Mr. Singh, usually these are the recommendations due to the low signal-to-noise ratio of ATR. But in your case there has no any other possibility for measurements than ATR. In transmission (or absorption) of aqueous media would be possible but in D2O. remains transmission measurements from thin film, if it is enough transparent.

For baseline correction please read the paper with its supporting info file:

Analyst, 2012,137, 3355-3364 , A quantitative solid-state Raman spectroscopic method for control of fungicides, Bojidarka Ivanova and Michael Spiteller

Thanks. But, we do subtract water spectra to get spectra of our sample.

You can do both.

Measuring in absorption mode has the advantage that you know how much intensity reaches the detector / is absorbed by the sample. You later have to correct for the wavelength dependent penetration depth of IR radiation. This can be done by simply converting the absorption spectrum into an ATR spectrum.

If you measure in ATR mode from the beginning the correction for penetration depth is done automatically and you'll never know the sample absorption.

It depends on the sample. When possible, it is always better to use the absorption mode. But when the sample is not suitable and absorption mode gives bad spectra, the ATR mode needs to be used. For what concerns the baseline correction: there are different methods, and there is no general rule for choosing one method or the other. In any case: i) the baseline correction should be neither “too strong” nor “too weak” ii) to compare different spectra, it is important to use the same baseline method for all the spectra.

I also recommend to use the (original) absorption mode (absorption units).

Using ATR mode ("ATR units") means, that in the OPUS Software of Bruker, your original spectrum is weighted by a linear function of the wavenumber, in such a way, that intensities at higher wavenumbers (having lower penetration depths) appear more intense compared to intensities at lower wavenumbers (having higher penetration depths).

So ATR correction is only a numerical correction (multiplication of your spectrum (vector) with linear function), but does not result in any improvement of signal/noise ratio etc.

Even, if you like to quantify adsorbed amounts etc. by ATR-FTIR spectroscopy, you MUST use the originally measured spectrum without post treatment like ATR correction.

You might look for quantitative access to magnitudes like concentration or surfac concentration  by ATR-FTIR in this reference and therein cited references.

M. Müller, B. Torger, E. Bittrich, E. Kaul,  L. Ionov, P. Uhlmann, M. Stamm, In-situ ATR-FTIR for characterization of thin biorelated polymer films Thin Solid Films, 556, 1-8 (2014)

Best regards,

Martin Müller

The absorbance is measured in far direct way after fourier transforms as compared to the ATR unit recalculations. The ATR units are calculated by linearly enhancing the signal inntensities but doesnot provide any improvement in S/N ratio for low intensity signals as well as higher wavenumbers.

why we do FTIR? one line purose of FTIR?

Attenuated total reflectance (ATR) is a special accessory unit which can be used with Fourier transform infrared (FTIR) spectrometers. It enables you to measure directly onto a solid state sample surface by pressing the sample towards an ATR crystal (e.g. diamond), including liquid and gas samples, thus avoiding the need to prepare pellets (small concentration of sample in e.g. KBr, mechanically and/or chemically dissolved) for use in the normal transmittance mode. The bulk of a sample is readily measured by first slicing off the top layer of the sample to the desired depth (as long as that is possible). The ATR-FTIR technique makes it possible to study materials which are non-transparent to infrared radiation in a pristine condition. Hence, the extensive, time-consuming and often cumbersome sample preparation by pressing thin KBr pellets as in traditional FTIR transmittance spectroscopy is avoided. The traditional technique might even change the sample material in question. The ATR technique is based on a special reflectance setup where the sample is pressed directly onto various crystals with high refractive indices, e.g. diamond. When interpreting ATR-FTIR spectra where quantitative results are important, note that non-corrected ATR spectra have much stronger absorbance bands at longer wavelengths (smaller wave numbers) than at shorter wavelengths (larger wave numbers) compared to normal FTIR transmittance spectra. The reason being that the penetration depth is dependent on (in addition to refractive indices of sample and ATR crystal, and angle of incident radiation) the radiation wavelength, and increases with increasing wavelength (decreasing wave number). The sample should be measured at different locations (where you can specify the FTIR instrument to run several spectra in one run, e.g. 32 scans, which are automatically averaged into one spectrum), but since you’re doing ATR-FTIR you should not make an average of the spectra taken at different locations with different contact variations between the sample and the ATR crystal. Relatively hard surfaces complicate accurate quantitative measurements (height of absorbance peaks) due to varying contact with the ATR crystal for the different samples. Air between sample and ATR crystal results in a weaker absorbance signal. Thus, unless other conditions indicate otherwise (e.g. inhomogenities, impurities, etc.), the FTIR spectrum with the largest absorbance peaks represents the most correct measurements on one and the same sample, and hence this spectrum should be chosen as it is assumed to be the most correct one. Qualitative measurements (location of absorbance peaks at wave numbers) do not represent a problem as long as the contact area is large enough to ensure a sufficient strong measurement signal. Various details about the ATR-FTIR technique which may be helpful may be found in the two following articles (may be requested through ResearchGate): (a) 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 (b) 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. Concerning baselines: In any case you should first run a baseline with no sample on your ATR crystal, then running a sample spectra with still no sample on your ATR crystal as a check to see if you then obtain the correct 100 % transmittance spectrum horizontal line with as little noise as possible on the spectrum line.

ATR correction is actually much more than just a multiplication with a linear factor. If you just do the latter, your intensities are still incorrect and have such strong bandshifts that database software will frequently fail to identify your compounds. Have a look at https://www.researchgate.net/post/ATR_correction-why_would_you_not_use_it ! Unfortunately the software provided by the vendors seems to be not very sophisticated...