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Michel Coene (michel)
Senior Member
Username: michel

Post Number: 45
Registered: 2-2002
Posted on Friday, March 07, 2008 - 4:13 am:   

You should look if the difference in absorbency is continuous over the whole wavelength range. If it is due to particle size, then the difference should become bigger for the longer wavelengths. As you approach visible wavelengths, the difference should go towards zero. But with such a big difference, my first guess would be somebody made a "small mistake" in sample presentation...
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dafna barkan (dafna)
Member
Username: dafna

Post Number: 12
Registered: 4-2003
Posted on Friday, February 15, 2008 - 7:24 am:   

Bruce,
As I pointed out earlier, I have currently no access to the instrument giving out the lower absorbances. I guess I will certainly have to do some clarifications before proceeding with the calibration work.
The intensity differences show through the whole spectrum, at every datapoint.
I will certainly take your suggestion and get the information about the wavelenght standards of both instruments. That could be a good start.
Anyway, thank you all for your posts.
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Bruce H. Campbell (campclan)
Moderator
Username: campclan

Post Number: 111
Registered: 4-2001
Posted on Friday, February 15, 2008 - 5:32 am:   

Dafna,
You mentioned differences in absorption. If you are working in that area you may have differences in the baseline. Is the intensity difference the same or not when using derivatives?
Do you have access to spectra from both instruments for contrrol standards or from wavelegth standards? Are the the same?
The values you cite are rather low if they are the major peaks in the spectra and they are absorbances. What is the intensity differences in other peaks?
Bruce
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Howard Mark (hlmark)
Senior Member
Username: hlmark

Post Number: 178
Registered: 9-2001
Posted on Friday, February 15, 2008 - 4:51 am:   

Dafna - soemtimes it's necessary to work under those constraints. For example, I sometimes had to do calibrations based on data supplied by a customer while I worked in the Applications department at Technicon, and I also never saw the actual samples.

But it's not an ideal situation. When there's something about the data to make you suspicious, then it's necessary to at least contact whoever made the measurements and discuss the data, and your misgivings, with them, before you try to do any calibration work. They might know something, or recall something, that can shed light on the question. Or they might themselves take another look at the samples, to see if, for example, the two grindings, and everything else are the same.

\o/
/_\
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dafna barkan (dafna)
Member
Username: dafna

Post Number: 11
Registered: 4-2003
Posted on Friday, February 15, 2008 - 3:16 am:   

Thank you for your reactions.
Just to make things a bit more clearer, I have no access to the samples to see the grinding quality. I was asked to develop a calibration for a product having received a spectra file and reference values only. While having a look at my previous applications for this same product, I saw that while my first datapoint read 0.13 in one instrument, I had now readings around 0.05. Both instruments are working in reflectance. What I am trying to understand is if I still can go ahead and do the calibration for the file with the lower absorptions without necessarily incurring in some terrible misjudgement. I have the feeling that either grinding or, as Howard mentioned, some instrument internal standard misalignement, may be negatively influencing the trustfulness of this spectgral information...
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Howard Mark (hlmark)
Senior Member
Username: hlmark

Post Number: 177
Registered: 9-2001
Posted on Thursday, February 14, 2008 - 9:16 am:   

I agree with Dave's assessment. 1:2.5 is a big change (but you knew that or you probably not have posted a question): about 20:1 in energy. Are you sure the grinding was done the same way? If it's grinding differences, you should be able to see that by eye, or by feel, for such a large difference.

I would also look for other possible causes, such as the sample cup not being in the same position, or the reference reflector having changed or being misaligned.

\o/
/_\
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David W. Hopkins (dhopkins)
Senior Member
Username: dhopkins

Post Number: 133
Registered: 10-2002
Posted on Thursday, February 14, 2008 - 9:03 am:   

Hi Dafna,

It is highly unlikely that you would see 2.5-fold difference in absorbances due to the differences in photometric response in any instruments. Unless, of course, you are working at low absorbances where small differences in absorbance lead to large relative differences.

You should be able to show that the difference in photometric response is relatively small by looking at control or standard samples run on the same instrument.

Are you working in transmission or reflection? Can you tell us what the material you are working with is?

Best wishes,
Dave
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dafna barkan (dafna)
Junior Member
Username: dafna

Post Number: 10
Registered: 4-2003
Posted on Thursday, February 14, 2008 - 8:48 am:   

I have two files of a same product. They were collected by two different Foss NIRSystems 5000 instruments. The ratio between the absorbances of the two files is about 1:2,5. Could this be due to the different particle sizes of the samples after grinding or are there other hardware reasons for such different absorbances?

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