One very common analysis run on soil and / or groundwater samples collected during a site characterization project is Total Petroleum Hydrocarbon (TPH). Many times, we focus this analysis on gasoline range (TPHg), diesel range (TPHd), motor oil range (TPHo), or the full carbon chain (TPHcc) depending on the historic site use or the findings of previous site investigations.
There several ways of analyzing TPH in soil and groundwater, but the general theory is to attempt to identify the portions of the carbon chain and length of the carbon chain present in the sample. This stems from the fact that these compounds originate from long-chain crude oil that is refined into shorter carbon chain segments. The, “Lighter-end” hydrocarbon chain with 5-12 carbons generally represent the gasoline range (TPHg). The carbon chains ranging between 10-28 are generally considered diesel (TPHd), with compounds such as kerosene and jet fuels somewhere in between. The actual number of carbons in the chain varies based on the specific type of petroleum product, it’s purity, and age (extent of degradation). The, “Heavier-end” of the carbon chain up to around 40 carbons is considered the motor oil range (TPHo) and in this range, we generally request that the laboratory perform a silica gel clean up on the sample to remove naturally-occurring lipids that would create a false-positive. One example of a naturally occurring compound that would be removed by the silica gel cleanup method would be eucalyptus oil. The only drawback of the silica gel cleanup process is a small amount of dilution and subsequently slightly higher detection limits. For this reason, we really only perform the silica gel cleanup process when performing a site investigation or characterization and avoid it in more long-term groundwater monitoring or remediation projects.