Interpretation of volatile organic compound measurements by proton-transfer-reaction mass spectrometry over the deepwater horizon oil spill

• Use GC–PTRMS to identify major masses in oil spill and crude oil samples.
• Cycloalkanes are important in PTR-MS mass spectra of crude oil samples.
• Evaporation of crude oil can affect the identification of PTR-MS masses.

A proton-transfer-reaction mass spectrometer (PTR-MS) was used onboard the NOAA WP-3D aircraft for atmospheric measurements over the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico in 2010. A mass spectrum obtained by PTR-MS downwind from the accident site revealed the presence of a complex mixture of volatile organic compounds (VOCs). Here, we interpret this mass spectrum by comparing it with mass spectra of crude oil, gasoline and diesel samples measured in the laboratory. Aromatics were less abundant over the spill than from crude oil samples, due to the dissolution of these species in the seawater. The mass spectra obtained from aircraft measurements and crude oil samples both show strong signals at masses with mass-to-charge ratio (m/z) of 14 × n ± 1, including 43, 57, 69, 71, 83, 85, 97 and 111Yamu. PTR-MS coupled with a gas chromatograph was used to identify major mass signals from crude oil samples. Cycloalkanes are important contributors to the signals of mass m/z 14 × n ± 1, especially for masses 69, 83, 97 and 111 amu. Aromatics could be interpreted from their specific masses without significant interference for crude oil vapors, but the interferences to benzene from higher aromatics can be significant as crude oil evaporates. The interpretation of DWH mass spectrum is not only helpful in understanding the atmospheric emissions associated with the DWH oil spill, but also for the interpretation of PTR-MS measurements in urban air, near natural oil seeps and oil as well as natural gas extraction activities.

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