You are using the raw coconut fiber? it will be quite difficult to understand the changes since the composition of each region of the fiber, or the place the coconut tree was planted, the soil, the amount of water, nutrients etc, will change the raw fiber, probably with you do a pre-treatment on the fiber this issue will cease to happen.

I had problems even with air plasma modified spruce fibers several years ago (master thesis). I have found two guidelines: 1. look for intensity decrease in skeletal vibrations (mostly breaking of C=C bonds, structural degradation) and 2. look for intensity increase in vibrations related to oxygen-containing groups (C=O).

In your case you can rely only on the first one. As you were useing argon plasma, there should be no other distinct peaks (dis)appearing in the spectra. The C-H related vibrations are everywhere in the spectrum and have yet never encountered measureable change in their intensity after plasma treatment. The natural variation in fibre surface's composition will make any comparison less reliable. Measuring the same fibre bundle before and after plasma treatment provided me the most accurate results.

I attached my master thesis, I hope it can provide you with some inspiration. Anyway FTIR is not most effective method of quantifying plasma made changes - check also my paper from 2012.

Vadym, run first simple tests, like wettability (there is few possibilities how to do it,  try LAC- I can send instructions to you,  test with color - the procedure you know from Belgrade, consult Mira which color would be acceptable in this case, eosin, ink or something else),  also after plasma treatment do sorption of silver, or cooper, or something else and measure amount of sorbed chemical (with ICPOES lets say, or titration method you can also use, depend what possibilities you have for measurements). Important thing- use different plasma treatment times to find optimal plasma treatment, before you go on FTIR, XPS ..

Dear Vadym,

I preassume you are trying to characterize the FTIR with ATR technique after Plasma treatment. You must keep the contact pressure on KRS5 crystal invariant. Secondly, use C-H stretching or allied band as reference band to calculate the optical density as reference band. Calculate  optical density of C=O band intensity as sensitive band. Take ratio of the Optical densities of both bands  to assess the changes due to plasma treatment. Further, you are using inert gas plasma. Instead  introduce O2 as carrier gas to intensify the effect of plasma and study the FTIR.

First of all thanks for all the answers, now I think I started to understand better that these fibers are not as easy material as pure metals or semiconducors. Zsolt, in the end I will try to follow your procedure. What I think happens was answered by Sergio as we detecting changes of somehow similar bands on some samples only (and detectabilty of those probably depends on sample itself more than on our treatment). Based on our numerous tests, I think that's the only reasonable explanation to be used when presenting the obtained results. Nina, we observe that wettability is changing, but it is rather simple technique that essentially do not tell us nothing. The final goal of my colleagues is to create a "better" composite with fibers and some polymer, and wettability in this case just demonstrates that our jet behaves as any other atmospheric plasma (more or less). Guddo, thanks for your note, we allowed to pressure be a bit different (not too much though), I will make sure to keep pressure value to a fixed value. Though we are using Ar plasma, of course we have ambient air that receives energy from plasma (plasma color is changed to pink due to nitrogen emission from air) close to the fibers, thus we have oxygen and hydroxyl radicals that are doing surface chemistry also. I am going to look for a procedure to calculate optical density also (I have never used it before).  

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