Chromatography to the Rescue!

Posted by on Sep 26, 2014 in C&T Thought Leadership

Did you know that diesel and heating oil spills can sometimes be age-dated? If you want to know if a release was recent or if it occurred years ago, we can use chromatography to find out the answer, if the degradation of specific compounds follows known patterns. This type of information is often used to determine liability for clean-up or confirm that any release concerns are not the responsibility of a new owner. Here’s an example of this type of a challenge brought to us by a client.

Challenge: 

We were recently asked by a client to determine whether the diesel range hydrocarbons we detected in a sample were from a recent release or something older. The “weathering” of fuels in the environment can occur through evaporation, leaching or microbial action (biodegradation), all of which have different effects, but in this case the sample chromatogram represents the archetype of biodegraded diesel.

Solution:

Diesel is a mixture of hundreds of compounds. In the fresh diesel chromatogram below, each compound is represented by a peak along the x-axis. The prominent, evenly-spaced peaks represent the C10 to C22 n-alkanes. Those straight-chain hydrocarbons are the first to be digested by microbes, followed by the cycloalkanes and aromatics that together make up the majority of diesel. But there is another class of compounds called isoprenoids which comprise less than 5% of diesel. These branched alkanes have a chemical structure that inhibits biodegradation. In the fresh diesel chromatogram, two of these compounds – pristane (C-19 isoprenoid) and phytane (C20 isoprenoid) – can be seen just to the right of the C17 and C18 alkanes respectively.

In the degraded diesel chromatogram below, the alkane peaks are not visible above the chromatographic “hump” of unresolved compounds. Instead, the prominent peaks represent various isoprenoids, including pristane and phytane.

Christensen and Larsen (1993) famously developed a model to age date diesel and heating oil spills in soil that was based on the peak-height ratio of n-C17 to pristane. The n-C18/phytane ratio can be similarly compared. Those ratios start at approximately 2 in fresh fuel and fall to 0 after about 20 years. The method was developed using data collected from various European sites where the age of the spill was known and where the release had not been weathered significantly except by biodegradation.

Results: 

In this case, the C17 and C18 alkanes are not detectable above the hump, so the ratios are 0 and we can be confident that this was not a recent release. While this model won’t always be applicable, we work with our clients to help find answers to their particular challenge using the appropriate models and testing procedures.