The Forgotten Compounds

One of the purposes of this blog is to offer quick review on peer-reviewed articles that I read recently and I found interesting. The thinking behind this is that 1) it gives me place to archive these quick summaries I sometimes write about paper I read, 2) it will encourage me to do more of these summaries, 3) but not least it well promote good research! I will be mainly positive in my reviews as they are other fora for criticism.

The paper I am writing about today was brought to my attention by a tweet by @EnvSci&Tech although it has been published by people of which I know and admire the work.

“Oil Weathering after the Deepwater Horizon Disaster Led to the Formation of Oxygenated Residues” by Aeppli et al. doi: 10.1021/es3015138 (ES&T)

Of course when you have an interest in Environmental Forensics or Environmental Contamination, you feel very jealous of people who were given the chance to work on the Deepwater Horizon oil spill. It was obviously a disaster but it is so well documented that you know where your oil comes from and when it was released which mean that you get a rare opportunity to establish principles that you will be able to apply to less documented spills.

The scientific approach of the paper is really exciting for an analytical chemist: GC-FID, GC-MS, TLC-FID, GCxGC (FID, MS), Elemental analysis, natural radiocarbon analysis, 1H NMR and FT-IR!!! A lot of the tools in the box! Using all these tools, they aimed at characterising the forgotten compounds of oil analysis. A tiered approach using GC and GC-FID is usually employed to characterise oil from spill (see any publication by godfather of environmental forensics Zhendi Wang) but this means that only “GC compatible” compounds are seen and quantified. One of the reasons behind this is that the compounds that were thought the most toxic are PAHs which are seen by GC analysis.  Recent findings show that other compounds might be toxic in oil slicks and notably oxygen-containing compounds. Aeppli et al. define a new family of compounds to look at when studying spills, the “oxygenated fraction” composed of what they coined the “oxyhydrocarbons” and in this article attempt to characterise these compounds compared to the other fractions and propose hypothesis regarding their origin.

This struck a chord with me because we published recently a paper on the hydroxylated compounds (or “oxyhydrocarbons”!)  identified in coal tar DNAPL samples (doi:10.1016/j.chroma.2012.06.093.) and that I am currently preparing a second paper on the potential origin of these compounds. Our approach was to derivatise the DNAPL prior to solvent extraction (but they will be more on this elsewhere). Although they also used derivatisation, the authors demonstrated the existence and the fate during weathering of the oxygenated fraction using mainly TLC-FID, FT-IR and elemental analysis. Figure 2 of the paper is wonderful! Comparing the trend of the GC traces with the TLC-FID traces and the FT-IR spectra! Delicious! It clearly shows that the more weathered the crude oil is the more important the portion of oxyhydrocarbons becomes, with the FT-IR trace clearly showing an increase in the peaks for C-OH and C=O. Using hopane normalisation they demonstrate that the phenomenon is due to the formation of the oxyhydrocarbon and not to the disappearance of the aromatic and aliphatic fractions (confirming their assumption using the Carbon Preference Index on the long chain carboxylic acids… which made me think: I can do that too!!). Their suggestion is that biodegradation and photo-oxidation are responsible for the oxyhydrocarbons but also that these compounds are more recalcitrant than the hydrocarbons and are note transitory states “en route” to mineralisation as they appear early in the weathering process but are still in the very weathered sample.

So on the negative side, what if they are toxic?? However, on the plus side, they are likely to be good forensic markers to evaluate degrees of weathering.

PS: I now really really want to use FT-IR for our coal tar samples, these spectra are so nice to look at, a delicacy for analytical chemists!

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