Intro to DIY Raman Spectroscopy

hey everyone I've been trying to get my Raman spectroscopy setup to work for months and finally tonight I was able to collect some real data so let me tell you about it first off what is Raman spectroscopy this is a technique where you shine a light onto an object and then determine things about the molecular structure of that object by the way that the light is reflected so specifically if you shine a light of one color onto an object you actually get light of different color coming back this seems counterintuitive if you shine a a green light on something it's not like you get red light back but actually you do it just happens to be like a billion times less intense so this this color shift is called a Raman shift has nothing to do with noodles and the the reason that you can't observe is because the normal reflection called the Rayleigh scattering is just so much more intense so you need a special set up to extract this color shift from the overpowering not color shifted light the neat thing is that the exact way that the color is shifted for example shining a red light on something might actually return a small amount of green light will indicate the type of molecular bonds in the structure that's being observed so depending on the vibrational modes and the rotational modes of the molecular bonds you can actually get a different color shift you actually get sort of a fingerprint for that specific molecule this this technique is really useful because you don't have to destroy the object all you're doing is shining light on it and you can actually figure quite a lot out about the molecular structure so here's my set up I've got a helium neon laser tube and this is a small amount of optics here which I'll talk about in a minute the sample is here it's a polystyrene cup styrofoam and the light travels through here into a diffraction grating and I'm observing it with this camera this is a DSLR camera that I've modified by removing its infrared filter so there's actually nothing between the sensor surface and the front of the camera body although I am using the stock lens in this case here's a schematic of my optical setup is the helium neon laser and the beam comes out and goes through a beam splitter some of it goes off to this side which is just lost we don't get to use that part of the beam the beam splitter is about a 50-50 beam splitter in my case so half the beam goes through the beam splitter into a microscope objective and the microscope objective focuses that beam which is basically colonnaded down to a point and that point is on the surface of the object that we want to investigate so when you focus all this down a lot of it returns from the object and it goes back through the microscope objective becomes collimated again because that's the same path the same distance in everything and that return path hits this beam splitter and we lose half of it again goes straight back into the laser but half of it comes out this way so the whole trick with Raman spectroscopy is that you have to get rid of the sort of activation light like if we're shining red light from a helium neon laser onto an object and we're looking for this this weird sort of color shift that's you know a billion times less intense what we need to do is get rid of the red light from the laser so that we can see all the shifted light so what we do is use a notch filter and this is a special optical filter that only blocks light from the laser and hopefully lets everything else pass the trick is that these notch filters are very difficult to manufacture so the one that I bought was kind of what I considered expensive enough already and it's not a particularly narrow notch filter this is about 30 nanometers wide I'll get into it later but we lose a bit of signal right around the laser beam because this notch filter is it also takes out some of our signal light we ideally like it to just get rid of the laser itself but this will also get rid of colors that are not in the laser after passing through the knotch filter we send the light through a very very narrow slit probably about 50 micron or something on that order and the very narrow shaft of light comes through here gets collimated by a here and then hits the diffraction grading so the diffraction grading is actually what separates out the different colors into different spatial locations and it sends those spatial locations in a cone like this and I'm just using this DSLR camera to to actually generate an image so the light comes out of here in theory focused at infinity because it's it's been collimated by this lens and then the camera lens focuses that infinity down to the image sensor here so here's my setup here I just got a black piece of Delrin plastic that I machined and it slips over the end of the laser tube like this and there's the beam splitter that I carefully lined and then hot glued in place that's the waste beam that we don't get to use because it's just shooting out the side here and if we look in the port here if I put an object in front of this hopefully you can see the intensity change because we're actually getting signal back so what's happening is is there's light coming out the end of this but if I put an object here especially a reflective one then light goes back through the system and we get it out the port here however what we add is this laser line filter which blocks the laser light but hopefully lets through that color shifted Raman signature through here and then we plug all that light into the spectrometer so the way I set this up was just to clamp it here like this and the light goes from here this is where the slit is located sorry about that there we go this is where the slit is located and the light travels into the spectrometer here hits the diffraction grating here and then goes into the camera I calibrated the system by putting the knotch filter into the front of the spectrometer and then just shining a tungsten light bulb through there so what this did was give me a full spectrum but with the knotch taken out so I that the width of the notch agreed with the angles that were coming out of the diffraction grating by actually using the protractor on the spectrometer and this seemed to agree pretty well with the Edmond spec sheet of having bandwidth of around 30 nanometers so then having that image of the notch allowed me to calibrate the images that I was getting out of my camera because I knew that notch was always going to be about 30 nanometers wide and I made the assumption that the entire scale was going to be linear so something that was the width of that knotch far away in the image would also be 30 nanometer separation so after a bit of fiddling I set the whole thing up with a polystyrene cup you know styrofoam sitting in front of the sensor head and collected this image so there it is you can actually see it in fact you can even see the colors in the lines what's weird here is that Raman spectroscopy works on both sides of the excitation frequency so it you should get infrared lines as well as lines in the color spectrum so I took the infrared filter off the front of this camera so that I could see the longer wavelength lines which are supposed to be higher in intensity but I don't see those for some reason nonetheless it's pretty cool to see orange and green light coming back from a subject that I've illuminated only with red light so I was pretty happy to see that I used Photoshop to combine my calibration image and the data image that I got from polystyrene and did a couple of quick calculations to figure out how many pixels per nanometer I had my image and then I loaded it up into octave which is sort of a MATLAB open source sort of copy of MATLAB and combines the image just using averaging for now into a graph and lo and behold after compensating for wave number which I'll talk about in a minute it actually looks just like the signature that I got off the web for polystyrene clearly I've got sort of an offset here there's there's a lot of noise and ghosting in the image but the signature is very clear I mean there's there's no doubt about it this matches the industry accepted polystyrene signature so I'm pretty psyched about that and in fact going even further one of the biggest peaks in this spectra is for ch2 and so taking a look at the polystyrene molecule you can see that there's quite a lot of CH to going on this molecule gets repeated over and over and over again and there's lots of ch2 bonds which explains why this peak is so high so what's all this wave number stuff for Raman spectroscopy you can use different wavelengths to excite the source so right now I'm using a helium neon laser which is six hundred thirty two point eight animators but you can use any wavelength you want in fact a lot of Raman spectroscopy is done with longer wavelength light because you want to avoid fluorescence so if I use the ultraviolet light yeah you still get the Raman phenomenon but you also get fluorescence which is actually a pain in the butt because that washes out your signal again so most as far as I know most Raman spectroscopy is done with longer wavelength laser diodes but I just happen to have a helium neon laser so that's what I'm using in any case the wave number is a generalized format so you don't have to tell someone oh well I use the six hundred and thirty two point eight and ammeter laser and you'll have to convert my numbers if you're using a you know 850 nanometer laser so the wave number is just a way of comparing results without trying to be tied to a specific source light okay well I'm pretty stoked about this so I think I'm going to build this into a better system kind of with fewer light leaks and better optics and then try to actually use this to collect real data you can do quite a bit with Raman spectroscopy so I'm going to see how far I can push this in my home shop here okay see you next time bye

Channel: Applied Science

Views: 136,492

Keywords: raman spectroscopy, raman, spectroscopy, 632.8, hene laser, diffraction grating, spectra, diy

Id: tRrOdKW06sk

Length: 10min 31sec (631 seconds)

Published: Mon May 27 2013

Reddit Comments

[u/the5andmany]

and he makes poprocks, aerogel, supercritical carbon dioxide CO2 and so on...

[u/zx321]

Krasnow works for Valve?

[u/Jamator01]

Wtf am I doing with my life?

[u/tookalook88]

Totally thought he was going to spectrophotometer ramen noodles, 43 seconds in, its clarified, haha.

[u/Foley1]

Is this why HL3 is taking so long? Valve engineers playing around in the garage?

[u/Slammy1]

I built one of these in grad school, very close to the technology. CCD was still new at the time, you had to use liquid N2 to cool it. I played MOO1 while running experiments, give you an idea of the time frame. Physical chemists always had money for nice PCs. Something like this but much cheaper. http://www.rdec.co.jp/e/image/SPR-e.pdf

I had to program my own data acquisition through an ancient RS-232 cable but, still, unlike him we made use of the lost signal he talks about (and in a less noisy system you have that as well as vibration controls). We had an optical bench so, not really a fair comparison (plus 3 detectors, the CCD to catch the raman signal). You could pump it up with air to reduce vibration.

With the death of R&D I'm just working as a union employee in an industrial lab where original thought is welcomed so long as it is aligned with current corporate thinking. Still, the only guy in the union with > A.A.S. I keep thinking some night I'll be jumped by 2-3 of them, but that's just me being paranoid. 3 years later I still stole their jobs.

[u/howaboutthis13]

That is pretty impressive for a home made spectrometer.

[u/Sagan_Paul_Narwhal]

I will be impressed when he makes a homemade ICP-MS

[u/Reputed]

Call me when he builds an anti-mass spectrometer.