Interpretation of NIR absorbance!!

Anonymous's picture

Dear NIR members,

I am using NIR to assess the ageing in seed for my research. I had taken spectra before and after ageing (1 Year of storage) of the same seeds and PCA showed a good seperation between them. I have wavenumbers from loading plot. The mean spectra of unaged and aged seeds have considerable differences in terms of absorbance log(1/R) (Figure is attached). Can I interpret the results on the basis of absorbance only. Literatures say- reduction in amount of lipids and proteins in aged seeds. I have constraints performing the reference test. Is it possible to refer the lower absorbance to the lower content of the biochemical (lipids/protein/carbohydrate) as per the spectral information or vice versa for the higher absorbance? I do have knowledge on the impact of particle size/concentration of object have influence on the absorbance and scattering could not be ruled out. If I can do, could you please suggest some literatures of this nature too??

Thanking you

Santosh Shrestha/PhD student/Aarhus University

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dardenne's picture


For this kind of interpretation, you have to consider at least these points :

  • You have to prove that your instrument is stable over the period.
  • If constituents decrease, others must increase: the sum of all components is always 100%.
  • The shift of the based line has likely no meaning; use MSC and/or second derivate for the interpretation, but the use of equations to quantity the constituents is always better. Even more, wet chemistry analyses one year ago and some today would have been the right strategy to confirm the changes in the spectra.

Best regards,




hwswien's picture

Dear Santosh,

looking at your spectra I have doubts whether biochemical changes took place in your sample. You may try the 2nd derivative spectra to find subtle changes but in my opinion the shift/change in baseline/slope (absorbance changes ?) are a consequence of sample presentation.

Kind regards and good luck for your further research

Heinz W. Siesler

ianm's picture

Hello Santosh. Your spectra look as if the seeds have been ground. Depending on the method of storage, the year of storage could cause changes in the moisture content, which would cause changes in all of the other constituents. Plastic bags look like a chicken-wire fence under the microscope, so if the samples are stored in plastic bags in the open at room temperature, or even under cool conditions, there would be some moisture loss, which could account for the change in spectral shape (absorbance). Depending on the method of grinding such difference in moisture content would also affect the particle size, which would change the spectra. A better test of possible biological change is the germination test. If seeds are not stored under optimum conditions the germination capacity will drop, and the seeds will become unreliable for planting. In the case of barley changes in the germination capacity also affect malting potential. If you can develop reliable NIR spectroscopy calibrations for germination potential this would be a useful contribution.

Greetings, and best wishes,

(posted on behalf of Phil Williams by Ian Michael)