Chromatography Forum

I have a sample that tested high for aliphatic c16-21 hydrocarbons, but low for everything else. Motor oil would be an example of a c16-21 aliphatic, but aromatic c16-21 hydrocarbons are also found in motor oils.

Is it common to only see c16-21 aliphatic hydrocarbons but not aromatic c16-21?

Aliphatic c16-21 was at 4400 ppm, aromatic c16-21 was at 40 ppm.

Aromatics may not be present in some synthetic oils (motor oils, or whatever else might be using C16-21 size hydrocarbons). Conventional motor oil is distilled from petroleum, thus chock full of both aromatic and aliphatic hydrocarbons. Some types of synthetic oil are created by GTL such as at Shell's Pearl plant in the ME.

Are there any other clues to your source in the soil sample?

Hi

Just a couple of thoughts

I would suspect that Motor oils would have a much higher chain length range.

Would diesel engine exhaust fumes or diesel fuel fit better with your observations? How close to a major road or factory were your soil samples taken and at what depth?

Edit 8/10/16

The aromatics could be lost leaving the less volatile aliphatics

Regards

Ralph

You will see various aromatic compounds in most heavy-range petroleum distillates (including motor oil). However, they should be present at lower concentrations than the aliphatics. In particular, look for ethylbenzene, xylenes, trimethylbenzenes and isomers of ethyltoluene. As mentioned previously, the aromatics are more vulnerable to weathering (i.e. evaporation), which could also explain their low abundance. Aromatics may not be present at all if the product has been dearomatised.

The actual hydrocarbon range of the substance detected will depend on the sampling technique. However, the pattern of the aliphatic compound peaks will be more indicative of the type of product present. Distillates have a characteristic Gaussian distribution of n-alkane peaks.