Hi, 

ATR is considered as a technique adapted for "surface" characterizations, with a penetration in the materials from a hundred nm to few microns depending on the crystal used, the wavenumber investigated, the nature of the material... So it could be adapted to the characterization of modified surfaces. 

Using ATR you should be able to determine the presence or not of Tetracycline by looking at the adsorbtion due to organic bonds such as CO, NH, .... Reference pattern for this compound can be easily found in litterature.

I agreed with Vantin Robin........