What does the increase or decrease in Peak intensities in ATR-FTIR spectra mean?

Dear Bernhard Torger

I am using ZnSe crystal with pressure clamp assembly to firmly keep in contact the skin sample with ZnSe crystal...is it not possible that any changes in the composition of skin cause by the treatment effects the transmission intensity to either increase or decrease???

Thanks sir....

First of all you should plot your ATR-FTIR spectra in absorbance mode.

A = -log(I/I0) I: single channel sample, I0: single channel background.

Only in absorbance (A) mode you have a quasilinear relationship to concentration

in distinct cases (see below).

Then you must distinguish between thin and thick films.

I guess animal skin samples are thick film samples

meaning thickness d > 300 nm.

If so, the observed intensity increases are due to an increase in concentration of the compounds of your skin sample (maybe by water loss ?)

Also check water band at 3400 and 1640 cm-1.

Best regards,

Martin

Thanks alot Martin Muller for clarification.

Regards

Generally, the characteristic bands shift toward a lower wave number region may be due to the increase in particles size. Because the frequency of

vibration is found to be inversely proportional to the mass

of vibrating molecule. So molecules with larger sizes have

less vibration frequency and lower the wave numbers.

And also please refer the following article.

As the absorption of electromagnetic radiation, e.g. IR radiation, follows the Beer-Lambert law, i.e. the radiation is decreasing exponentially with the penetration depth in the actual material, it is often helpful to plot the spectra on a logarithmic absorbance scale versus wavelength or wave number. Hence, a representative spectrum is chosen from each of the samples and plotted on a logarithmic absorbance scale for quantitative studies. Mathematically and physically it follows that a doubling of the logarithmic absorbance, also called optical density, is interpreted as a doubling of material thickness or a doubling of concentration of absorption active agents. In general, a spectrometer measures either the transmittance or the reflectance of a sample, i.e. measuring the radiation which is collected by the detector from either the transmitted or the reflected beam. Furthermore: T + A + R = 1 (100 %) where T = transmittance, A = absorbance and R = reflectance between 0 to 1 (or between 0 to 100 %). That is, the absorbance is not measured directly, but is calculated from the measured transmittance and reflectance (if both can be measured). When a spectrometer gives you the absorbance A’ on the logarithmic form, i.e. the optical density OD which is a common output from many spectrophotometers, it is in reality a calculation from the measured transmittance ignoring the reflectance. Note that OD = A’ = log10(1/T) = alog10(e)x = a’x, "10" supposed to be as a subscript here, i.e. the 10-log, where T is the transmittance, x is the penetration length or depth x, and a and a’ denote the absorption coefficients depending what form is used, which is deduced from the Beer-Lambert law. For further information see e.g. chapter 7.6.3 in the following article (may be requested through Research Gate): B. P. Jelle, ”Solar Radiation Glazing Factors for Window Panes, Glass Structures and Electrochromic Windows in Buildings - Measurement and Calculation”, Solar Energy Materials and Solar Cells, 116, 291-323, 2013. In addition, the following articles may also be of interest: (a) B. P. Jelle, A. Gustavsen, T.-N. Nilsen and T. Jacobsen, ”Solar Material Protection Factor (SMPF) and Solar Skin Protection Factor (SSPF) for Window Panes and other Glass Structures in Buildings”, Solar Energy Materials and Solar Cells, 91, 342-354, 2007, (b) B. P. Jelle, T.-N. Nilsen, P. J. Hovde and A. Gustavsen, ”Accelerated Climate Aging of Building Materials and their Characterization by Fourier Transform Infrared Radiation Analysis”, Journal of Building Physics, 36, 99-112, 2012, and (c) B. P. Jelle and T.-N. Nilsen, ”Comparison of Accelerated Climate Ageing Methods of Polymer Building Materials by Attenuated Total Reflectance Fourier Transform Infrared Radiation Spectroscopy”, Construction and Building Materials, 25, 2122-2132, 2011.

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