Public Lab Research note

Oil testing kit: Testing

by ethanbass | October 29, 2015 18:59 29 Oct 18:59 | #12344 | #12344

ethanbass was awarded the Basic Barnstar by stevie for their work in this research note.

What I want to do

I wanted to see if I could use the oil testing kit to fulfill its goal of reliably distinguishing between different grades of oil.

My attempt and results

When my spectrometer is placed flat on the table facing the opening of the oil testing kit, the alignment does not produce a reading. My first, quick and dirty solution was to wedge a pipette under the front of the spectrometer to adjust the angle of its interface with the cuvette holder. I used this method for all of the data I gathered on October 23rd, but I became concerned that the instability of the angle was adversely impacting my results. After some experimentation, I designed a simple platform for the spectrometer, made out of paper, which succeeded in improving the alignment. I determined the proper height for the stand based on trial and error. Yagiz has just pointed out to me a different solution to this problem.

paper_platform.jpg spectrometer propped up on paper platform to improve slit alignment

A. Documenting known samples:

crude_oil.jpg (, crude oil spectra

diesel_oil.jpg (, diesel oil spectra

diesel_and_crude_crop.jpg (, a direct comparison between diesel and crude highlights the differences between their fluorescence profiles. Diesel has one large peak at 475 nm whereas crude has a much broader distribution, with several peaks spread out from 480-530 nm and an additional red peak at 580-590 nm.

marpol_diesel_and_marpol_crude.jpg This is a figure from Hengstermann and Reuter, 1990, comparing emission spectra from diesel and crude oil. I was excited to see that the shapes of the spectra are a good match for my results! According to the LIF literature, it is to be expected that the wavelengths of the fluorescence emission spectrum will vary according to the wavelength of the excitation source that is used (in this case, our excitation source is a 405 nm blue-ray laser). However, we see here that the shapes of the fluorescence emission spectra are consistent with the results published in the literature.

diesel_oil_RGB.jpg crude_oil_RGB.jpg The additional red peak of the crude oil (bottom) pops out even more in RGB mode.

80w90.jpg 80w90_RGB.jpg (, 80w-90 spectra on the top with RGB mode on the bottom

80w90_and_crude.jpg (, comparison of 80w-90 with crude oil. The distributions here are much closer. The biggest difference I see is the strong red peak for crude oil at ~580 nm which does not show up as strongly in the 80w-90 spectra.

20w50.jpg 20w50_rgb.jpg (, spectra for 20w-50

20w50_and_80w90.jpg (, comparison of 20w-50 and 80w-90 -- 80w-90 appears to have a broader distribution with more emissions from 500-550 nm whereas 20w-50 has most of its emissions from 470-515 nm.

5w30_area_equalization.jpg (, 5w-30 had by far the weakest signal, resulting in this squashed looking graph when I tried to use area equalization.

5w30_height_equalization__sub_s4.jpg The height equalization macro yielded a much better looking graph for 5w-30. I'm still a bit suspicious of this data because of the weak signal. I plan to test some dilutions of the sample to see if the signal strength can be improved.

B. Investigating unknown samples:

All of the unknowns gave me fairly weak fluorescence compared to the rest of the oil samples (though not as weak as 5w-20).

unknown_25.png (, unknown 25

unknown_29.png (, unknown 29

unknown_93.png (, unknown 93

unknown_25r1-3_comparison.jpg The fluorescence emission profile of unknown 25 matches most closely with 20w-50

unknown_29r1-3_comparison.jpg The emission profile of unknown 29 is slightly to the left of unknown 25. It falls between the spectra of 20w-50 and diesel fuel.

80w90_and_crude_with_unknown_93.jpg Unknown 93 shows good segregation with 80w-90.

unknown_93r1-3_comparison.jpg Also, here (the green line is 80w-90 and the three lines clustered around it are the 3 captures of unknown 93).

Questions and next steps

I want to test dilutions of the samples to see if some of the spectra can be improved by making dilutions and, most importantly, whether the shapes of the spectra stay consistent for each sample when they are diluted.


Very cool. We'll have to check our 'secret file' to see how well you did on your proposed matches.

I was interested in the difference in diesel scans between #2 and #5, and the rest. Any idea why they're different but consistent?

I also really liked your comparison between diesel and crude -- as expected they're quite different and it's great to see such differentiation. Interesting also to think about the fact that diesel is refined from crude, so in theory crude is a mixture of all these samples, and other components.

Thanks, Ethan! Would you be able to post your "guesses" in a clear list, or table?

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Hi, I'm not sure why scans #2 and #5 came out so much rounder than the rest of the spectra. I think it has to do either with the angle of the spectrometer with the cuvette holder or with the flexure of the housing. I've noticed that both of these variables can effect the shape of a spectrum.

The cuvette cover-flap doesn't stay folded over the cuvette on its own, so I always press it down with my finger when I am capturing a spectrum. I think that the force from my fingers might be the cause of some variability. I would prefer it if i didn't have to be touching the spectrometer while capturing spectra.

I took 2 more scans of the diesel sample today and they both seemed to segregate with the majority of my scans from 10/28. OTK_diesel_replicability.png (

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Have you tried determining the presence of motor oil in a gasoline solution? Do we have enough resolution to do this reliably? jim in Sweden

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Hi, Ethan - can you demonstrate the variability by taking two scans while pressing vs. not pressing, or pressing differently? That'd be very helpful.

I also like the idea of a motor oil/diesel blend, or perhaps a dilution study of motor or crude oil. It was great seeing you at the Barnraising, Ethan -- great work so far!

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Ethan, the flat spot is clipping of the spectra based on blowing out the blue channel: clipping

this is one of the problematic issues mentioned in the instructions, we might need to bump it up: in the wiki: in my initial research:

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which spectrum are you looking at?

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I may be wrong. this spectra in the RGB channels isn't clipped, but there does appear to be a flat spot that looks a bit like clipping:

can't find any actual clipping

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Yes, I see what you mean. But I don't understand why it would be clipped to the left of the peak and not at the peak itself, where the intensity is higher.

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I haven't seen it on the camera we're using in the current spectrometer, but many other cameras have in-camera software that filters peaks. I'm not sure that is what is going on here, but a series of tests of varying brightness on the sample may narrow the problem down.

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I agree -- doing several different brightnesses to back off of the highest peaks, then equalizing by height, may help us check if the curve shape is an artifact of overexposure. I do think it's possible though that this is just the composite shape of the multiple oil components, overlapping to create that distinct shape.

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