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Eduardo Zamora (edzaro)
New member
Username: edzaro

Post Number: 2
Registered: 2-2009
Posted on Sunday, December 12, 2010 - 2:49 am:   

Thank you very much for your comments. The questions help to think in aspects that I dont normally consider. I will go through them.

Dear Dr. Hopkins, thank you for tell me your experience. The world is a small place!
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Jerry Jin (jcg2000)
Senior Member
Username: jcg2000

Post Number: 38
Registered: 1-2009
Posted on Thursday, December 09, 2010 - 1:06 pm:   

Hi, David:

Could you elaborate "and therefore the light scattered from the sample that is missed by the detectors increases with increasing wavelengths, causing the upturn in the Foss spectra"?

It sounds to me Howard Mark's explanation is more plausible if the two instruments use different measurement modes. But I would like to listen to other perspectives.

Cheers!

Jerry Jin
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David W. Hopkins (dhopkins)
Senior Member
Username: dhopkins

Post Number: 179
Registered: 10-2002
Posted on Thursday, December 09, 2010 - 8:55 am:   

Hi Eduardo,

Nice to hear from you again! The last time we met was in NY Penn Station, a week after the IDRC meeting. For you statisticians, what is the probability of that happening?

Anyway, I believe your prototype is giving very believable spectra, if your sample is highly scattering. I have seen a similar effect with an instrument with a probe design with the light source in the center and the anulus for the detector. I also compared the Foss 6500 spectra. I believe your prototype is doing an excellent job of sampling the diffusely reflected light. In comparison, the Foss loses comparatively more of the reflected light because the four detectors are at a significant distance from the sample, and therefore the light scattered from the sample that is missed by the detectors increases with increasing wavelengths, causing the upturn in the Foss spectra.

It is interesting, MSC or SNV work equally well under both optical systems, and both systems give equally useful spectra for calibration and subsequent measurements. I was as surprised as you were, and I expect you will find, as I did, that your new instrument will perform very well.

Best wishes,
Dave
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Howard Mark (hlmark)
Senior Member
Username: hlmark

Post Number: 372
Registered: 9-2001
Posted on Thursday, December 09, 2010 - 8:42 am:   

Eduardo - since you are using what amounts to a quasi-transmission measurement through a scattering sample, you have to realize that short wavelengths undergo more scattering and would therefore be more highly reflected from the illuminated side of the sample. This leaves less energy to pass through the sample, giving lower transmission readings and therefore higher absorbance readings than at the long wavelengths.

"Normal" spectra, as you call them, of scattering samples are generally measurements of the reflectance from the illuminated side, which therefore have the higher energy at the short wavelengths and consequently lower absorbance values.

This is pretty basic stuff. I think you need to review the information contained in some of the elementary books on NIR that are available.

\o/
/_\
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Fernando Morgado (fmorgado)
Junior Member
Username: fmorgado

Post Number: 9
Registered: 12-2005
Posted on Thursday, December 09, 2010 - 8:09 am:   

Hello :
First check if the Teflon you are using have high reflectance. Foss use reference with more than 99% of reflectance. Diferent teflon ( normaly named spectralom) have diferent spectra in the Y axis.
See this link :

http://www.labsphere.com/data/userFiles/Diffuse%20Reflectance%20Standards%20Product%20Sheet_7.pdf

A comment , in your spectra I see only Moisture bands, for me will be dificult predict other parameters, I am almost sure your sample have maybe more than 50% Moisture, and in this conditions see other parameter will be only Pls Correlation, sometimes not realistic correlation.

Is my opinion, maybe is wrong.

Fernando Morgado
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Eduardo Zamora (edzaro)
New member
Username: edzaro

Post Number: 1
Registered: 2-2009
Posted on Thursday, December 09, 2010 - 5:20 am:   

Hi all,

I am new in this forum, but I have read a lot of things here and I think that it is really useful. I have a question regarding the NIRS spectra shape.

I am working with a NIRS prototype where the optical arrangement is that source and detector are arranged concentrically with the detector in the core and source at the annulus. The spectrum that I obtain has a shape were the absorbance values showed a decreasing tendency for longer wavelengths, while a normal spectrum should be on the other way, as the Foss spectrum showed. The sample analyzed is intact pig adipose tissue in interactance mode. The reference that I use is Teflon.

Is this spectrum shape normal? Can be due to the fiber optic design?spectra

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