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Techniques for CollectorsGeneral Raman Question

15th Sep 2015 14:22 UTCBob Rock

I have done some raman spectroscopy of minerals using a basic diy spectrometer. What are other peoples experiences like, particularly in the area of optimizing signal quality? In my experience, transparent minerals that are chemically pure give the best signals. If optical alignment is correct and the laser and spectrometer are correctly configured, the nature of the sample being tested seems the next most important aspect. I get a lot of variation in results, according to the quality of the sample. Theoretically, is it true stokes shifted light will have trouble escaping the surface in opaque samples, thereby giving none or weak signal? If yes, that would explain a lot of the results I am getting.

Bob

15th Sep 2015 16:35 UTCUwe Kolitsch Manager

The colour of your sample also plays an important role.

Using a green laser on a red mineral usually is not good (possibly beam damage, poor spectrum), while a red laser on a red mineral will be ok. Biotite is notorious for giving poor spectra; you have to strongly turn down the laser power (down to 1% in cases) and use longer measurement times.

16th Sep 2015 02:33 UTCBob Rock

I am finding red garnets with a green laser difficult. Also, as you say biotite is difficult. Some of the coloured minerals also fluoresce, which masks the Raman signal somewhat, especially with a green laser.

16th Sep 2015 02:43 UTCDoug Daniels

So, this almost sounds like you get different spectra with different wavelengths (laser color). Maybe another, but much more complicated way to identify minerals? What about using blue lasers (do they exist?), or near-infrared lasers, or ultraviolet lasers? I admit I don't know much about the method, just throwing out some thoughts.

16th Sep 2015 10:49 UTCBob Rock

The spectra are the same with different laser wavelengths, but the quality of the spectra varies according to the geometry, nature of sample (chemical composition, rough or polished). Some minerals have a weak Raman cross section to begin with, so will always be difficult to acquire good quality spectra. Other minerals give strong spectra. Some of the strong minerals I have analyzed are calcite, diamond, emerald, sapphire, zoisite, crocoite, quartz. Some of the weaker minerals I have had trouble with are non-fluorescent garnet, jadeite, mica, orthoclase. Just some examples with my own setup. Green lasers at 532nm are usually preferred for minerals, as they give stronger Raman signals, although NIR lasers at 785nm are sometimes used when fluorescence is a problem.

16th Sep 2015 16:13 UTCReiner Mielke Expert

I have another question. Could RAMAN be used to identify talc in drill core without having to make a polished section? If so how much would a RAMAN unit cost that could do that?

17th Sep 2015 03:06 UTCBob Rock

I have not done that, so I do not know. The main requirement would be a flat surface.

20th Sep 2015 08:58 UTCBob Rock

A few spectra from my DIY Raman...

20th Sep 2015 20:41 UTCHenry Barwood

Bob,

How is your spectrometer configured?

21st Sep 2015 02:31 UTCBob Rock

I am using an optical bench configuration.

A 532nm DPSS laser is directed towards a 542nm DRLP Dichroic beamsplitter. The beamsplitter reflects the laser into a 20x objective, which focuses the beam onto the sample. The Raman backscatter (and Rayleigh scattered light) is collected by the objective, passed through the beamsplitter, through a notch filter (to block the unwanted 532nm laser backscatter) into a plano-convex collecting lens fl 1inch. The collecting lens focuses the light into a 600um sma fiber cable. Finally, the signal is collected by a TEC cooled 532nm to 750nm spectrometer.

This is DIY Raman spectrometer V1.0

21st Sep 2015 02:48 UTCHenry Barwood

Interesting. When I tried a very similar configuration I got basically noise with no useful signal.

21st Sep 2015 05:08 UTCBob Rock

Did you use a notch filter and dichroic beamsplitter? Without a notch filter, the laser will overwhelm the Raman signal. Also, if you don't use a proper dichroic beamsplitter, you won't get much Raman signal passed back to the spectrometer. The other important aspect is optical alignment, even if it is off 1mm you won't get a signal. Even with everything correct, you may only get a signal from minerals with a strong Raman cross section, depending on your spectrometer.

21st Sep 2015 17:19 UTCHenry Barwood

I'm using cut-off filters. They work, but the noise factor was overwhelming. I suspect that the combination of misalignment and fluorescence from my optical train were the main culprits. I'm trying a different set-up and hope to get a workable device. I'm using OO spectrometers and software.

22nd Sep 2015 07:17 UTCBob Rock

Sounds like the problem lies with your filters. Your OO spectrometer should be Ok.

22nd Sep 2015 13:38 UTCHenry Barwood

While it could be the filters, I'm basically using a configuration/components that another amateur got good results with. In a few weeks, when I've finished it, I'll let you know what happens.

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