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Spectrometer FAQ

This is a revision from May 17, 2011 23:19. View all revisions
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Answers to some common questions about using a spectrometer (thanks to Katie Gradowski, who asked them!):

UV -- why is this a better method than white light? (clearer lines? materials absorb UV "better" than other wavelengths? some particle-level difference in how light is absorbed?)

The difference is that for poly aromatic hydrocarbons (and some other contaminants) there isn't a lot of well-known spectral identifying features (like absorbance lines) in the visible or near-infrared range. There's good literature about fluorescence spectroscopy, though, which is where UV light excites the material to actually glow, usually in the UV range, though always at a longer wavelength than the source UV. So for oil contamination, we're focusing on UV-fluorescence instead of visible light or infrared spectroscopy.

Reflectance vs. absorption -- is there a qualitative difference in what you can observe with these two methods that can be seen at the level of CD spectrometers?

Yes, probably a quantitative difference too. When light reflects off a material and into your spectrometer, it only interacts with the top layer of atoms or so. With transmission spectroscopy, light passes through the material, interacting with anywhere between a few microns and (with a liquid sample) a few centimeters of material before entering your instrument. There's just more sample to absorb light.

Spectral workbench currently measures "intensity" -- translating to "brightness"? What do we see from looking at the peaks that we don't see from just looking at the image itself?

Well, the graph of intensity is easier to compare against another spectrum. It's also turning your perception of brightness into some numbers which you could use to automatically match with another spectrum from our library of spectra. But sometimes I do feel like you can visually pick out bright or dark lines with the naked eye much better.

I think what I'm supposed to be looking for are absorption lines -- but also seems like with spectra of materials (pancetta, etc) it's hard to get the kind of specificity you get with gas tubes (say, in pictures on the internet). If not lines, what's useful to see here?

This is where the graphs can help. If you can see a generally brighter region between a sample and control spectra, there's a broadband absorption. But that doesn't hold a lot of very specific information. With the pancetta, we could say, it's just blocking a lot of UV. Meaning it'd be a good sunscreen ;-)

When I see dark lines in the CFL spectra, am I seeing absorption lines? Or am I just seeing the lack of emitted wavelengths? (and if so, am I looking at "emission lines" as the converse of "absorption lines," and does that even make sense when I'm looking at a full spectrum light?)

The CFL spectra are actually from fluorescence, hence "fluorescent light". So you're looking at the brightness lines where light is produced, not the missing dark lines where it's absorbed. Yes, you're right, it's kind of the inverse.