even more problematic. These diffi culties are inherent to characterizing PAH levels in samples of used engine oils, which not only gives insight into engine combustion effi ciencies but also helps ensure proper environmentally-conscious waste disposal.


Acquisition Parameters Table 1. GC×GC-TOFMS (Pegasus®


Gas Chromatograph Injection Carrier Gas Primary Column Secondary Column Temperature Program BT 4D) Conditions


Agilent 7890 with Dual Stage Quad Jet Modulator and LECO LPAL-3 Autosampler


Liquid injection, split 20:1 @ 320°C


He @ 1.4 mL/min, Corrected Constant Flow


Rxi-PAH, 60 m x 0.25 mm i.d. x 0.10 µm coating (Restek, Bellefonte, PA, USA)


Rxi-1HT, 0.6 m x 0.25 mm x 0.10 µm coating (Restek, Bellefonte, PA, USA)


1.5 min at 80°C, ramped 10°C/min to 300°C, then ramped 3°C/min to 320°C and held 10 min


Secondary oven maintained +10 °C relative to primary oven


Figure 1. Chromatographic contour plots displaying characteristic masses for PAHs are shown for the PAH Standard at 1000 ppb concentration level and a sample of used engine oil from a car routinely driven short distances. GCxGC provides a clear separation of the PAH band, which elutes before the large mass of hydrocarbon interferences in the 2nd dimension.


Using a combination of comprehensive two-dimensional gas chromatography (GCxGC) and high performance time-of-fl ight mass spectrometry (TOFMS) found in the LECO Pegasus BT 4D, PAHs are separated from matrix interferences using both orthogonal column phase selectivity and additional extracted ion mass precision. Identifi cation of specifi c compounds is accomplished by retention time correlation with standard mixes and full-mass range spectral matching with commercial libraries. With common quantitation challenges overcome, PAH levels in used engine oils are compared between gasoline-powered engines


in cars that routinely travel short vs. long distances, providing insight into the nature of combustion by-products that occur when engines are routinely operated under different conditions.


Experimental Samples and Standards


PAH Calibration Standards (Restek #31874 EPA Method 8310 PAH Mixture) were made at concentrations of 5, 10, 25, 50, 100, 250, 500, 1000, and 2500 pg/µL in toluene and spiked with 100 pg/µL of PAH Internal Standard (Restek #31206 SV Internal Standard Mix).


Samples of used engine oil were collected from the dipsticks of various cars: one car routinely driven short distances with an average of 5 miles/trip before and after an oil change, labelled Used Oil (S) and New Oil (S); one car routinely driven medium distances with an average of less than 50 miles/trip labelled Used Oil (M); and one car routinely driven long distances with an average of greater than 100 miles per trip labelled Used Oil (L). A sample of unused oil was also collected from a newly opened bottle of commercially available SAE 30 engine oil. Figure 2 shows the icons and description for each sample type.


Each sample was diluted to 10 mg/mL in toluene and spiked with the PAH internal standard.


Modulation Transfer Line


Mass Spectrometer Ion Source Temperature Mass Range


Acquisition Rate Data Processing


Sample fi les were processed using the Target Analyte Find (TAF) and Non-Target Deconvolution Peak Find (PF) features of ChromaTOF. Calibration curves for a quantitation method were created using peaks from the TAF processing within the Quantitation section of the ChromaTOF software. Identifi cations of peaks were confi rmed by matching spectra from the NIST 17 commercial libraries to the deconvoluted peaks returned by PF processing.


Results and Discussion


Linear calibration curves were built using peak areas of each standard analyte with concentrations of 5-2500 pg/µL. Figure 3 shows an example of the calibration curve generated for fl uoranthene, with excellent linearity demonstrated. Table 2 shows the linear least-squares correlation coeffi cients for each calibrated analyte, with values of greater than 0.995 for each.


2.5 s with temperature maintained +10°C relative to 2nd oven


350 °C


LECO Pegasus BT 4D 300 °C


45-500 m/z 200 spectra/s


Figure 2: Sample key showing icons representing various samples of engine oil. OCTOBER / NOVEMBER • WWW.PETRO-ONLINE.COM


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