What I want to do
I wanted to investigate how the spectral profile an oil sample is affected by the dilution of the sample in mineral oil.
Why I'm interested
If we want to use the oil testing kit to analyze samples collected from the environment, we will need to know how emission spectra are affected by dilution. It may be difficult to measure the concentration of oil in environmental samples with any degree of accuracy. The question arises: how carefully must be control for dilution when we prepare our samples?
I made a series of dilutions of diesel fuel in mineral oil to see how the spectral distribution would be affected. I first loaded approximately 1 ml of pure mineral oil into a cuvette and double checked that it did not exhibit any detectable fluorescence. I then began adding diesel fuel into the cuvette, five drops at a time. I mixed the diesel fuel into the diluent by inverting the cuvette several times before taking measurements of the spectral distribution.
My data set can be viewed here. I found that the neat diesel fuel had a red shifted and spectrally broadened emission spectrum in comparison with the most dilute sample tested (containing 5 drops of diesel in ~ 1 ml of mineral oil or ~1:5). With the exception of a single outlier, the spectra shift monotonically to the right with each increase in diesel concentration across the whole set (after equalizing either by area or by height).
This general trend is in good agreement with the published literature. The figure below, reproduced in Analysis of Crude Petroleum Oils Using Fluorescence Spectroscopy (Reviews in Fluorescence 2005) shows the spectral shifting that occurs in a lighter and a heavier grade of crude oil at various dilution factors.
This shifting of the emission profile is generally attributed to “collisional energy transfer” — the process whereby an excited molecule transfers energy to a larger molecule by colliding with it, which then causes it to fluorescence Visible and Near-Infrared Fluorescence of Crude Oils.
Questions and next steps
I intend to expand upon this area of study by further investigating the effects of dilution on different types of oils included with the OTK. I would also like to see this result replicated by other experimenters in the Public Lab community. If these findings are accurate, they could have important consequences for how we think about the preparation of samples from the environment.
The identification of environmental samples may require the development of methods to control for the dilution of the analyte within a sample. Absorption spectroscopy may prove useful as a tool to help quantify the concentration of oil in environmental samples, so that we can compare them to known standards.