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 Post subject: Spectrometry
PostPosted: Mon Jan 02, 2023 2:33 am 
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Hello all.

I am new to this board and wanted to both introduce myself as well as get some guidance if possible.

I am an American engineer living in Pakistan working at a medical device company that I helped start here. Living here where getting out is sometimes difficult has left me with a lot of time to kill on nights and weekends. Since there are a lot of gemstones that are mined here (I have purchased many from Swat Emeralds to Kashimiri Pink Sapphire) All of this has rekindled my love of gemology which I have had since I was very young.

A trip to Bangkok last summer found me buying another gem microscope and all of the standard tools one would find in a gem lab.

I have had an interest in more advanced equipment, and so have cobbled together a UV-VIS spectrometer and a Raman Spectrometer. Both of these built around ThunderOptics components. The UV-VIS is pretty straightforward. The Raman not so much. I can get spectra from Topaz and Spessartine. The resolution is not great so looking to improve the performance of the laser and the spectrometer.

Raman spectra for corundum eludes me. The laser is 532 nm and fluorescence starting at about 1400 cm-1 drowns out the raman signals completely. I know that 532 nm is pretty efficient at exciting Cr3+ hence the strong fluorescence. But when I look at spectra from the RRUFF database I can find plenty of Sapphire and Ruby showing proper peaks even when using a 532nm laser.

Does anyone have any experience in this and why some are able to seemingly avoid strong fluorescence even with a 532 nm?


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sapphire raman plot.png
sapphire raman plot.png [ 256.14 KiB | Viewed 21157 times ]
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 Post subject: Re: Spectrometry
PostPosted: Mon Jan 02, 2023 9:37 pm 
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I believe the board admin Barbra and moderator Stephen Challener are very experienced in this subject.

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 Post subject: Re: Spectrometry
PostPosted: Mon Jan 02, 2023 10:52 pm 
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first, thanks for sharing some results from this system! I think anyone who's looked for a Raman has been curious how they perform.
As a first thought have you tried just running it longer? You don't need anything up at 1400 or higher inverse centimeters, that's well above the fingerprint zone.

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 Post subject: Re: Spectrometry
PostPosted: Tue Jan 03, 2023 12:15 am 
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Thanks Stephen,

The standard exposure time for my system is 100 ms. But I have tried exposures up to 2 full seconds. It does not change the fluorescence. There isnt a way to have the system only scan up to 1400 cm-1. I can cut it off post scan but it doesn't really help.

Everyone I have spoken with says change to a 785 nm laser. I can do that, but good narrow band lasers are expensive and there are plenty of RRUFF spectra for corundum at 532 which do not exhibit this. I have read about photobleaching. And tried that with no results.

This is only one of my issues that I am dealing with. For now I think i will set aside the fluorescence issue and concentrate on improving the spectra of gems that I can get Raman signatures from.

Attached is a spectra from Topaz mined here in Pakistan. As you can see the three peaks which should appear around 300 cm-1 are smudged. So that is the first issue to fix.

Next will be fluorescence spectroscopy. GIA has a good paper on a relatively simple system to construct but again it requires a good spectrometer with good resolution.

My end goal in this is to develop a spectral database of UV-UIS, Fluorescence and Raman Spectra. I have a pretty good collection of natural reference stones. Plus reference stones of Ruby, Sapphire, Emerald and Alexandrite of each synthetic process along with diffused Sapphire and glass filled Ruby.

All interesting stuff which keeps me occupied and out of trouble :)

Thanks for allowing me to join this group!

Eric

Thanks for your help.


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Raman Spectra of Topaz.pdf [830.68 KiB]
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 Post subject: Re: Spectrometry
PostPosted: Thu Jan 19, 2023 6:45 am 
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Did the uv-vis spectrometer give good results with gems? I have heard some stories that the spectra where almost unusable, but it is a tempting instrument.


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 Post subject: Re: Spectrometry
PostPosted: Thu Jan 19, 2023 7:12 am 
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Yes I have had good results with the UV-Vis system. There is a learning curve and it took a couple of emails to some of the GIA research folks for me to understand a few things.

There are two issues. The first is a reference database. Much of the spectra that can be found on the web are FTIR. I do not have an FTIR system now but that will be in the pipeline at some point, likely along with a UV PL system. Caltech has a fairly extensive UV-Vis section but the images are poor. The data is there but it would be a project to import everything into software like Spectragryph and replot the spectra and create a database. I will likely do that at some point and include spectra of my reference stone collection.

The second issue is getting sufficient light through heavily included or translucent stones. I had spectra of one ruby that was very unruby like, but all of its standard gemological information said ruby. The peaks were there they were just hidden. Once as suggested by one of the GIA folks that I modify my setup to get more light through, low and behold a ruby spectra was born!

Of the two the Raman system seems to have the most promise but its also the most expensive and demanding of equipment. My laser will be characterized next week at a local university so I may know more after that.

But in answer to your question the UV Vis stuff is fairly simple and delivers good results. Perhaps not definitive but in conjunction with standard gemological testing its fantastic.

Eric


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 Post subject: Re: Spectrometry
PostPosted: Fri Jan 20, 2023 11:26 am 
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Thank you for your answer. May I ask which version of the hardware you use, and what kind of slit? I understand getting the reference spectra can be somewhat daunting, but for occasional use it seems also some fun to dive deeper. Lighting is of course always a problem. I tinkered with a spectroscope and a webcam, got some results but the resolution was very poor. So if thunderoptics was able to make this better, it looks like a fine tool to experiment.


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 Post subject: Re: Spectrometry
PostPosted: Fri Jan 20, 2023 4:18 pm 
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The Cr3+ fluorescence always peaks in the red, so an easy solution to reduce the input signal in that region is to place a shortpass filter with cutoff edge around 600 nm between the stone and light collection.

You can spend a lot on high-end filters, with as flat a transmission and as sharp a cutoff edge as you want, from Chroma or Omega Optical. But, just to try it out, you can purchase an Edmund OD 2.0 shortpass filter for less than $50.


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 Post subject: Re: Spectrometry
PostPosted: Sat Jan 21, 2023 12:50 am 
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@HenkRijneveld I am using the Thunderoptics SMA-E for Uv Vis. The slit width is 100 um. It's a comparitively very low cost Spectrometer, and so far seems perfectly adequate for UV-vis applications. I did wind up 3d printing a series of cuvettes to direct the light through the stone. And am still tinkering with that, the smaller the stone the more difficult. So looking at ways to collimate the light source.

@Brian. Interesting. I am an American but working in Pakistan so access to stuff from Edmunds or Thorlabs is a pain. But on the bright side access to Swat Emeralds and Kashmiri pink sapphires is easy! I will try to figure out something on the filter as an experiment. At the moment my laser will go to a local universities optics lab so they can align (totally not trivial!) and characterize singlemode vs multimode etc.

All in all this is fun stuff and has caused me to learn a lot more about optics than I knew previously.

Eric


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Cuvette.jpg
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Figure 1.jpg
Figure 1.jpg [ 1.7 MiB | Viewed 20605 times ]
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 Post subject: Re: Spectrometry
PostPosted: Fri Feb 10, 2023 11:38 pm 
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Hi eflachbart, what intensity values do you get on spectragryph with your light set up? One thing that I would change about the set up is that the stone is actually siting on the fiber optic end so that no direct light has the chance of entering and also no waste of passed through light. Here a pic of my set up. Also I noticed you have a double fiber optic light, try to get a singular one instead like that you will get also more light.


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gs4.jpg
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 Post subject: Re: Spectrometry
PostPosted: Sat Feb 11, 2023 5:30 am 
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Thankyou @earthling.

You are correct on getting enough light through. I had two GIA certified rubies that I got very different spectra from. I quieried one of the GIA researchers and he suggested exactly what you did. So I modified the cuvette such that the stones sit directly on the fiber. After that the spectra matched. One stone had far more inclusions and I was not getting enough light through.

In terms of dual versus single fiber. That is a good point. I am going to Bangkok next week and will pick up a single fiber cable.

Since I moved to a new apartment I have not set up my lab in my home again. So I cannot check intensity. Mostly the plots I do are absorption.

Once I get the lab set up I will get back on to the Raman system. My laser is now out at a local universities optics lab getting characterized in terms of bandwidth.

Eric


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