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Silicate and Borosilicate glass

NC State University's picture
Forums: 

I would like to take some transmittance NIR measurements of samples of pine resin passing through two glass coverslips.  Quartz coverslips are too expensive, so I want to use either silicate glass slips or borosilicate glass slips.  I suppose as long as I am consistent with the type of glass in the future, I could develop useful models, but is there any reason to prefer one or the other?
Thanks,
Gary Hodge
[email protected]
 

hlmark's picture

Gary - the main problem with glass (compared to NIR-quality quartz) in the NIR region is the presence of absorbance bands due to water, or at least,  to-OH bands. I suggest that you can make the decision of which glass to use by measuring the spectrum of each type and using whichever one exhibits less absorbance. Since cover slips are very thin, you may want to include several pieces (thicknesses) in each measurement, just make sure you use the same number of thicknesses.
A better question, however, is how do you plan to keep the sample thickness constant? Under the circumstances, that is more likely to vary from one spectrum to another, than the cover slip thickness.
\o/
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NC State University's picture

Thanks for the feedback.  That is a good suggestion to test both types.
The slips themselves will likely be the #1 thickness, which are supposed to be 0.15 mm thick and range from 0.13 to 0.17 mm thick.  Since there will be two slips for each sample (upper and lower), the total amount of glass will be 0.30 mm +- some variation.  For the resin sample, we hope to put a uniform volume of resin on the slip and try to get even spread to the edges.  I think we will also want to do multiple readings (i.e., multiple slides) of resin from the same tree to deal with some of the variability.  Regardless, I expect we will have to do some kind of math pretreatment of the spectra prior to model building.
GRH

hlmark's picture

Gary - I think I'm not the only person who will be interested in the results of your testing the different kinds of glass. Could you post them here on this discussion thread when the results are all in? Much grass.
As for keepng pathlength constant, this is not a new problem. The Mid-IR world dealt (and still deals) with that same issue for at least the past 50-60 years. They solved it by putting spacers between the windows. In their case they needed pathlengths of about 0.1 mm, and spacers were made commercially for their use. You have a much easier problem, since you'll probably be using sample pathlengths on the order of up to 1 mm, and maybe even more. Still, you could look into what's available for the mid-IR community, I suspect you'll find something suitable.
\o/
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hlmark's picture

BTW - Teflon sheet in various thicknesses, easily cut into whatever shape spacer you want, is available commercially from many places. One suppllier that I like to use, because they sell in small quantities, is Small Parts, Inc. They can supply a 1 x 1 foot piece. You should specify that you want it packed flat for shipping, otherwise they mght roll it up, and then it's difficult to work with.
\o/
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ianm's picture

Reply from Phil Williams posted by Ian Michael

Gary, if you are from North Carolina State you could call Fred McClure, retired from NC State. He would know the answer. His number is [e-mail [email protected] for the number]. There is probably not much difference between the two types of glass anyway for what you need to do. You may have more trouble in getting the resin to pass through the glass with an even layer, to say nothing of clean-up between samples.

P.

shileyda's picture

Gary, try using deep well microscope slides to hold your resin. Place the material into the indention then cover them with a standard microscope slide instead of a cover slip. I don't like to use cover slips because they are so fragile and thus represent a safety hazard in the lab. Depending upon the properties of your resin, you can either use a single deep well slide or invert the second slide to provide a thicker sample. Cleaning is simple even with viscous materials. As with any non-quartz materials, I would recommend that you purchase a sufficient quantity from the same product lot.
Regards,
Dan