I have been facing some problems with the Raman imaging of natural rocks. At many locations of the rock (even polished rock samples) , the baseline shifts upwards drastically (suggestive of fluorescence) , totally engulfing the peaks of interest. I cannot use a higher wavelength because the spatial resolution of the CCD gets limited (upto 1800 cm-1), which means I have to do two scans to search for peaks in the other end of the spectrum. Moreover the intensity is too low . The rock is rough , which means using a 100 x objective makes me lose out on z resolution. But using a lower magnification causes a greater baseline shift. I have seen many images of rocks showing the different minerals in it clearly demarcated and I was wondering what I can do to the rocks to make the imaging better.
Dear Dr.Kenneth, I haven’t done a lot of Raman studies but generally the material is just purified in case of liquids and just treated for organics in terms of solids. For the rock sample I have, I just plasma clean the sample to get rid of adsorbed organics. I know contamination is the problem, because chemically synthesized/industrially available rock constituents (Quartz,clay) don’t show this problem of upward baseline shift. But the problem is that one cannot mimick the complex intricacies of a million year process by chemical synthesis, hence characterization of natural (dirty) rocks is of importance to me.
Dear Tamas, thank you for the reply. I have noticed all the problems that you have mentioned above, and I understand there is no easy way to circumvent this problem. However, if the fluorescence does come from organics, I was wondering if there is some chemical treatment that should help solve this problem without disturbing the rock crystals. I am also convinced these are organics because the baseline shifts upwards with time at the same pixel, and later (probably) due to bleaching, it comes back down, but now with two prominent carbon peaks.
Its interesting to know about the surface roughness follower. Pure, optically pleasing crystals like quartz in the rock give no fluorescence. But if you go in between these grains, in the gaps of the rocks, I notice this baseline shift. It's annoying because I can't create a nice false colour map of the different minerals in the rock, because 50% of the pixels have large baseline shifts, which engulf the peaks.
Thanks again for the reply.
Hi Aurelio, I worked with thin sections of the sample. I also tried a bulk sample which was polished into a cylindrical shape. Why do you ask?
Hi Prof. Kenneth, I don't use any chemical treatment to get rid of the organics. I was wondering if chemical treatment is possible, without affecting the structure of these crystals and the clay composition in them.
Yes, I have an idea about what minerals are present. The problem comes in recording the spectra for imaging. I will try to chemically treat the rocks using a commercial bleach and see if the spectra get better.
And the rock is primarily Quartz, with little grains of clay. The Quartz as such doesn't cause the baseline shift and can be scanned quite nicely. The clay, in addition to being really difficult to find, can also show these baseline shifts.
Thank you for your time!
Dear Aurelio, I scanned both thin sections and bulk sections too. Thin sections also gave the same problem, so I assume the luminescence is coming from the rock (organics, clays) etc. I state this because these baseline shifts are usually associated with the CH stretching band between 2700-3000 cm-1. And thank you for your time!
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